This part of the reference documentation covers all the technologies that are absolutely integral to the Spring Framework.

参考文档的这一部分涵盖了Spring框架中所有必不可少的技术。

Foremost amongst these is the Spring Framework’s Inversion of Control (IoC) container. A thorough treatment of the Spring Framework’s IoC container is closely followed by comprehensive coverage of Spring’s Aspect-Oriented Programming (AOP) technologies. The Spring Framework has its own AOP framework, which is conceptually easy to understand and which successfully addresses the 80% sweet spot of AOP requirements in Java enterprise programming.

其中最重要的就是Spring框架中的IoC容器。当对Spring IoC容器充分理解后,紧随其来的就是对Spring AOP(面向切面编程)相关的全面介绍。Spring框架拥有它自己的AOP框架,该框架从概念上易于理解,它成功解决了Java企业编程中80%AOP需求相关痛点。

Coverage of Spring’s integration with AspectJ (currently the richest — in terms of features — and certainly most mature AOP implementation in the Java enterprise space) is also provided.

其中也涵盖了Spring与AspectJ的集成(就功能而言,目前它是最丰富也是Java企业领域中最成熟的AOP实现)。

IoC容器(The IoC Container)

This chapter covers Spring’s Inversion of Control (IoC) container.

本章节涵盖了Spring IoC容器相关内容。

Spring IoC容器与Bean简介(Introduction to the Spring IoC Container and Beans)

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This chapter covers the Spring Framework implementation of the Inversion of Control (IoC) principle. IoC is also known as dependency injection (DI). It is a process whereby objects define their dependencies (that is, the other objects they work with) only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean. This process is fundamentally the inverse (hence the name, Inversion of Control) of the bean itself controlling the instantiation or location of its dependencies by using direct construction of classes or a mechanism such as the Service Locator pattern.

本章节涵盖了Spring框架对于控制反转(IoC)原理的实现。IoC也被称为依赖注入(DI) 。在此过程中,对象只能通过构造函数参数,工厂方法的参数来定义其依赖项(即与之一起使用的其他对象),或者在通过构造器构造或从工厂方法返回得到对象实例之后,在对象实例上对其属性进行设置。然后,容器在创建bean时注入那些依赖项。此过程从本质上来讲是通过使用类的直接构造或者例如服务定位模式之类的机制,从根本上反转了bean自身对它依赖项的定位(知秋注:和其他bean建立依赖联系)或实例化(因此称为控制反转)。

The org.springframework.beans and org.springframework.context packages are the basis for Spring Framework’s IoC container. The BeanFactory interface provides an advanced configuration mechanism capable of managing any type of object. ApplicationContext is a sub-interface of BeanFactory. It adds:

  • Easier integration with Spring’s AOP features

  • Message resource handling (for use in internationalization)

  • Event publication

  • Application-layer specific contexts such as the WebApplicationContext for use in web applications.

org.springframework.beansorg.springframework.context 两个包是Spring框架中IoC容器的基础。 BeanFactory 接口提供了一种能够管理任何类型对象的高级配置机制。 ApplicationContextBeanFactory 的子接口,它添加了以下特性:

  • 更易于与Spring AOP特性相整合

  • 消息资源处理(用于国际化处理)

  • 事件发布

  • 应用层特定上下文,例如:在Web应用中所使用的 WebApplicationContext

In short, the BeanFactory provides the configuration framework and basic functionality, and the ApplicationContext adds more enterprise-specific functionality. The ApplicationContext is a complete superset of the BeanFactory and is used exclusively in this chapter in descriptions of Spring’s IoC container. For more information on using the BeanFactory instead of the ApplicationContext, see The BeanFactory.

简单来说,BeanFactory 提供了配置框架以及基础功能,而 ApplicationContext 添加了更多企业特定的功能。 ApplicationContextBeanFactory 的一个完整超集,它在本章中只用来描述Spring IoC容器。如果想了解更多 BeanFactory 使用上的信息而不是 ApplicationContext 的话,请查看 The BeanFactory 这一章节。

In Spring, the objects that form the backbone of your application and that are managed by the Spring IoC container are called beans. A bean is an object that is instantiated, assembled, and otherwise managed by a Spring IoC container. Otherwise, a bean is simply one of many objects in your application. Beans, and the dependencies among them, are reflected in the configuration metadata used by a container.

在Spring中,构成你应用程序主干并通过Spring IoC容器进行管理的对象被称为bean。bean是一个由Spring IoC容器实例化,组装并管理的一个对象。否则的话,bean只是你应用中众多对象中的一个而已。Bean及其之间的依赖关系都反映在容器使用的配置元数据中。

容器概述(Container Overview)

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The org.springframework.context.ApplicationContext interface represents the Spring IoC container and is responsible for instantiating, configuring, and assembling the beans. The container gets its instructions on what objects to instantiate, configure, and assemble by reading configuration metadata. The configuration metadata is represented in XML, Java annotations, or Java code. It lets you express the objects that compose your application and the rich interdependencies between those objects.

org.springframework.context.ApplicationContext 接口表示Spring IoC容器,并且负责初始化、配置和组装bean。Spring容器通过读取配置元数据来获取初始化、配置和组装对象的指令。可以使用XML文件、Java注解或者Java代码来配置元数据。以此来让你在你应用程序中对这些配置bean对象进行组合并得到丰富的依赖关系。

Several implementations of the ApplicationContext interface are supplied with Spring. In stand-alone applications, it is common to create an instance of ClassPathXmlApplicationContext or FileSystemXmlApplicationContext. While XML has been the traditional format for defining configuration metadata, you can instruct the container to use Java annotations or code as the metadata format by providing a small amount of XML configuration to declaratively enable support for these additional metadata formats.

Spring提供了 ApplicationContext 的几种实现。在单体应用中,通常会创建 ClassPathXmlApplicationContext 或者 FileSystemXmlApplicationContext 的实例。尽管XML文件是定义配置文件的传统格式,但你可以通过使用少量的XML配置声明来对Java注解或代码这些额外的元数据格式进行支持。

In most application scenarios, explicit user code is not required to instantiate one or more instances of a Spring IoC container. For example, in a web application scenario, a simple eight (or so) lines of boilerplate web descriptor XML in the web.xml file of the application typically suffices (see Convenient ApplicationContext Instantiation for Web Applications). If you use the Spring Tool Suite (an Eclipse-powered development environment), you can easily create this boilerplate configuration with a few mouse clicks or keystrokes.

在大多数应用场景中,用户不需要显式的通过代码初始化一个或者多个Spring IoC容器。例如,在web应用程序场景中,它的 web.xml 文件中一个简单的8行样板web描述XML通常就足够了(详情请见 context-create )。如果你使用 Spring Tool Suite (Eclipse驱动的开发环境),那么你只需要点击几次鼠标就可以创建这种样板配置文件了。

The following diagram shows a high-level view of how Spring works. Your application classes are combined with configuration metadata so that, after the ApplicationContext is created and initialized, you have a fully configured and executable system or application.

下图高度概括展示了Spring是如何工作的。你的应用程序中的类通过配置元数据从而整合到一起,这样,在 ApplicationContext 创建并初始化之后,你就会有一个配置好的可执行的系统或应用程序。

container magic
Figure 1. The Spring IoC container

配置元数据(Configuration Metadata)

As the preceding diagram shows, the Spring IoC container consumes a form of configuration metadata. This configuration metadata represents how you, as an application developer, tell the Spring container to instantiate, configure, and assemble the objects in your application.

如上图所示,是一种表示Spring IoC容器将配置元数据进行消费使用的形式。作为应用程序开发者的你,通过配置元数据来告诉Spring容器,在你的应用程序中应该如何去实例化、配置和组装对象。

Configuration metadata is traditionally supplied in a simple and intuitive XML format, which is what most of this chapter uses to convey key concepts and features of the Spring IoC container.

传统的,我们通过简单且直观的XML格式来配置元数据,本章大部分关于Spring IoC容器的关键概念和特性也会以XML的形式来展现。

XML-based metadata is not the only allowed form of configuration metadata. The Spring IoC container itself is totally decoupled from the format in which this configuration metadata is actually written. These days, many developers choose Java-based configuration for their Spring applications.

: 基于XML的元数据不是配置元数据的唯一格式。Spring IoC容器本身与使用XML配置元数据的格式完全分离。如今,许多开发者在他们的应用程序中选择基于Java 代码的形式 Java-based configuration 来进行配置。

For information about using other forms of metadata with the Spring container, see:

Thus, you can define beans external to your application classes by using Java rather than XML files. To use these new features, see the @Configuration, @Bean, @Import, and @DependsOn annotations.

对于在Spring容器中使用其他方式配置元数据的相关信息,请查阅:

因此,你可以通过使用Java代码替代XML文件来在你的应用程序类外部定义bean。想要使用这些新特性,请查看 @Configuration, @Bean, @Import, and @DependsOn annotations.

Spring configuration consists of at least one and typically more than one bean definition that the container must manage. XML-based configuration metadata configures these beans as <bean/> elements inside a top-level <beans/> element. Java configuration typically uses @Bean-annotated methods within a @Configuration class.

Spring配置由容器管理的至少一个、通常是多个的bean定义组成。基于XML的配置元数据会将这些bean配置在 <beans/> 这个最顶级元素内部的 <bean/> 元素中。Java代码配置通常是在使用 @Configuration 注解的类中使用 @Bean 注解方式来对bean进行配置。

These bean definitions correspond to the actual objects that make up your application. Typically, you define service layer objects, data access objects (DAOs), presentation objects such as Struts Action instances, infrastructure objects such as Hibernate SessionFactories, JMS Queues, and so forth. Typically, one does not configure fine-grained domain objects in the container, because it is usually the responsibility of DAOs and business logic to create and load domain objects. However, you can use Spring’s integration with AspectJ to configure objects that have been created outside the control of an IoC container. See Using AspectJ to dependency-inject domain objects with Spring.

这些bean定义对应于实际组成应用程序的对象。通常,你定义服务层对象、数据访问层对象(DAO)、表现层对象如Struts的 Action 实例、基础结构对象如Hibernate的 SessionFactories 和JMS的 Queues 等等。通常不会在容器中配置细粒度的域对象,因为这一般是由DAOs和业务逻辑来创建和加载域对象的。然而,你可以使用Spring集成的AspectJ来对已创建的外部对象(知秋注:未在配置元数据中定义的bean)进行IoC容器控制配置。详情请看 Using AspectJ to dependency-inject domain objects with Spring

The following example shows the basic structure of XML-based configuration metadata:

下例展示了基于XML的配置元数据的基本结构:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="..." class="..."> (1) (2)
        <!-- collaborators and configuration for this bean go here -->
    </bean>

    <bean id="..." class="...">
        <!-- collaborators and configuration for this bean go here -->
    </bean>

    <!-- more bean definitions go here -->

</beans>
1 The id attribute is a string that identifies the individual bean definition.
2 The class attribute defines the type of the bean and uses the fully qualified classname.

The value of the id attribute refers to collaborating objects. The XML for referring to collaborating objects is not shown in this example. See Dependencies for more information.

1 id 属性是一个用于标识单独bean定义的字符串。
2 class 属性使用全类名的方式定义一个bean的类型。

id 属性的值用于指向协作对象。用于参考的协作对象的XML未在本例中进行展示。更多信息请参考Dependencies

实例化一个容器(Instantiating a Container)

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The location path or paths supplied to an ApplicationContext constructor are resource strings that let the container load configuration metadata from a variety of external resources, such as the local file system, the Java CLASSPATH, and so on.

将本地一个或多个资源路径字符串提供给 ApplicationContext 构造器,可使 container 从外部资源中加载配置需要的元数据。资源文件路径可以为本地文件、CLASSPATH 等。

Java
ApplicationContext context = new ClassPathXmlApplicationContext("services.xml", "daos.xml");
Kotlin
val context = ClassPathXmlApplicationContext("services.xml", "daos.xml")

After you learn about Spring’s IoC container, you may want to know more about Spring’s Resource abstraction (as described in Resources), which provides a convenient mechanism for reading an InputStream from locations defined in a URI syntax. In particular, Resource paths are used to construct applications contexts, as described in Application Contexts and Resource Paths.

在学习了Spring IoC container容器的相关知识后,你可能想了解更多有关Spring 中 Resource 抽象的知识 (详见 Resources ),该抽象提供了一个从URI语法定义的位置中读取InputStream的便捷机制。特别是,如 Application Contexts and Resource Paths 中所述,Resource 路径用于构造应用程序上下文。

The following example shows the service layer objects (services.xml) configuration file:

下面的例子是一个展示了 service 层对象的配置文件 (services.xml):

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

    <!-- services -->

    <bean id="petStore" class="org.springframework.samples.jpetstore.services.PetStoreServiceImpl">
        <property name="accountDao" ref="accountDao"/>
        <property name="itemDao" ref="itemDao"/>
        <!-- additional collaborators and configuration for this bean go here -->
    </bean>

    <!-- more bean definitions for services go here -->

</beans>

The following example shows the data access objects daos.xml file:

下面的例子是一个展示了数据访问层对象的配置文件 daos.xml:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="accountDao"
        class="org.springframework.samples.jpetstore.dao.jpa.JpaAccountDao">
        <!-- additional collaborators and configuration for this bean go here -->
    </bean>

    <bean id="itemDao" class="org.springframework.samples.jpetstore.dao.jpa.JpaItemDao">
        <!-- additional collaborators and configuration for this bean go here -->
    </bean>

    <!-- more bean definitions for data access objects go here -->

</beans>

In the preceding example, the service layer consists of the PetStoreServiceImpl class and two data access objects of the types JpaAccountDao and JpaItemDao (based on the JPA Object-Relational Mapping standard). The property name element refers to the name of the JavaBean property, and the ref element refers to the name of another bean definition. This linkage between id and ref elements expresses the dependency between collaborating objects. For details of configuring an object’s dependencies, see Dependencies.

在前面的示例中,服务层由 PetStoreServiceImpl 类和两个类型为 JpaAccountDaoJpaItemDao 的数据访问对象组成(基于JPA对象关系映射标准)。 property name 元素引用JavaBean中属性的name,而 ref 元素引用另一个bean定义的名称(即id名称)。 idref 元素之间的这种联系表达了协作对象之间的依赖性。 有关配置一个对象的依赖项的详细信息,请参见 Dependencies

组合基于XML的配置元数据(Composing XML-based Configuration Metadata)

It can be useful to have bean definitions span multiple XML files. Often, each individual XML configuration file represents a logical layer or module in your architecture.

在横跨多个XML文件中定义Bean是很有用的,往往,每个单独的XML配置文件代表你项目架构中的一个逻辑层或一个逻辑模块(译者注:比如我们经常使用的一个项目,多个模块开发模式)

You can use the application context constructor to load bean definitions from all these XML fragments. This constructor takes multiple Resource locations, as was shown in the previous section. Alternatively, use one or more occurrences of the <import/> element to load bean definitions from another file or files. The following example shows how to do so:

你可以使用应用程序上下文构造函数从所有这些XML片段中加载bean定义。该构造函数可以接收多个 Resource 位置,如 上一节所示。或者,使用一个或多个 <import/> 元素来从另一个或多个xml文件中加载bean的定义。以下示例显示了该如何执行此操作:

<beans>
    <import resource="services.xml"/>
    <import resource="resources/messageSource.xml"/>
    <import resource="/resources/themeSource.xml"/>

    <bean id="bean1" class="..."/>
    <bean id="bean2" class="..."/>
</beans>

In the preceding example, external bean definitions are loaded from three files: services.xml, messageSource.xml, and themeSource.xml. All location paths are relative to the definition file doing the importing, so services.xml must be in the same directory or classpath location as the file doing the importing, while messageSource.xml and themeSource.xml must be in a resources location below the location of the importing file. As you can see, a leading slash is ignored. However, given that these paths are relative, it is better form not to use the slash at all. The contents of the files being imported, including the top level <beans/> element, must be valid XML bean definitions, according to the Spring Schema.

在上面的示例中,从 services.xml, messageSource.xmlthemeSource.xml 这三个文件来加载外部bean定义。导入的bean定义文件使用的都是本地相对路径。所以 services.xml 与执行导入的文件必须在同一个文件目录或同一个classpath 位置。而 messageSource.xmlthemeSource.xml 必须在一个叫 resources 的位置下。如你所见,/ 被忽略,鉴于 这些路径是相对的,最好不要使用 /。关于这些文件被导入的内容,包括顶级的 <beans/> 元素,根据Spring Schema,必须是有效的XML bean定义。

It is possible, but not recommended, to reference files in parent directories using a relative "../" path. Doing so creates a dependency on a file that is outside the current application. In particular, this reference is not recommended for classpath: URLs (for example, classpath:../services.xml), where the runtime resolution process chooses the “nearest” classpath root and then looks into its parent directory. Classpath configuration changes may lead to the choice of a different, incorrect directory.

You can always use fully qualified resource locations instead of relative paths: for example, file:C:/config/services.xml or classpath:/config/services.xml. However, be aware that you are coupling your application’s configuration to specific absolute locations. It is generally preferable to keep an indirection for such absolute locations — for example, through "${…​}" placeholders that are resolved against JVM system properties at runtime.

使用"../" 这个相对路径来指向父目录是可行的,但不推荐。这样做会为当前应用程序创建一个外部文件的依赖。 特别是,不建议在 classpath 的URLs(例如 classpath:../services.xml)中使用。在这些URL中,程序在运行解析过程会选择“ 最近 ”classpath 根路径,然后查看其父级目录。classpath配置的更改可能导致选择其他错误的目录。

你可以一直使用绝对资源路径来代替相对路径:例如,file:C:/config/services.xmlclasspath:/config/services.xml。 但是,请注意,你会将应用程序的配置耦合到特定的绝对路径位置。通常最好为这样的绝对路径保留一个间接寻址—​例如,通过设定一个针对JVM系统属性的 "${…​}" 占位符,在运行时对它进行解析。

The namespace itself provides the import directive feature. Further configuration features beyond plain bean definitions are available in a selection of XML namespaces provided by Spring — for example, the context and util namespaces.

命名空间自身提供了import指令功能。在Spring提供的一系列XML命名空间中除了提供了除普通bean的定义,它还提供了其他配置功能。例如,contextutil 这两种命名空间。

The Groovy Bean Definition DSL

As a further example for externalized configuration metadata, bean definitions can also be expressed in Spring’s Groovy Bean Definition DSL, as known from the Grails framework. Typically, such configuration live in a ".groovy" file with the structure shown in the following example:

作为外部化配置元数据的另一个示例,Bean定义也可以在Spring的Groovy Bean定义DSL中表达,这从Grails框架中可以知道。 通常,此类配置位于一个 ".groovy"文件中,其结构如下例所示:

beans {
    dataSource(BasicDataSource) {
        driverClassName = "org.hsqldb.jdbcDriver"
        url = "jdbc:hsqldb:mem:grailsDB"
        username = "sa"
        password = ""
        settings = [mynew:"setting"]
    }
    sessionFactory(SessionFactory) {
        dataSource = dataSource
    }
    myService(MyService) {
        nestedBean = { AnotherBean bean ->
            dataSource = dataSource
        }
    }
}

This configuration style is largely equivalent to XML bean definitions and even supports Spring’s XML configuration namespaces. It also allows for importing XML bean definition files through an importBeans directive.

这种配置风格在很大程度上等同于XML bean定义,甚至支持Spring的XML配置命名空间。它还允许通过“importBeans”指令导入XML bean定义文件。

容器使用(Using the Container)

The ApplicationContext is the interface for an advanced factory capable of maintaining a registry of different beans and their dependencies. By using the method T getBean(String name, Class<T> requiredType), you can retrieve instances of your beans.

ApplicationContext 是一个能够维护不同bean的注册信息及其依赖关系的高级工厂接口。 通过使用方法 T getBean(String name, Class<T> requiredType),可以检索查找bean的实例。

The ApplicationContext lets you read bean definitions and access them, as the following example shows:

通过 ApplicationContext ,你可以读取bean定义并使用它们,如下所示:

Java
// create and configure beans
ApplicationContext context = new ClassPathXmlApplicationContext("services.xml", "daos.xml");

// retrieve configured instance
PetStoreService service = context.getBean("petStore", PetStoreService.class);

// use configured instance
List<String> userList = service.getUsernameList();
Kotlin
import org.springframework.beans.factory.getBean

// create and configure beans
val context = ClassPathXmlApplicationContext("services.xml", "daos.xml")

// retrieve configured instance
val service = context.getBean<PetStoreService>("petStore")

// use configured instance
var userList = service.getUsernameList()

With Groovy configuration, bootstrapping looks very similar. It has a different context implementation class which is Groovy-aware (but also understands XML bean definitions). The following example shows Groovy configuration:

对于Groovy配置来说,它的引导方式看起来和我们之前接触的其他同类引导非常相似。它有一个与众不同且专为Groovy而设计的上下文实现类(该类同样也可以识别XML中的bean定义)。Groovy配置示例如下:

Java
ApplicationContext context = new GenericGroovyApplicationContext("services.groovy", "daos.groovy");
Kotlin
val context = GenericGroovyApplicationContext("services.groovy", "daos.groovy")

The most flexible variant is GenericApplicationContext in combination with reader delegates — for example, with XmlBeanDefinitionReader for XML files, as the following example shows:

对应于上面的例子,最灵活的方式是将 GenericApplicationContext 与对应的reader代理(知秋注:可以是XML文件,也可以是Groovy配置文件)一起结合使用。比如,对于XML配置文件,它可以使用 XmlBeanDefinitionReader ,具体使用如下例所示:

Java
GenericApplicationContext context = new GenericApplicationContext();
new XmlBeanDefinitionReader(context).loadBeanDefinitions("services.xml", "daos.xml");
context.refresh();
Kotlin
val context = GenericApplicationContext()
XmlBeanDefinitionReader(context).loadBeanDefinitions("services.xml", "daos.xml")
context.refresh()

You can also use the GroovyBeanDefinitionReader for Groovy files, as the following example shows:

同样,针对 Groovy 文件也可以使用 GroovyBeanDefinitionReader ,示例如下:

Java
GenericApplicationContext context = new GenericApplicationContext();
new GroovyBeanDefinitionReader(context).loadBeanDefinitions("services.groovy", "daos.groovy");
context.refresh();
Kotlin
val context = GenericApplicationContext()
GroovyBeanDefinitionReader(context).loadBeanDefinitions("services.groovy", "daos.groovy")
context.refresh()

You can mix and match such reader delegates on the same ApplicationContext, reading bean definitions from diverse configuration sources.

你可以在 ApplicationContext 中混合组合对应匹配 reader ,从不同的配置源读取bean的定义。

You can then use getBean to retrieve instances of your beans. The ApplicationContext interface has a few other methods for retrieving beans, but, ideally, your application code should never use them. Indeed, your application code should have no calls to the getBean() method at all and thus have no dependency on Spring APIs at all. For example, Spring’s integration with web frameworks provides dependency injection for various web framework components such as controllers and JSF-managed beans, letting you declare a dependency on a specific bean through metadata (such as an autowiring annotation).

然后,你可以使用 getBean 方法去检索你的bean实例。 ApplicationContext 接口为检索bean实例也提供了一些其他方法。但理想情况下,你的应用程序代码中应该永远用不到它们。确实,你的应用程序代码应该根本不会调用 getBean() 方法。因此,这里根本不依赖于Spring的API。例如,集成了web框架的Spring对于不同的web框架组件(例如controller和JSF所管理的bean)提供了依赖注入。让你通过元数据(例如 @Autowired 注解)来对一个特定的bean进行依赖声明。

Bean Overview

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A Spring IoC container manages one or more beans. These beans are created with the configuration metadata that you supply to the container (for example, in the form of XML <bean/> definitions).

Spring的 IoC容器管理着一个或者多个 bean 。这些beans是通过你给容器所提供的元数据创建的。(例如:在XML中通过 <bean/> 的形式进行的定义)

Within the container itself, these bean definitions are represented as BeanDefinition objects, which contain (among other information) the following metadata:

在IoC容器内部,这些bean被表示为 BeanDefinition 对象,这个对象包含了以下元数据:

  • A package-qualified class name: typically, the actual implementation class of the bean being defined.

  • Bean behavioral configuration elements, which state how the bean should behave in the container (scope, lifecycle callbacks, and so forth).

  • References to other beans that are needed for the bean to do its work. These references are also called collaborators or dependencies.

  • Other configuration settings to set in the newly created object — for example, the size limit of the pool or the number of connections to use in a bean that manages a connection pool.

  • 包限定的类名:通常是所定义bean的实际实现类

  • bean行为配置元素,用来定义bean在容器中的行为(作用域,生命周期回调等。译者注:比如定义这个对象是不是以单例(sington)形式存在)。

  • 该bean想要正常工作需要对其他bean进行引用。这些引用也被称为协作bean或者依赖项(知秋注:这些依赖项指bean定义内的相关引用类型的字段所引用的容器里的bean)

  • 在新创建的对象中设置其他配置项—​例如:管理连接池bean的大小或者连接数的限制。

This metadata translates to a set of properties that make up each bean definition. The following table describes these properties:

这些元数据转化成了每个bean中的定义的属性。下面是这些属性的描述:

Table 1. The bean definition
Property Explained in…​

Class

初始化Beans(Instantiating Beans)

Name

Bean的命名(Naming Beans)

Scope

Bean作用域(Bean Scopes)

Constructor arguments

依赖注入(Dependency Injection)

Properties

依赖注入(Dependency Injection)

Autowiring mode

自动装配协作者(对bean的相关依赖进行自动注入)(Autowiring Collaborators)

Lazy initialization mode

bean的懒加载(Lazy-initialized Beans)

Initialization method

Initialization Callbacks

Destruction method

Destruction Callbacks

In addition to bean definitions that contain information on how to create a specific bean, the ApplicationContext implementations also permit the registration of existing objects that are created outside the container (by users). This is done by accessing the ApplicationContext’s BeanFactory through the getBeanFactory() method, which returns the BeanFactory DefaultListableBeanFactory implementation. DefaultListableBeanFactory supports this registration through the registerSingleton(..) and registerBeanDefinition(..) methods. However, typical applications work solely with beans defined through regular bean definition metadata.

容器中除了包含用于创建特定bean的bean定义信息外,ApplicationContext 实现也允许将容器外部(由用户)创建的现有对象注册其中。这是通过 ApplicationContext 中的 BeanFactory 调用 getBeanFactory() 方法来完成的。该方法返回BeanFactory 的 DefaultListableBeanFactory 实现。DefaultListableBeanFactory通过 registerSingleton(..)registerBeanDefinition(..) 方法来对此注册提供支持。但是,通常情况下,应用程序只能通过常规的bean的元数据定义来得到容器里的bean。

Bean metadata and manually supplied singleton instances need to be registered as early as possible, in order for the container to properly reason about them during autowiring and other introspection steps. While overriding existing metadata and existing singleton instances is supported to some degree, the registration of new beans at runtime (concurrently with live access to the factory) is not officially supported and may lead to concurrent access exceptions, inconsistent state in the bean container, or both.

Bean元数据和手动提供的单例实例需要尽早注册,以便容器在自动注入和其他自省步骤中对它们进行正确地推理。虽然在某种程度上支持覆盖现有元数据和现有单例实例,但是这种操作在运行时(会同时对bean工厂访问)对新bean的注册不被支持,并且可能导致并发访问异常,或容器中bean状态不一致。

Bean的命名(Naming Beans)

Every bean has one or more identifiers. These identifiers must be unique within the container that hosts the bean. A bean usually has only one identifier. However, if it requires more than one, the extra ones can be considered aliases.

每个bean有一个或多个标识符。这些标识符在承载bean的容器中必须是唯一的。通常情况下一个bean只有一个标识符。然而,如果需要多个,多余的可以视作别名。

In XML-based configuration metadata, you use the id attribute, the name attribute, or both to specify the bean identifiers. The id attribute lets you specify exactly one id. Conventionally, these names are alphanumeric ('myBean', 'someService', etc.), but they can contain special characters as well. If you want to introduce other aliases for the bean, you can also specify them in the name attribute, separated by a comma (,), semicolon (;), or white space. As a historical note, in versions prior to Spring 3.1, the id attribute was defined as an xsd:ID type, which constrained possible characters. As of 3.1, it is defined as an xsd:string type. Note that bean id uniqueness is still enforced by the container, though no longer by XML parsers.

在基于XML的配置元数据中,你可以使用 id 属性,name 属性,或同时使用这两者来作为bean标识符。id 属性是唯一的。通常情况下,这些名称是字母或数字( myBeansomeService 等),但是也可以包含特殊字符。如果要为bean引入其他别名,也可以在 name 属性中指定它们,并用逗号(,),分号(;)或空格分隔。在Spring的历史中,Spring 3.1之前的版本中,id 属性被定义为一个叫 xsd:ID 的类型,该类型限制了很多字符。从3.1开始,它被定义为 xsd:string 类型。特别强调,bean id 的唯一性仍由容器强制执行,不再由XML解析器执行。

You are not required to supply a name or an id for a bean. If you do not supply a name or id explicitly, the container generates a unique name for that bean. However, if you want to refer to that bean by name, through the use of the ref element or a Service Locator style lookup, you must provide a name. Motivations for not supplying a name are related to using inner beans and autowiring collaborators.

Springidname 不是必须提供的。如果你不显式提供 nameid ,容器将为该bean生成一个唯一的名称。但是,如果要通过 name 引用该bean,则通过使用ref元素或 Service Locator 方式查找,此时你必须提供一个名称。如果你使用inner beansautowiring collaborators,则无须为bean提供一个 name。

Bean Naming Conventions

The convention is to use the standard Java convention for instance field names when naming beans. That is, bean names start with a lowercase letter and are camel-cased from there. Examples of such names include accountManager, accountService, userDao, loginController, and so forth.

Naming beans consistently makes your configuration easier to read and understand. Also, if you use Spring AOP, it helps a lot when applying advice to a set of beans related by name.

Bean命名约定

我们约定使用Java对实例字段的标准命名规范。也就是说,bean名称以小写字母开头,使用驼峰命名法。举个例子比如: accountManageraccountServiceuserDaologinController ,等等。

一致地命名方式可以使你的配置可读性更好,更容易理解。另外,如果你使用Spring AOP,则在将advice应用于一组与name相关的bean时,它会很有帮助。

With component scanning in the classpath, Spring generates bean names for unnamed components, following the rules described earlier: essentially, taking the simple class name and turning its initial character to lower-case. However, in the (unusual) special case when there is more than one character and both the first and second characters are upper case, the original casing gets preserved. These are the same rules as defined by java.beans.Introspector.decapitalize (which Spring uses here).
通过在类路径中进行组件扫描,Spring会按照前面描述的规则为未命名的组件生成Bean名称:本质上,采用简单的类名称并将其首字母转换为小写。但是,在一些(不寻常的)特殊情况下,如果有多个字符并且第一个和第二个字符均为大写字母,则会保留原始大小写。这些规则与 java.beans.Introspector.decapitalize (由Spring在此处使用)定义的规则相同。
定义bean的别名(Aliasing a Bean outside the Bean Definition)

In a bean definition itself, you can supply more than one name for the bean, by using a combination of up to one name specified by the id attribute and any number of other names in the name attribute. These names can be equivalent aliases to the same bean and are useful for some situations, such as letting each component in an application refer to a common dependency by using a bean name that is specific to that component itself.

对于bean的定义本身来说,可以通过使用最多一个 id 属性和任意数量的其他名称的组合来为 bean 提供多个名称 name 。这些name可以是同一个 bean 的等效别名,并且在某些情况下很有用,例如让应用程序中的每个组件通过使用特定于该组件的bean名称来引用这个公共依赖项。

Specifying all aliases where the bean is actually defined is not always adequate, however. It is sometimes desirable to introduce an alias for a bean that is defined elsewhere. This is commonly the case in large systems where configuration is split amongst each subsystem, with each subsystem having its own set of object definitions. In XML-based configuration metadata, you can use the <alias/> element to accomplish this. The following example shows how to do so:

然而,在实际开发过程中我们并不能将所有别名一下子全都定义出来(译者注:我们很多时候并不可能将未来的模块全都考虑进去)。有时需要在别处为定义的bean引入别名。在拆分配置的大型系统中经常需要用到这种情况。每个子系统都有自己的对象定义集。在基于XML的配置元数据中,你可以使用 <alias/> 元素来完成此任务。以下示例显示了该如何执行此操作:

<alias name="fromName" alias="toName"/>

In this case, a bean (in the same container) named fromName may also, after the use of this alias definition, be referred to as toName.

在这个的例子中,一个名叫 fromNamebean (在同一个容器中),在使用了这个别名定义后也能作为 toName 来在容器中被引用到。

For example, the configuration metadata for subsystem A may refer to a DataSource by the name of subsystemA-dataSource. The configuration metadata for subsystem B may refer to a DataSource by the name of subsystemB-dataSource. When composing the main application that uses both these subsystems, the main application refers to the DataSource by the name of myApp-dataSource. To have all three names refer to the same object, you can add the following alias definitions to the configuration metadata:

例如,子系统A的配置元数据引用一个叫 SubsystemA-dataSource 的数据源。 子系统B的配置元数据引用一个叫 SubsystemB-dataSource 的数据源。 组合并使用这两个子系统的主程序所使用的DataSource的名称为 myApp-dataSource 。要使所有三个名称都引用同一个对象,可以将以下别名定义添加到配置元数据中:

<alias name="myApp-dataSource" alias="subsystemA-dataSource"/>
<alias name="myApp-dataSource" alias="subsystemB-dataSource"/>

Now each component and the main application can refer to the dataSource through a name that is unique and guaranteed not to clash with any other definition (effectively creating a namespace), yet they refer to the same bean.

现在,每个组件和主应用程序都可以通过唯一的名称引用数据源,并保证不与任何其他定义冲突(有效地创建了一个命名空间),但它们引用的是同一个bean。

Java-configuration

If you use Javaconfiguration, the @Bean annotation can be used to provide aliases. See Using the @Bean Annotation for details.

Java配置

如果通过Java代码来写配置, @Bean 这个注解提供了注册别名的方式 可以通过查看 Using the @Bean Annotation 来了解细节.

初始化Beans(Instantiating Beans)

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A bean definition is essentially a recipe for creating one or more objects. The container looks at the recipe for a named bean when asked and uses the configuration metadata encapsulated by that bean definition to create (or acquire) an actual object.

bean的定义本质上是为创建一个或多个对象提供配方。容器在被询问时,则会通过所问的bean名字来找配方,并使用该bean定义所封装的配置元数据来创建(或获取)一个实际对象。

If you use XML-based configuration metadata, you specify the type (or class) of object that is to be instantiated in the class attribute of the <bean/> element. This class attribute (which, internally, is a Class property on a BeanDefinition instance) is usually mandatory. (For exceptions, see 通过使用实例工厂方法进行初始化(Instantiation by Using an Instance Factory Method) and Bean Definition Inheritance.) You can use the Class property in one of two ways:

如果你使用基于XML的配置元数据,需要在 <bean/> 元素的 class 属性中指定要实例化对象的类型(或class)。 这个 class 属性( 在内部是 BeanDefinition 实例的 Class 属性)通常是必须的。(也有一些例外情况,可以查看 通过使用实例工厂方法进行初始化(Instantiation by Using an Instance Factory Method) 以及 Bean Definition Inheritance)。 你可以通过以下两种方式之一使用 Class 属性:

  • Typically, to specify the bean class to be constructed in the case where the container itself directly creates the bean by calling its constructor reflectively, somewhat equivalent to Java code with the new operator.

  • To specify the actual class containing the static factory method that is invoked to create the object, in the less common case where the container invokes a static factory method on a class to create the bean. The object type returned from the invocation of the static factory method may be the same class or another class entirely.

  • 通常,在指定了bean的class之后,容器可以通过反射直接调用它的构造方法来创建bean实例,这等同于在Java代码中使用 new 来创建bean。

  • 如果指明了实际的类中包含一个 static 的工厂方法来创建对象。在这种不太常见的情况下,容器会调用类中的 static 工厂方法来创建一个bean实例。从 static 工厂方法中返回的可能与工厂方法所在的类是同一个对象,也可以是另外一个实例。

Inner class names

If you want to configure a bean definition for a static nested class, you have to use the binary name of the nested class.

For example, if you have a class called SomeThing in the com.example package, and this SomeThing class has a static nested class called OtherThing, the value of the class attribute on a bean definition would be com.example.SomeThing$OtherThing.

Notice the use of the $ character in the name to separate the nested class name from the outer class name.

Inner class names

如果想要为 static 嵌套类来配置bean定义,则必须使用该嵌套类的二元名称。

例如,如果在 com.example 包中有一个叫做 SomeThing 的类,并且 SomeThing 这个类有一个 static 的嵌套子类叫做 OtherThing, OtherThing 在bean定义中的 class 属性则应该是 com.example.SomeThing$OtherThing

需要注意名称中使用了 $ 符号来分隔外部类名与嵌套类名。

通过构造器进行初始化(Instantiation with a Constructor)

When you create a bean by the constructor approach, all normal classes are usable by and compatible with Spring. That is, the class being developed does not need to implement any specific interfaces or to be coded in a specific fashion. Simply specifying the bean class should suffice. However, depending on what type of IoC you use for that specific bean, you may need a default (empty) constructor.

当你使用构造器的方式来创建bean时,所有的普通类都可以通过Spring使用,并且与Spring兼容。也就是说,正在开发中的类不需要实现任何特定的接口或者以某种特定的方式进行编码。只需要简单的声明bean的class就可以了。但是,因为使用了IOC的方式来声明具体的bean,你需要给该类创建一个默认的(空的)构造方法。

The Spring IoC container can manage virtually any class you want it to manage. It is not limited to managing true JavaBeans. Most Spring users prefer actual JavaBeans with only a default (no-argument) constructor and appropriate setters and getters modeled after the properties in the container. You can also have more exotic non-bean-style classes in your container. If, for example, you need to use a legacy connection pool that absolutely does not adhere to the JavaBean specification, Spring can manage it as well.

Spring IoC 容器实际上可以管理任何你想要它管理的类。它并不仅限于可以管理真正的 JavaBeans 。大多数的Spring使用者更喜欢用只有默认的(无参)构造方法和针对属性的get与set方法的普通 JavaBeans。在容器中,也可以拥有一个非普通 bean 样式的类。例如,如果你有一个并不符合 JavaBean声明的数据库连接池,Spring 同样可以很好的管理它。

With XML-based configuration metadata you can specify your bean class as follows:

使用基于XML的配置元数据,你可以通过以下方式进行bean的声明:

<bean id="exampleBean" class="examples.ExampleBean"/>

<bean name="anotherExample" class="examples.ExampleBeanTwo"/>

For details about the mechanism for supplying arguments to the constructor (if required) and setting object instance properties after the object is constructed, see Injecting Dependencies.

关于向构造函数传递参数(如果需要)以及在对象被构造之后设置对象实例的属性机制的详细信息,可以查看 Injecting Dependencies

通过静态工厂方法进行初始化(Instantiation with a Static Factory Method)

When defining a bean that you create with a static factory method, use the class attribute to specify the class that contains the static factory method and an attribute named factory-method to specify the name of the factory method itself. You should be able to call this method (with optional arguments, as described later) and return a live object, which subsequently is treated as if it had been created through a constructor. One use for such a bean definition is to call static factories in legacy code.

当定义用静态工厂方法创建bean时,使用 class 属性指定包含静态工厂方法的类,用 factory-method 属性指定 工厂方法本身的名称。你应该可以调用这个方法(带可选参数,如后面所述)并返回一个活动对象,该对象随后被认为是通过 构造函数创建的对象。这种bean定义的一个用途是可以在遗留代码中调用 static 工厂。

The following bean definition specifies that the bean be created by calling a factory method. The definition does not specify the type (class) of the returned object, only the class containing the factory method. In this example, the createInstance() method must be a static method. The following example shows how to specify a factory method:

下面的bean定义指定了通过工厂方法创建bean。该定义并不指定返回对象的类型(class),只指定包含工厂方法的类。在本例中, createInstance() 方法必须是静态方法。下面的示例中演示如何指定工厂方法:

<bean id="clientService"
    class="examples.ClientService"
    factory-method="createInstance"/>

The following example shows a class that would work with the preceding bean definition:

以下为与上面bean定义一起工作的类代码:

Java
public class ClientService {
    private static ClientService clientService = new ClientService();
    private ClientService() {}

    public static ClientService createInstance() {
        return clientService;
    }
}
Kotlin
class ClientService private constructor() {
    companion object {
        private val clientService = ClientService()
        fun createInstance() = clientService
    }
}

For details about the mechanism for supplying (optional) arguments to the factory method and setting object instance properties after the object is returned from the factory, see Dependencies and Configuration in Detail.

关于向工厂方法提供可选参数以及在对象实例从工厂方法返回之后设置属性的详情,参见 Dependencies and Configuration in Detail

通过使用实例工厂方法进行初始化(Instantiation by Using an Instance Factory Method)

Similar to instantiation through a static factory method, instantiation with an instance factory method invokes a non-static method of an existing bean from the container to create a new bean. To use this mechanism, leave the class attribute empty and, in the factory-bean attribute, specify the name of a bean in the current (or parent or ancestor) container that contains the instance method that is to be invoked to create the object. Set the name of the factory method itself with the factory-method attribute. The following example shows how to configure such a bean:

与通过静态工厂方法 static factory method 进行的实例化类似,使用实例工厂方法进行实例化是通过调用容器中一个存在bean的非静态方法来创建一个新的bean。 要使用这种机制,需要使 class 属性为空,并在 factory-bean 属性中配置当前(或父辈或祖先)容器中一个bean的名称,该bean包含要被用来创建对象实例的方法。使用 factory-method 属性设置factory方法本身的名称。 下面的例子中展示了如何配置这样一个bean:

<!-- the factory bean, which contains a method called createInstance() -->
<bean id="serviceLocator" class="examples.DefaultServiceLocator">
    <!-- inject any dependencies required by this locator bean -->
</bean>

<!-- the bean to be created via the factory bean -->
<bean id="clientService"
    factory-bean="serviceLocator"
    factory-method="createClientServiceInstance"/>

The following example shows the corresponding class:

下面的例子中展示了相应的类:

Java
public class DefaultServiceLocator {

    private static ClientService clientService = new ClientServiceImpl();

    public ClientService createClientServiceInstance() {
        return clientService;
    }
}
Kotlin
class DefaultServiceLocator {
    companion object {
        private val clientService = ClientServiceImpl()
    }
    fun createClientServiceInstance(): ClientService {
        return clientService
    }
}

One factory class can also hold more than one factory method, as the following example shows:

一个工厂类可以拥有多个工厂方法,如下面的例子中所示:

<bean id="serviceLocator" class="examples.DefaultServiceLocator">
    <!-- inject any dependencies required by this locator bean -->
</bean>

<bean id="clientService"
    factory-bean="serviceLocator"
    factory-method="createClientServiceInstance"/>

<bean id="accountService"
    factory-bean="serviceLocator"
    factory-method="createAccountServiceInstance"/>

The following example shows the corresponding class:

下面的示例中展示了相应的类:

Java
public class DefaultServiceLocator {

    private static ClientService clientService = new ClientServiceImpl();

    private static AccountService accountService = new AccountServiceImpl();

    public ClientService createClientServiceInstance() {
        return clientService;
    }

    public AccountService createAccountServiceInstance() {
        return accountService;
    }
}
Kotlin
class DefaultServiceLocator {
    companion object {
        private val clientService = ClientServiceImpl()
        private val accountService = AccountServiceImpl()
    }

    fun createClientServiceInstance(): ClientService {
        return clientService
    }

    fun createAccountServiceInstance(): AccountService {
        return accountService
    }
}

This approach shows that the factory bean itself can be managed and configured through dependency injection (DI). See Dependencies and Configuration in Detail.

这种方式表明 factory bean 本身可以通过依赖注入(DI)进行管理和配置。详见Dependencies与Configuration细节

In Spring documentation, “factory bean” refers to a bean that is configured in the Spring container and that creates objects through an instance or static factory method. By contrast, FactoryBean (notice the capitalization) refers to a Spring-specific FactoryBean .
在Spring文档中, factory bean 指的是在Spring容器中配置的bean,它通过 instance 或者 static factory method 创建对象。相比之下, FactoryBean (注意用大写), 指的是一个Spring专用的bean FactoryBean

依赖(Dependencies)

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A typical enterprise application does not consist of a single object (or bean in the Spring parlance). Even the simplest application has a few objects that work together to present what the end-user sees as a coherent application. This next section explains how you go from defining a number of bean definitions that stand alone to a fully realized application where objects collaborate to achieve a goal.

一个典型的企业应用不会只包含单个对象(Spring 中的 bean)。哪怕是最简单的应用程序,也会由多个对象协同工作,最终由用户看起来好似是一个对象在应用中进行某项工作。下一章部分会阐述你该怎样从定义一些独立的 bean 定义开始,到完全实现对象间的协作,最终实现目标的应用程序。

依赖注入(Dependency Injection)

Dependency injection (DI) is a process whereby objects define their dependencies (that is, the other objects with which they work) only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean. This process is fundamentally the inverse (hence the name, Inversion of Control) of the bean itself controlling the instantiation or location of its dependencies on its own by using direct construction of classes or the Service Locator pattern.

依赖注入(DI)是一个过程,通过该过程,对象只能通过构造函数参数,工厂方法参数或者在已被创建或工厂方法返回的实例对象上设置属性来定义依赖关系(一起协作的其他对象)。 然后容器在创建 bean 时注入这些依赖项。从根本上来说,此过程是对bean本身使用类的直接构造或Service Locator模式来逆向控制其依赖项的实例化或位置(因此被称为控制反转)。

Code is cleaner with the DI principle, and decoupling is more effective when objects are provided with their dependencies. The object does not look up its dependencies and does not know the location or class of the dependencies. As a result, your classes become easier to test, particularly when the dependencies are on interfaces or abstract base classes, which allow for stub or mock implementations to be used in unit tests.

通过使用依赖注入,代码更为简洁,当对象之间存在依赖关系时也更容易解耦。对象不需要去查找它的依赖项,并且也不知道依赖项的位置或者类。其结果是,你的类变得更容易测试, 尤其是当依赖项是接口或者抽象基类时,它们允许在单元测试中使用 stub 或者 mock。

依赖注入(DI)主要有 2 种方式:基于构造函数的依赖注入 以及 基于Setter方法的依赖注入

基于构造函数的依赖注入(Constructor-based Dependency Injection)

Constructor-based DI is accomplished by the container invoking a constructor with a number of arguments, each representing a dependency. Calling a static factory method with specific arguments to construct the bean is nearly equivalent, and this discussion treats arguments to a constructor and to a static factory method similarly. The following example shows a class that can only be dependency-injected with constructor injection:

基于构造函数的DI 是由容器调用具有多个参数的构造函数完成的,每个参数代表一个依赖项。这与通过指定参数来调用具有参数的静态工厂方法来构造 bean 几乎是等效的,本讨论在处理构造函数或者静态工厂方法上的参数时是类似的。以下展示一个只能通过构造函数注入的类:

Java
public class SimpleMovieLister {

    // the SimpleMovieLister has a dependency on a MovieFinder
    private MovieFinder movieFinder;

    // a constructor so that the Spring container can inject a MovieFinder
    public SimpleMovieLister(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // business logic that actually uses the injected MovieFinder is omitted...
}
Kotlin
// a constructor so that the Spring container can inject a MovieFinder
class SimpleMovieLister(private val movieFinder: MovieFinder) {
    // business logic that actually uses the injected MovieFinder is omitted...
}

Notice that there is nothing special about this class. It is a POJO that has no dependencies on container specific interfaces, base classes or annotations.

请注意,这个类并无任何特殊之处。它只是一个 POJO,不依赖于容器内特定的接口、类或者注解。

构造函数参数解析(Constructor Argument Resolution)

Constructor argument resolution matching occurs by using the argument’s type. If no potential ambiguity exists in the constructor arguments of a bean definition, the order in which the constructor arguments are defined in a bean definition is the order in which those arguments are supplied to the appropriate constructor when the bean is being instantiated. Consider the following class:

构造函数参数解析是通过参数的类型来进行匹配的。如果定义 bean 的构造函数参数中不存在潜在的歧义,当实例化 bean 时,构造函数中的参数会按照合适的次序来依次实例化。考虑下面这个类:

Java
package x.y;

public class ThingOne {

    public ThingOne(ThingTwo thingTwo, ThingThree thingThree) {
        // ...
    }
}
Kotlin
package x.y

class ThingOne(thingTwo: ThingTwo, thingThree: ThingThree)

Assuming that ThingTwo and ThingThree classes are not related by inheritance, no potential ambiguity exists. Thus, the following configuration works fine, and you do not need to specify the constructor argument indexes or types explicitly in the <constructor-arg/> element.

假设 ThingTwoThingThree 之间没有继承关系,也不存在其他潜在的歧义。因此,以下配置可以正常工作,并且你无需在 <constructor-arg/> 中显式指定构造函数参数的索引或者类型。

<beans>
    <bean id="beanOne" class="x.y.ThingOne">
        <constructor-arg ref="beanTwo"/>
        <constructor-arg ref="beanThree"/>
    </bean>

    <bean id="beanTwo" class="x.y.ThingTwo"/>

    <bean id="beanThree" class="x.y.ThingThree"/>
</beans>

When another bean is referenced, the type is known, and matching can occur (as was the case with the preceding example). When a simple type is used, such as <value>true</value>, Spring cannot determine the type of the value, and so cannot match by type without help. Consider the following class:

当引用另一个 bean 时,如果类型是已知的,那么可以自动匹配(和上面的例子一样)。如果使用简单类型,比如 <value>true</value>,Spring 在没有其他帮助的情况下无法判断值是什么类型,因此也就不能按类型匹配。考虑下面的这个类:

Java
package examples;

public class ExampleBean {

    // Number of years to calculate the Ultimate Answer
    private int years;

    // The Answer to Life, the Universe, and Everything
    private String ultimateAnswer;

    public ExampleBean(int years, String ultimateAnswer) {
        this.years = years;
        this.ultimateAnswer = ultimateAnswer;
    }
}
Kotlin
package examples

class ExampleBean(
    private val years: Int, // Number of years to calculate the Ultimate Answer
    private val ultimateAnswer: String// The Answer to Life, the Universe, and Everything
)
构造器参数类型匹配(Constructor argument type matching)

In the preceding scenario, the container can use type matching with simple types if you explicitly specify the type of the constructor argument by using the type attribute. as the following example shows:

在上面这个例子中,可以通过使用 type 来显式的为构造函数中的参数指定类型,那么容器就可以实现按类型匹配。如以下示例所示:

<bean id="exampleBean" class="examples.ExampleBean">
    <constructor-arg type="int" value="7500000"/>
    <constructor-arg type="java.lang.String" value="42"/>
</bean>
构造器参数索引(Constructor argument index)

You can use the index attribute to specify explicitly the index of constructor arguments, as the following example shows:

你也可以使用 index 属性来显式指定构造函数参数的索引,如下所示:

<bean id="exampleBean" class="examples.ExampleBean">
    <constructor-arg index="0" value="7500000"/>
    <constructor-arg index="1" value="42"/>
</bean>

In addition to resolving the ambiguity of multiple simple values, specifying an index resolves ambiguity where a constructor has two arguments of the same type.

指定索引的方式,不仅可以消除多个简单值引发的歧义,还可以消除构造函数中存在相同类型的参数时引起的歧义。

The index is 0-based.
index从0开始走起。
构造函数参数名称(Constructor argument name)

You can also use the constructor parameter name for value disambiguation, as the following example shows:

你也可以通过构造函数参数名称来消除歧义,如以下示例所示:

<bean id="exampleBean" class="examples.ExampleBean">
    <constructor-arg name="years" value="7500000"/>
    <constructor-arg name="ultimateAnswer" value="42"/>
</bean>

Keep in mind that, to make this work out of the box, your code must be compiled with the debug flag enabled so that Spring can look up the parameter name from the constructor. If you cannot or do not want to compile your code with the debug flag, you can use the @ConstructorProperties JDK annotation to explicitly name your constructor arguments. The sample class would then have to look as follows:

请记住,为确保这种方式可以正常运行,你必须在 debug 模式下编译你的代码,Spring 才能根据参数名字查找。如果你不想或不能在 debug 模式下编译代码,你可以通过 @ConstructorProperties 注解来明确的指定构造函数参数名称。就像下面这样:

Java
package examples;

public class ExampleBean {

    // Fields omitted

    @ConstructorProperties({"years", "ultimateAnswer"})
    public ExampleBean(int years, String ultimateAnswer) {
        this.years = years;
        this.ultimateAnswer = ultimateAnswer;
    }
}
Kotlin
package examples

class ExampleBean
@ConstructorProperties("years", "ultimateAnswer")
constructor(val years: Int, val ultimateAnswer: String)
基于Setter方法的依赖注入(Setter-based Dependency Injection)

Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or a no-argument static factory method to instantiate your bean.

通过调用无参构造函数或无参静态工厂方法实例化 bean 之后,容器再通过调用 bean 上的 setter 方法来完成基于 setter 的依赖注入。

The following example shows a class that can only be dependency-injected by using pure setter injection. This class is conventional Java. It is a POJO that has no dependencies on container specific interfaces, base classes, or annotations.

下面的示例展示了一个只能通过使用 setter 方式来依赖注入的类。此类是常规的 Java 类。它是一个 POJO,不依赖于容器中特定接口,基类或注解。

Java
public class SimpleMovieLister {

    // the SimpleMovieLister has a dependency on the MovieFinder
    private MovieFinder movieFinder;

    // a setter method so that the Spring container can inject a MovieFinder
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // business logic that actually uses the injected MovieFinder is omitted...
}
Kotlin
class SimpleMovieLister {

    // a late-initialized property so that the Spring container can inject a MovieFinder
    lateinit var movieFinder: MovieFinder

    // business logic that actually uses the injected MovieFinder is omitted...
}

The ApplicationContext supports constructor-based and setter-based DI for the beans it manages. It also supports setter-based DI after some dependencies have already been injected through the constructor approach. You configure the dependencies in the form of a BeanDefinition, which you use in conjunction with PropertyEditor instances to convert properties from one format to another. However, most Spring users do not work with these classes directly (that is, programmatically) but rather with XML bean definitions, annotated components (that is, classes annotated with @Component, @Controller, and so forth), or @Bean methods in Java-based @Configuration classes. These sources are then converted internally into instances of BeanDefinition and used to load an entire Spring IoC container instance.

ApplicationContext 对其管理的 bean 同时支持基于构造函数的方式以及基于 setter 方法的方式进行依赖注入。如果已经通过构造函数方式注入了一些依赖项,它也支持在之后通过 setter 方式继续注入。你可以使用 BeanDefinition 来配置你的依赖项,它可以与 PropertyEditor 实例配合来转换属性的格式。然而,大多数 Spring 用户不会直接(即以编程方式)使用这些类,而是使用 XML bean 定义,或者带注解的组件(比如带有 @Component, @Controller 以及其他注解的类),或者基于 java 代码对类添加 @Configuration 注解,并在此配置类中方法上添加 @Bean 注解等方式。之后再将这些定义元数据在内部转换为 BeanDefinition 实例,一起加载到 Spring IoC 容器中。

选择基于构造函数注入还是 setter 方法注入?(Constructor-based or setter-based DI?)

Since you can mix constructor-based and setter-based DI, it is a good rule of thumb to use constructors for mandatory dependencies and setter methods or configuration methods for optional dependencies. Note that use of the @Required annotation on a setter method can be used to make the property be a required dependency; however, constructor injection with programmatic validation of arguments is preferable.

你可以将基于构造函数的注入以及基于 setter 的注入进行混合使用,一个很好的经验是,把强制性依赖通过构造函数方式注入,把其他可选项依赖通过基于 setter 方法或者配置方法注入。请注意,虽然在 setter 方法上设置 @Required 注解也可以使得该属性变为必需项;但是,更合理的做法是通过构造函数注入, 同时在构造函数中对参数进行验证。

The Spring team generally advocates constructor injection, as it lets you implement application components as immutable objects and ensures that required dependencies are not null. Furthermore, constructor-injected components are always returned to the client (calling) code in a fully initialized state. As a side note, a large number of constructor arguments is a bad code smell, implying that the class likely has too many responsibilities and should be refactored to better address proper separation of concerns.

Spring 团队通常建议采用构造函数方式注入,因为这可以确保你实现的应用组件是不可变对象,并且确保必需的依赖项不为 null 。此外,基于构造函数方式注入还能确保返回给调用方的是完全初始化的组件。附带说明下,如果构造函数拥有大量的参数,通常不是很好的设计,意味着该类可能承担了太多的责任,最好进行重构以更好的处理关注点分离问题。

Setter injection should primarily only be used for optional dependencies that can be assigned reasonable default values within the class. Otherwise, not-null checks must be performed everywhere the code uses the dependency. One benefit of setter injection is that setter methods make objects of that class amenable to reconfiguration or re-injection later. Management through JMX MBeans is therefore a compelling use case for setter injection.

Setter 注入主要仅用于那些可以有合理默认值的可选依赖项。否则,如果在代码方法中使用了这些依赖项,就必须对它们执行非空检查。setter 注入的一个好处是,通过setter 方法可使该类的对象在后面得以重新配置或重新注入。通过使用 setter注入进行管理有一个很值得关注的例子就是 JMX MBeans

Use the DI style that makes the most sense for a particular class. Sometimes, when dealing with third-party classes for which you do not have the source, the choice is made for you. For example, if a third-party class does not expose any setter methods, then constructor injection may be the only available form of DI.

你应该为特定的类选择最合理的注入方式。有时候,当处理一些未知来源的第三方类时,你别无选择。比如,如果一个第三方类不对外暴露 setter 方法,那么基于构造函数方式的注入就是你唯一的选择。

依赖关系解析(Dependency Resolution Process)

The container performs bean dependency resolution as follows: 容器执行 bean 依赖解析,如下所示:

  • The ApplicationContext is created and initialized with configuration metadata that describes all the beans. Configuration metadata can be specified by XML, Java code, or annotations.

  • 通过配置元数据创建和初始化 ApplicationContext ,其中,配置元数据是对所有bean的描述(知秋注:所谓的描述可以理解为该bean的基因配置)。配置元数据可以通过 XML,Java 代码或者注解的形式进行指定。

  • For each bean, its dependencies are expressed in the form of properties, constructor arguments, or arguments to the static-factory method (if you use that instead of a normal constructor). These dependencies are provided to the bean, when the bean is actually created.

  • 对每个 bean,其依赖项都可以用属性、构造函数参数或者静态工厂方法参数(如果你使用这个替代常规构造函数的话)来表示。当 bean 被实际创建时,这些依赖项会提供给 bean。

  • Each property or constructor argument is an actual definition of the value to set, or a reference to another bean in the container.

  • 每个属性或者构造函数参数是一个实际的值,或者对容器中另一个 bean 的引用。

  • Each property or constructor argument that is a value is converted from its specified format to the actual type of that property or constructor argument. By default, Spring can convert a value supplied in string format to all built-in types, such as int, long, String, boolean, and so forth.

  • 每个属性或者构造函数参数的值都会根据它所指定的格式转换为该属性或者构造函数参数实际的类型。默认情况下,Spring 可以转换字符串格式到所有内置类型,比如 int,long, String, boolean, 以及其他。

The Spring container validates the configuration of each bean as the container is created. However, the bean properties themselves are not set until the bean is actually created. Beans that are singleton-scoped and set to be pre-instantiated (the default) are created when the container is created. Scopes are defined in Bean作用域(Bean Scopes). Otherwise, the bean is created only when it is requested. Creation of a bean potentially causes a graph of beans to be created, as the bean’s dependencies and its dependencies' dependencies (and so on) are created and assigned. Note that resolution mismatches among those dependencies may show up late — that is, on first creation of the affected bean.

当容器创建的时候,Spring 容器会校验每个 bean 的配置。然而,直到 bean 被真正创建时才会设置属性本身。创建容器时,会创建单例作用域并默认预先初始化。bean 的作用域在 Bean作用域(Bean Scopes) 定义。否则, bean 只有在请求时才会创建。创建 bean 可能会导致更多的 bean 被创建,因为其依赖项以及依赖项的依赖项(以此类推)都会被创建并分配。请注意,在bean首次创建的时候,它的依赖项之间可能会出现依赖解析匹配问题。

循环依赖(Circular dependencies)

If you use predominantly constructor injection, it is possible to create an unresolvable circular dependency scenario.

如果你主要通过构造函数方式注入,那么有可能会出现无法解决的循环依赖场景。

For example: Class A requires an instance of class B through constructor injection, and class B requires an instance of class A through constructor injection. If you configure beans for classes A and B to be injected into each other, the Spring IoC container detects this circular reference at runtime, and throws a BeanCurrentlyInCreationException.

比如:A 类需要 B 类的一个实例通过构造函数来注入,并且 B 类也需要 A 类的一个实例通过构造函数注入。如果你配置 A 和 B 需要彼此注入,那么 Spring IoC 容器在运行时会监测到循环依赖,并抛出一个 BeanCurrentlyInCreationException 异常。

One possible solution is to edit the source code of some classes to be configured by setters rather than constructors. Alternatively, avoid constructor injection and use setter injection only. In other words, although it is not recommended, you can configure circular dependencies with setter injection.

一个可能的解决方案是编辑某些类的代码,将由构造函数注入方式更改为 基于setter 注入方式。或者,避免构造函数方式注入,而只使用 setter 方式注入。换句话说,尽管不推荐这样做,但是通过 setter 注入你可以解决循环依赖问题。

Unlike the typical case (with no circular dependencies), a circular dependency between bean A and bean B forces one of the beans to be injected into the other prior to being fully initialized itself (a classic chicken-and-egg scenario).

不同于常规场景(即没有循环依赖),bean A 和 bean B 之间循环依赖会迫使其中一方在还没完全初始化之前强制注入另一方(典型的先有鸡还是先有蛋的问题)。

You can generally trust Spring to do the right thing. It detects configuration problems, such as references to non-existent beans and circular dependencies, at container load-time. Spring sets properties and resolves dependencies as late as possible, when the bean is actually created. This means that a Spring container that has loaded correctly can later generate an exception when you request an object if there is a problem creating that object or one of its dependencies — for example, the bean throws an exception as a result of a missing or invalid property. This potentially delayed visibility of some configuration issues is why ApplicationContext implementations by default pre-instantiate singleton beans. At the cost of some upfront time and memory to create these beans before they are actually needed, you discover configuration issues when the ApplicationContext is created, not later. You can still override this default behavior so that singleton beans initialize lazily, rather than being pre-instantiated.

通常,你可以相信 Spring 会做正确的事情。在容器加载时,它检测配置问题,比如引用了不存在的 bean 或者循环依赖。Spring 会尽可能晚的设置属性以及解决依赖问题,即在 bean 被实际创建时。这意味着当你请求一个对象时,如果创建对象或者其依赖项时出现问题,可能会报异常,尽管 Spring 容器在此之前已被正确加载了 —— 比如,由于属性缺失或者无效,bean 抛出异常。正是由于会延迟暴露某些配置问题,所以 ApplicationContext 的默认实现是预先实例化单例 bean。花费一些时间和内存在实际需要之前就预先创建好 bean,你就能在创建 ApplicationContext 时发现配置问题,而不是以后。当然,你也可以覆盖此默认行为,改成单例 bean 延迟初始化,而不是预先实例化。

If no circular dependencies exist, when one or more collaborating beans are being injected into a dependent bean, each collaborating bean is totally configured prior to being injected into the dependent bean. This means that, if bean A has a dependency on bean B, the Spring IoC container completely configures bean B prior to invoking the setter method on bean A. In other words, the bean is instantiated (if it is not a pre-instantiated singleton), its dependencies are set, and the relevant lifecycle methods (such as a configured init method or the InitializingBean callback method) are invoked.

如果不存在循环依赖,当一个或者多个 bean 作为依赖项注入到 bean 时,每个 bean 在注入之前就已被完全配置。也就是说,如果 bean A 有个依赖项 bean B,在调用 bean A 的 setter 方法前,Spring IoC 容器就已经对 bean B 完成了配置。换句话说,实例化 bean 时(如果它不是预先实例化的单例 bean),会先设置它的依赖项,并调用相关生命周期方法(比如 configured init method 或者 InitializingBean callback method)。

依赖注入示例(Examples of Dependency Injection)

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文档整理

The following example uses XML-based configuration metadata for setter-based DI. A small part of a Spring XML configuration file specifies some bean definitions as follows:

下面的案例基于XML的DI(依赖注入)容器配置元数据进行Setter注入。如下所示的代码是在Spring XML配置文件中定义Bean的一部分片段内容。

<bean id="exampleBean" class="examples.ExampleBean">
    <!-- setter injection using the nested ref element -->
    <property name="beanOne">
        <ref bean="anotherExampleBean"/>
    </property>

    <!-- setter injection using the neater ref attribute -->
    <property name="beanTwo" ref="yetAnotherBean"/>
    <property name="integerProperty" value="1"/>
</bean>

<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>

The following example shows the corresponding ExampleBean class:

下面的例子展示了对应的ExampleBean类的Java和Kotlin代码:

Java
public class ExampleBean {

    private AnotherBean beanOne;

    private YetAnotherBean beanTwo;

    private int i;

    public void setBeanOne(AnotherBean beanOne) {
        this.beanOne = beanOne;
    }

    public void setBeanTwo(YetAnotherBean beanTwo) {
        this.beanTwo = beanTwo;
    }

    public void setIntegerProperty(int i) {
        this.i = i;
    }
}
Kotlin
class ExampleBean {
    lateinit var beanOne: AnotherBean
    lateinit var beanTwo: YetAnotherBean
    var i: Int = 0
}

In the preceding example, setters are declared to match against the properties specified in the XML file. The following example uses constructor-based DI:

在上面的例子中,setter声明要和XML文件中的property元素的name属性相匹配。下面的例子是基于构造方法进行依赖注入:

<bean id="exampleBean" class="examples.ExampleBean">
    <!-- constructor injection using the nested ref element -->
    <constructor-arg>
        <ref bean="anotherExampleBean"/>
    </constructor-arg>

    <!-- constructor injection using the neater ref attribute -->
    <constructor-arg ref="yetAnotherBean"/>

    <constructor-arg type="int" value="1"/>
</bean>

<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>

The following example shows the corresponding ExampleBean class:

下面的例子展示了对应的ExampleBean类的Java和Kotlin代码:

Java
public class ExampleBean {

    private AnotherBean beanOne;

    private YetAnotherBean beanTwo;

    private int i;

    public ExampleBean(
        AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
        this.beanOne = anotherBean;
        this.beanTwo = yetAnotherBean;
        this.i = i;
    }
}
Kotlin
class ExampleBean(
        private val beanOne: AnotherBean,
        private val beanTwo: YetAnotherBean,
        private val i: Int)

The constructor arguments specified in the bean definition are used as arguments to the constructor of the ExampleBean.

在Bean的定义中所指定的构造函数参数,将会赋值给ExampleBean类的成员变量。

Now consider a variant of this example, where, instead of using a constructor, Spring is told to call a static factory method to return an instance of the object:

现在考虑下这个示例变种,不使用构造函数,Spring通过调用静态工厂方法来返回一个对象实例:

<bean id="exampleBean" class="examples.ExampleBean" factory-method="createInstance">
    <constructor-arg ref="anotherExampleBean"/>
    <constructor-arg ref="yetAnotherBean"/>
    <constructor-arg value="1"/>
</bean>

<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>

The following example shows the corresponding ExampleBean class:

下面的例子展示了对应的 ExampleBean 类的Java和Kotlin代码:

Java
public class ExampleBean {

    // a private constructor
    private ExampleBean(...) {
        ...
    }

    // a static factory method; the arguments to this method can be
    // considered the dependencies of the bean that is returned,
    // regardless of how those arguments are actually used.
    public static ExampleBean createInstance (
        AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {

        ExampleBean eb = new ExampleBean (...);
        // some other operations...
        return eb;
    }
}
Kotlin
class ExampleBean private constructor() {
    companion object {
        // a static factory method; the arguments to this method can be
        // considered the dependencies of the bean that is returned,
        // regardless of how those arguments are actually used.
        fun createInstance(anotherBean: AnotherBean, yetAnotherBean: YetAnotherBean, i: Int): ExampleBean {
            val eb = ExampleBean (...)
            // some other operations...
            return eb
        }
    }
}

Arguments to the static factory method are supplied by <constructor-arg/> elements, exactly the same as if a constructor had actually been used. The type of the class being returned by the factory method does not have to be of the same type as the class that contains the static factory method (although, in this example, it is). An instance (non-static) factory method can be used in an essentially identical fashion (aside from the use of the factory-bean attribute instead of the class attribute), so we do not discuss those details here.

静态工厂方法的参数是由 <constructor-arg/> 元素提供的,与实际上使用的构造函数完全相同。静态工厂方法返回的类的类型,无须和它所属的工厂类的类型相同(虽然本案例中是相同的)。实例工厂方法(非静态)在使用上和静态工厂方法本质相同(除了使用 factory-bean 属性替代 class 属性),所以我们就不在此详细探讨。

依赖和配置详解(Dependencies and Configuration in Detail)

As mentioned in the previous section, you can define bean properties and constructor arguments as references to other managed beans (collaborators) or as values defined inline. Spring’s XML-based configuration metadata supports sub-element types within its <property/> and <constructor-arg/> elements for this purpose.

前一章节 所提到的,你可以将Bean的属性和构造参数定义为容器里其他所管理的Bean(即协作者),或者内联使用value属性定义值。为此,Spring的XML配置元数据支持在 <property/><constructor-arg/> 的元素中定义子元素。

直接赋值(基本类型,字符串等等)(Straight Values (Primitives, Strings, and so on))

The value attribute of the <property/> element specifies a property or constructor argument as a human-readable string representation. Spring’s conversion service is used to convert these values from a String to the actual type of the property or argument. The following example shows various values being set:

通过为 <property/> 元素的 value 属性使用一个人类可读的字符串来指定一个property 或 constructor的参数。通过Spring的 conversion service 将这些值由 String 字符串转换为property或者参数的实际类型。 下面的例子将展示各种值如何设置:

<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
    <!-- results in a setDriverClassName(String) call -->
    <property name="driverClassName" value="com.mysql.jdbc.Driver"/>
    <property name="url" value="jdbc:mysql://localhost:3306/mydb"/>
    <property name="username" value="root"/>
    <property name="password" value="masterkaoli"/>
</bean>

The following example uses the p-namespace for even more succinct XML configuration:

下面的例子使用 p-namespace 进行更简洁的XML配置:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource"
        destroy-method="close"
        p:driverClassName="com.mysql.jdbc.Driver"
        p:url="jdbc:mysql://localhost:3306/mydb"
        p:username="root"
        p:password="masterkaoli"/>

</beans>

The preceding XML is more succinct. However, typos are discovered at runtime rather than design time, unless you use an IDE (such as IntelliJ IDEA or the Spring Tool Suite) that supports automatic property completion when you create bean definitions. Such IDE assistance is highly recommended.

上面的XML配置更简洁。然而,除你在创建bean定义时所使用的IDE支持自动化完成属性装填 (比如 IntelliJ IDEA 或者 Spring Tool Suite),否则配置拼写错误将在运行时,而不是设计过程中发现。强烈建议使用此类IDE来为开发提供帮助。

You can also configure a java.util.Properties instance, as follows:

你也可以配置一个 java.util.Properties 的实例,如下:

<bean id="mappings"
    class="org.springframework.context.support.PropertySourcesPlaceholderConfigurer">

    <!-- typed as a java.util.Properties -->
    <property name="properties">
        <value>
            jdbc.driver.className=com.mysql.jdbc.Driver
            jdbc.url=jdbc:mysql://localhost:3306/mydb
        </value>
    </property>
</bean>

The Spring container converts the text inside the <value/> element into a java.util.Properties instance by using the JavaBeans PropertyEditor mechanism. This is a nice shortcut, and is one of a few places where the Spring team do favor the use of the nested <value/> element over the value attribute style.

Spring容器通过JavaBeans PropertyEditor 机制将 <value/> 元素中的text文本转换为 java.util.Properties 的一个实例。这是一步不错的捷径,并且这是Spring团队少有的钟情的地方之一,即比起使用 value 属性方式,更偏爱使用嵌套的 <value/> 元素。

idref 元素(The idref element)

The idref element is simply an error-proof way to pass the id (a string value - not a reference) of another bean in the container to a <constructor-arg/> or <property/> element. The following example shows how to use it:

idref 元素只是将容器中的另一个Bean的 id (一个非引用的字符串)传递给 <constructor-arg/> 或者 <property/> 元素的一种简单的防错方法。下面的例子将展示如何使用:

<bean id="theTargetBean" class="..."/>

<bean id="theClientBean" class="...">
    <property name="targetName">
        <idref bean="theTargetBean"/>
    </property>
</bean>

The preceding bean definition snippet is exactly equivalent (at runtime) to the following snippet:

前面Bean定义代码片段(在运行时)完全等同于下面的代码段:

<bean id="theTargetBean" class="..." />

<bean id="client" class="...">
    <property name="targetName" value="theTargetBean"/>
</bean>

The first form is preferable to the second, because using the idref tag lets the container validate at deployment time that the referenced, named bean actually exists. In the second variation, no validation is performed on the value that is passed to the targetName property of the client bean. Typos are only discovered (with most likely fatal results) when the client bean is actually instantiated. If the client bean is a prototype bean, this typo and the resulting exception may only be discovered long after the container is deployed.

第一种形式优于第二种,因为使用 idref 标签可以使容器在部署时校验被引用的命名Bean是否真实存在。在第二个变种中,不会对传递给id为 client 的Bean属性 targetName 的value值进行校验。只有在真正实例化 client Bean的时候,错字才会被发现(极有可能造成致命后果)。如果 client Bean是一个 prototype Bean,这个错字产生的异常可能只有在部署容器后很长时间后才能发现。

The local attribute on the idref element is no longer supported in the 4.0 beans XSD, since it does not provide value over a regular bean reference any more. Change your existing idref local references to idref bean when upgrading to the 4.0 schema.
4.0 beans XSD不再支持 idref 元素的 local 属性,因为它不再通过之前形式的 bean 的引用来提供值。升级到4.0模式后,需要将现有的 idref local 引用改为 idref bean

A common place (at least in versions earlier than Spring 2.0) where the <idref/> element brings value is in the configuration of AOP interceptors in a ProxyFactoryBean bean definition. Using <idref/> elements when you specify the interceptor names prevents you from misspelling an interceptor ID.

<idref/> 元素所带来价值,一个常见地方(至少在Spring 2.0之前的版本中)是在 ProxyFactoryBean Bean定义的 AOP interceptors 配置中。在指定拦截器名称时,使用 <idref/> 元素可以防止拼写错误的拦截器ID。

引用其他Bean(协作者)(References to Other Beans (Collaborators))

The ref element is the final element inside a <constructor-arg/> or <property/> definition element. Here, you set the value of the specified property of a bean to be a reference to another bean (a collaborator) managed by the container. The referenced bean is a dependency of the bean whose property is to be set, and it is initialized on demand as needed before the property is set. (If the collaborator is a singleton bean, it may already be initialized by the container.) All references are ultimately a reference to another object. Scoping and validation depend on whether you specify the ID or name of the other object through the bean, local, or parent attributes.

ref 元素是 <constructor-arg/> 或者 <property/> 中决定性的元素。在这里,你设置它的属性用以引用容器管理的另一个bean。引用的bean就是要设置属性bean的依赖,在设置属性值之前这个引用bean就要被初始化。(如果协作者是单例Bean,它会在容器初始化时完成初始化。)所有这些引用(知秋注:构造器参数或<property/>中的引用)最终都会引用到另一个对象。 ref 元素的作用域和校验取决于你是否通过 beanlocal 或者 parent 属性来指定另一个对象的id或name。

Specifying the target bean through the bean attribute of the <ref/> tag is the most general form and allows creation of a reference to any bean in the same container or parent container, regardless of whether it is in the same XML file. The value of the bean attribute may be the same as the id attribute of the target bean or be the same as one of the values in the name attribute of the target bean. The following example shows how to use a ref element:

通过 <ref/> 标签的 bean 属性指定引用Bean是最常见的方式,它允许引用本容器或者父容器中任意的Bean,不用管他们是否在同一XML文件中。bean 属性的值可以与引用Bean的 id 属性相同,也可以与引用Bean的 name 属性中的值其中一个相同。 下面的例子将展示如何使用 ref 元素:

<ref bean="someBean"/>

Specifying the target bean through the parent attribute creates a reference to a bean that is in a parent container of the current container. The value of the parent attribute may be the same as either the id attribute of the target bean or one of the values in the name attribute of the target bean. The target bean must be in a parent container of the current one. You should use this bean reference variant mainly when you have a hierarchy of containers and you want to wrap an existing bean in a parent container with a proxy that has the same name as the parent bean. The following pair of listings shows how to use the parent attribute:

通过指定引用Bean的 parent 属性来引用当前容器的父容器中的Bean。 parent 属性的值可以和引用Bean的 id 或者 name 属性值相同。引用的Bean必须存在于当前容器的父容器中。容器存在结构层次的关系时,你想要使用一个代理将父容器中已存在bean包装起来,并在子容器中通过相同名字使用。可以参考下面两段XML配置展示的 parent 属性使用方式:

<!-- in the parent context -->
<bean id="accountService" class="com.something.SimpleAccountService">
    <!-- insert dependencies as required as here -->
</bean>
<!-- in the child (descendant) context -->
<bean id="accountService" <!-- bean name is the same as the parent bean -->
    class="org.springframework.aop.framework.ProxyFactoryBean">
    <property name="target">
        <ref parent="accountService"/> <!-- notice how we refer to the parent bean -->
    </property>
    <!-- insert other configuration and dependencies as required here -->
</bean>
The local attribute on the ref element is no longer supported in the 4.0 beans XSD, since it does not provide value over a regular bean reference any more. Change your existing ref local references to ref bean when upgrading to the 4.0 schema.
在4.0 beans XSD 版本中, ref 元素上的 local 属性不再支持,因为它不再通过之前形式的 bean 的引用来提供值。当你升级到4.0版本时,将现有的 ref local 更改为 ref bean
Inner Beans

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集合、Null和空字符串、带p-namespace的XML快捷方式

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文档整理

A <bean/> element inside the <property/> or <constructor-arg/> elements defines an inner bean, as the following example shows:

<property/> 或者 <constructor-arg/> 元素内部定义的 <bean/> 叫做内部Bean,如下面的例子所示:

<bean id="outer" class="...">
    <!-- instead of using a reference to a target bean, simply define the target bean inline -->
    <property name="target">
        <bean class="com.example.Person"> <!-- this is the inner bean -->
            <property name="name" value="Fiona Apple"/>
            <property name="age" value="25"/>
        </bean>
    </property>
</bean>

An inner bean definition does not require a defined ID or name. If specified, the container does not use such a value as an identifier. The container also ignores the scope flag on creation, because inner beans are always anonymous and are always created with the outer bean. It is not possible to access inner beans independently or to inject them into collaborating beans other than into the enclosing bean.

内部bean不需要定义ID或名称。即使指定了,容器也不会使用这个值作为唯一标识符。容器在创建时同样也会忽略 scope 标签,因为内部beans始终是匿名的,并且随外部bean一起创建。内部bean不可能被独立的访问,或者将其注入除封闭的Bean之外的其他Bean(知秋注:其实就是给构造器参数新建一个匿名对象传入其中,这个匿名对象作用域仅限于它所用于的这个构造器对象中,不能用来被其他bean使用,否则就会造成逃逸,也就是产生依赖关系,也就脱离了它设计的初衷)。

As a corner case, it is possible to receive destruction callbacks from a custom scope — for example, for a request-scoped inner bean contained within a singleton bean. The creation of the inner bean instance is tied to its containing bean, but destruction callbacks let it participate in the request scope’s lifecycle. This is not a common scenario. Inner beans typically simply share their containing bean’s scope.

作为特殊情况,内部bean可以从一个自定义作用域接收销毁回调,例如,对于单例bean中包含request-scoped作用域的 inner bean。 inner bean实例的创建与包含它的bean绑定在一起,但是销毁回调使它可以参与request-scoped的生命周期。 这不是常见的情况。 Inner bean通常只简单共享其被包含bean的作用域。

集合(Collections)

The <list/>, <set/>, <map/>, and <props/> elements set the properties and arguments of the Java Collection types List, Set, Map, and Properties, respectively. The following example shows how to use them:

Spring中 <list/> , <set/> , <map/><props/> 元素用来设置Java Collection 类型的 List , Set , Map , 和 Properties 的属性和参数。以下示例显示了如何使用它们:

<bean id="moreComplexObject" class="example.ComplexObject">
    <!-- results in a setAdminEmails(java.util.Properties) call -->
    <property name="adminEmails">
        <props>
            <prop key="administrator">[email protected]</prop>
            <prop key="support">[email protected]</prop>
            <prop key="development">[email protected]</prop>
        </props>
    </property>
    <!-- results in a setSomeList(java.util.List) call -->
    <property name="someList">
        <list>
            <value>a list element followed by a reference</value>
            <ref bean="myDataSource" />
        </list>
    </property>
    <!-- results in a setSomeMap(java.util.Map) call -->
    <property name="someMap">
        <map>
            <entry key="an entry" value="just some string"/>
            <entry key ="a ref" value-ref="myDataSource"/>
        </map>
    </property>
    <!-- results in a setSomeSet(java.util.Set) call -->
    <property name="someSet">
        <set>
            <value>just some string</value>
            <ref bean="myDataSource" />
        </set>
    </property>
</bean>

The value of a map key or value, or a set value, can also be any of the following elements:

map的键和值,或者set的值,也可以是以下任意元素:

bean | ref | idref | list | set | map | props | value | null
集合合并(Collection Merging)

The Spring container also supports merging collections. An application developer can define a parent <list/>, <map/>, <set/> or <props/> element and have child <list/>, <map/>, <set/> or <props/> elements inherit and override values from the parent collection. That is, the child collection’s values are the result of merging the elements of the parent and child collections, with the child’s collection elements overriding values specified in the parent collection.

Spring容器还支持合并集合。应用程序开发人员可以定义一个父 <list/>, <map/>, <set/><props/> 元素,并有对应 <list/> , <map/>, <set/><props/> 的子元素从父元素继承并覆盖相应的值。也就是说,子集合的值是合并父集合和子集合元素的结果,子集合的元素会覆盖父集合中指定的值。

This section on merging discusses the parent-child bean mechanism. Readers unfamiliar with parent and child bean definitions may wish to read the relevant section before continuing.

这部分讨论了合并父子bean机制。不熟悉父bean和子bean的读者可能希望先阅读 相关部分 然后再继续。

The following example demonstrates collection merging:

下面的示例演示了集合合并:

<beans>
    <bean id="parent" abstract="true" class="example.ComplexObject">
        <property name="adminEmails">
            <props>
                <prop key="administrator">[email protected]</prop>
                <prop key="support">[email protected]</prop>
            </props>
        </property>
    </bean>
    <bean id="child" parent="parent">
        <property name="adminEmails">
            <!-- the merge is specified on the child collection definition -->
            <props merge="true">
                <prop key="sales">[email protected]</prop>
                <prop key="support">[email protected]</prop>
            </props>
        </property>
    </bean>
<beans>

Notice the use of the merge=true attribute on the <props/> element of the adminEmails property of the child bean definition. When the child bean is resolved and instantiated by the container, the resulting instance has an adminEmails Properties collection that contains the result of merging the child’s adminEmails collection with the parent’s adminEmails collection. The following listing shows the result:

注意,child bean定义的 adminEmails 内关于属性的 <props/> 元素上使用了 merge = true 。 当 child bean被容器解析并实例化时,生成的实例具有 adminEmails Properties 集合,其中包含将child的 adminEmails 集合与parent对象的 adminEmails 集合合并的结果。 以下清单显示了结果:

The child Properties collection’s value set inherits all property elements from the parent <props/>, and the child’s value for the support value overrides the value in the parent collection.

Properties 集合的值继承父 <props/> 的所有属性元素值。并且子元素通过重写 support 的值,以此来覆盖父集合中的对应值。

This merging behavior applies similarly to the <list/>, <map/>, and <set/> collection types. In the specific case of the <list/> element, the semantics associated with the List collection type (that is, the notion of an ordered collection of values) is maintained. The parent’s values precede all of the child list’s values. In the case of the Map, Set, and Properties collection types, no ordering exists. Hence, no ordering semantics are in effect for the collection types that underlie the associated Map, Set, and Properties implementation types that the container uses internally.

与这种合并行为相似,关于 <list/> , <map/>, 和 <set/> 这些集合类型也是这样应用的。在 <list/> 元素有个特殊情况,将维护与 List 集合类型相关联的语义(即值的有序集合的概念)。父列表的值优先于子列表的值。在集合类型是 Map, Set, 和 Properties 的情况下,无需排序。因此,容器内部使用以 Map, Set, 和 Properties 为基础的相关集合类型,是没有排序语义的。

集合合并的局限性(Limitations of Collection Merging)

You cannot merge different collection types (such as a Map and a List). If you do attempt to do so, an appropriate Exception is thrown. The merge attribute must be specified on the lower, inherited, child definition. Specifying the merge attribute on a parent collection definition is redundant and does not result in the desired merging.

你不能合并不同的集合类型(例如一个 Map 和一个 List)。如果你企图那么做,会有一个对应的 Exception 被抛出。merge 属性必须在下面的继承子定义中指定。在父集合中指定 merge 属性是多余的并且也不能达到预期的合并效果。

强类型集合(Strongly-typed collection)

With the introduction of generic types in Java 5, you can use strongly typed collections. That is, it is possible to declare a Collection type such that it can only contain (for example) String elements. If you use Spring to dependency-inject a strongly-typed Collection into a bean, you can take advantage of Spring’s type-conversion support such that the elements of your strongly-typed Collection instances are converted to the appropriate type prior to being added to the Collection. The following Java class and bean definition show how to do so:

随着Java 5中泛型类型的引入,你可以使用强类型集合。也就是说,可以声明一个 Collection 类型,使其只能包含(例如) String 类型。如果你使用Spring依赖注入一个强类型 Collection 到一个bean中,则可以利用Spring提供的类型转换支持,来使你的强类型 Collection 元素实例在被添加到 Collection 之前,转换为适当的类型。 以下Java类和bean定义显示了该如何执行此操作:

Java
public class SomeClass {

    private Map<String, Float> accounts;

    public void setAccounts(Map<String, Float> accounts) {
        this.accounts = accounts;
    }
}
Kotlin
class SomeClass {
    lateinit var accounts: Map<String, Float>
}
<beans>
    <bean id="something" class="x.y.SomeClass">
        <property name="accounts">
            <map>
                <entry key="one" value="9.99"/>
                <entry key="two" value="2.75"/>
                <entry key="six" value="3.99"/>
            </map>
        </property>
    </bean>
</beans>

When the accounts property of the something bean is prepared for injection, the generics information about the element type of the strongly-typed Map<String, Float> is available by reflection. Thus, Spring’s type conversion infrastructure recognizes the various value elements as being of type Float, and the string values (9.99, 2.75, and 3.99) are converted into an actual Float type.

something bean的 accounts 属性准备注入时,可以通过反射获得有关强类型Map <String,Float>元素类型的泛型信息。因此,Spring类型转换设施可以识别出类型为 Float 的各种元素值 ,并将字符串值(9.99, 2.75, 和 3.99)转换为实际的 Float 类型。

Null和空字符串(Null and Empty String Values)

Spring treats empty arguments for properties and the like as empty Strings. The following XML-based configuration metadata snippet sets the email property to the empty String value ("").

Spring将属性为空的参数视为空的 String。以下基于XML的配置元数据代码段设置 email 属性为空的 String 值("")。

<bean class="ExampleBean">
    <property name="email" value=""/>
</bean>

The preceding example is equivalent to the following Java code:

上面的示例等效于下面的Java代码:

Java
exampleBean.setEmail("");
Kotlin
exampleBean.email = ""

The <null/> element handles null values. The following listing shows an example:

<null/> 元素用于 null 值的处理。下面的代码清单展示了一个例子:

<bean class="ExampleBean">
    <property name="email">
        <null/>
    </property>
</bean>

The preceding configuration is equivalent to the following Java code:

上面的配置等效于下面的Java代码:

Java
exampleBean.setEmail(null);
Kotlin
exampleBean.email = null
带p-namespace的XML快捷方式(XML Shortcut with the p-namespace)

The p-namespace lets you use the bean element’s attributes (instead of nested <property/> elements) to describe your property values collaborating beans, or both.

p-namespace允许你使用bean元素的属性(代替内嵌的 <property/> 元素)来描述协作bean的属性值,或同时使用这两者。

Spring supports extensible configuration formats with namespaces, which are based on an XML Schema definition. The beans configuration format discussed in this chapter is defined in an XML Schema document. However, the p-namespace is not defined in an XSD file and exists only in the core of Spring.

Spring支持带有命名空间的可扩展配置格式 namespaces,这些命名空间基于XML Schema定义。 本章讨论的 beans 配置格式是在XML Schema文档中定义的。 但是,p-namespace 并未在XSD文件中定义,仅存在于Spring的核心中。

The following example shows two XML snippets (the first uses standard XML format and the second uses the p-namespace) that resolve to the same result:

下面的示例显示了两个XML代码段(第一个使用标准XML格式,第二个使用了p-namespace),它们可以解析为相同的结果:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean name="classic" class="com.example.ExampleBean">
        <property name="email" value="[email protected]"/>
    </bean>

    <bean name="p-namespace" class="com.example.ExampleBean"
        p:email="[email protected]"/>
</beans>

The example shows an attribute in the p-namespace called email in the bean definition. This tells Spring to include a property declaration. As previously mentioned, the p-namespace does not have a schema definition, so you can set the name of the attribute to the property name.

这个例子展示了在bean定义中使用p-namespace 给 email 属性赋值。以此来告诉Spring包含一个属性说明。如前所述,p-namespace没有schema定义,因此你可以通过使用<property/>的name元素来设定想要设定属性的名字。

This next example includes two more bean definitions that both have a reference to another bean:

下面这个例子包含两个bean定义,它们都引用了另一个bean:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean name="john-classic" class="com.example.Person">
        <property name="name" value="John Doe"/>
        <property name="spouse" ref="jane"/>
    </bean>

    <bean name="john-modern"
        class="com.example.Person"
        p:name="John Doe"
        p:spouse-ref="jane"/>

    <bean name="jane" class="com.example.Person">
        <property name="name" value="Jane Doe"/>
    </bean>
</beans>

This example includes not only a property value using the p-namespace but also uses a special format to declare property references. Whereas the first bean definition uses <property name="spouse" ref="jane"/> to create a reference from bean john to bean jane, the second bean definition uses p:spouse-ref="jane" as an attribute to do the exact same thing. In this case, spouse is the property name, whereas the -ref part indicates that this is not a straight value but rather a reference to another bean.

该示例不仅包括使用p-namespace的属性值,而且还使用一种特殊的格式来声明属性引用。 第一个bean定义使用 <property name="spouse" ref="jane"/> 创建从bean john到bean jane的引用,而第二个bean定义使用 p:spouse-ref="jane" 作为属性来做完全相同的事情。 在这种情况下,spouse 是属性名,而 -ref 部分表明这不是一个直接值,而是对另一个bean的引用。

The p-namespace is not as flexible as the standard XML format. For example, the format for declaring property references clashes with properties that end in Ref, whereas the standard XML format does not. We recommend that you choose your approach carefully and communicate this to your team members to avoid producing XML documents that use all three approaches at the same time.
p-namespace不如标准XML格式灵活。 例如,用于声明属性引用的格式与以 Ref 结尾的属性发生冲突,而标准XML格式则没有。 我们建议你谨慎选择,并与团队成员进行交流,以避免同时使用这三种途径生成XML文档。
c-namespace的XML快捷配置XML(Shortcut with the c-namespace)

Similar to the 带p-namespace的XML快捷方式(XML Shortcut with the p-namespace), the c-namespace, introduced in Spring 3.1, allows inlined attributes for configuring the constructor arguments rather then nested constructor-arg elements.

带p-namespace的XML快捷方式(XML Shortcut with the p-namespace) 类似,在Spring 3.1中引入的c-namespace允许使用内联属性来配置构造函数参数,而不是嵌套的 constructor-arg 元素。

The following example uses the c: namespace to do the same thing as the from 基于构造函数的依赖注入(Constructor-based Dependency Injection):

下面的例子使用 c: 命名空间做了和 基于构造函数的依赖注入(Constructor-based Dependency Injection) 相同的事情:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:c="http://www.springframework.org/schema/c"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="beanTwo" class="x.y.ThingTwo"/>
    <bean id="beanThree" class="x.y.ThingThree"/>

    <!-- traditional declaration with optional argument names -->
    <bean id="beanOne" class="x.y.ThingOne">
        <constructor-arg name="thingTwo" ref="beanTwo"/>
        <constructor-arg name="thingThree" ref="beanThree"/>
        <constructor-arg name="email" value="[email protected]"/>
    </bean>

    <!-- c-namespace declaration with argument names -->
    <bean id="beanOne" class="x.y.ThingOne" c:thingTwo-ref="beanTwo"
        c:thingThree-ref="beanThree" c:email="[email protected]"/>

</beans>

The c: namespace uses the same conventions as the p: one (a trailing -ref for bean references) for setting the constructor arguments by their names. Similarly, it needs to be declared in the XML file even though it is not defined in an XSD schema (it exists inside the Spring core).

c: 命名空间使用与 p: 相同的约定(后跟 -ref 用于bean引用),用于通过名称设置构造函数参数。 同样,即使未在XSD模式中定义它(也存在于Spring core中),也需要在XML文件中声明它。

For the rare cases where the constructor argument names are not available (usually if the bytecode was compiled without debugging information), you can use fallback to the argument indexes, as follows:

对于极少数情况下无法使用构造函数参数名称的情况(通常,是指如果字节码是在没有调试信息的情况下编译的),可以对参数索引使用后备设定,如下所示:

<!-- c-namespace index declaration -->
<bean id="beanOne" class="x.y.ThingOne" c:_0-ref="beanTwo" c:_1-ref="beanThree"
    c:_2="[email protected]"/>
Due to the XML grammar, the index notation requires the presence of the leading _, as XML attribute names cannot start with a number (even though some IDEs allow it). A corresponding index notation is also available for <constructor-arg> elements but not commonly used since the plain order of declaration is usually sufficient there.
由于XML语法的原因,索引符号前面要有 _ 为前导,因为XML属性名不能以数字开头(即使某些IDE允许)。相应的索引符号也可用于 <constructor-arg> 元素,但是不常用,因为通常只需简单的声明顺序就足够了。

In practice, the constructor resolution mechanism is quite efficient in matching arguments, so unless you really need to, we recommend using the name notation through-out your configuration.

实际上,构造函数解析 机制在匹配参数方面非常有效,因此,除非你确实需要,否则我们建议在整个配置过程中使用名称表示法。

复合属性名称(Compound Property Names)

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You can use compound or nested property names when you set bean properties, as long as all components of the path except the final property name are not null. Consider the following bean definition:

当你需要设置bean属性的时候,你可以使用复合或者嵌套属性名称进行设置,除了最终属性名称,只要所有路径中的所有组件不为null那就可以,考虑如下所示的bean定义:

<bean id="something" class="things.ThingOne">
    <property name="fred.bob.sammy" value="123" />
</bean>

The something bean has a fred property, which has a bob property, which has a sammy property, and that final sammy property is being set to a value of 123. In order for this to work, the fred property of something and the bob property of fred must not be null after the bean is constructed. Otherwise, a NullPointerException is thrown.

something bean具有一个 fred 属性,该属性具有一个 bob 属性,该属性具有一个 sammy 属性,并且最终的 sammy 属性值被设置为 123。 为了使它起作用,在构造bean之后,somethingfred 属性和 fredbob 属性一定不能为null。 否则,将抛出 NullPointerException

使用 depends-on 属性(Using depends-on

If a bean is a dependency of another bean, that usually means that one bean is set as a property of another. Typically you accomplish this with the <ref/> element in XML-based configuration metadata. However, sometimes dependencies between beans are less direct. An example is when a static initializer in a class needs to be triggered, such as for database driver registration. The depends-on attribute can explicitly force one or more beans to be initialized before the bean using this element is initialized. The following example uses the depends-on attribute to express a dependency on a single bean:

如果一个bean是另一个bean的依赖,这通常意味着一个bean被设置为另一个bean的一个属性。通常,你可以使用基于XML的配置元数据中的 <ref/> 元素 来完成此操作。 但是,有时bean之间的依赖性不太直接。譬如何时需要触发类中的静态初始值设定项,例如数据库驱动的注册。 depends-on 属性可以在使用此元素的bean初始化之前显式强制一个或多个bean初始化。 以下示例使用 depends-on 属性来表示对单个bean的依赖关系:

<bean id="beanOne" class="ExampleBean" depends-on="manager"/>
<bean id="manager" class="ManagerBean" />

To express a dependency on multiple beans, supply a list of bean names as the value of the depends-on attribute (commas, whitespace, and semicolons are valid delimiters):

为了表达对于多个bean的依赖关系,可以提供一个包含多个bean name的列表来作为 depends-on 的属性(该列表内name间可用逗号,空格或分号进行分隔):

<bean id="beanOne" class="ExampleBean" depends-on="manager,accountDao">
    <property name="manager" ref="manager" />
</bean>

<bean id="manager" class="ManagerBean" />
<bean id="accountDao" class="x.y.jdbc.JdbcAccountDao" />
The depends-on attribute can specify both an initialization-time dependency and, in the case of singleton beans only, a corresponding destruction-time dependency. Dependent beans that define a depends-on relationship with a given bean are destroyed first, prior to the given bean itself being destroyed. Thus, depends-on can also control shutdown order.
depends-on 属性既可以指定初始化时(initialization-time)的相关依赖,也可以仅在bean为单例( singleton )的情况下指定相应的销毁时(destruction-time)的相关依赖。 在销毁给定bean本身之前,先销毁与给定bean通过 depends-on 建立依赖关系的相关beans。 因此, depends-on 也可以控制销毁结束顺序。

bean的懒加载(Lazy-initialized Beans)

By default, ApplicationContext implementations eagerly create and configure all singleton beans as part of the initialization process. Generally, this pre-instantiation is desirable, because errors in the configuration or surrounding environment are discovered immediately, as opposed to hours or even days later. When this behavior is not desirable, you can prevent pre-instantiation of a singleton bean by marking the bean definition as being lazy-initialized. A lazy-initialized bean tells the IoC container to create a bean instance when it is first requested, rather than at startup.

In XML, this behavior is controlled by the lazy-init attribute on the <bean/> element, as the following example shows:

默认情况下, ApplicationContext 实现会在初始化过程中立即创建并配置所有单例bean( singleton )。 通常,这种预初始化正和我们的需求,因为在配置中或者周围环境中的错误会被立即发现,而不是在几个小时甚至几天之后。。 如果不需要这种行为,则可以通过将bean definition 标记为lazy-initialized(延迟初始化,俗称懒加载)来防止对singleton bean的预先实例化。标记为 lazy-initialized 的bean告诉IoC容器在首次请求时(而不是在启动时)创建一个bean实例。 在XML配置中,这种行为是通过 <bean/> 元素的 lazy-init 的属性控制的,如以下示例所示:

<bean id="lazy" class="com.something.ExpensiveToCreateBean" lazy-init="true"/>
<bean name="not.lazy" class="com.something.AnotherBean"/>

When the preceding configuration is consumed by an ApplicationContext, the lazy bean is not eagerly pre-instantiated when the ApplicationContext starts, whereas the not.lazy bean is eagerly pre-instantiated.

当上面的配置被 ApplicationContext 拿到处理时,当ApplicationContext启动时,就不会急于对 lazy Bean进行实例化,而对 not.lazy Bean则会快速进行实例化。

However, when a lazy-initialized bean is a dependency of a singleton bean that is not lazy-initialized, the ApplicationContext creates the lazy-initialized bean at startup, because it must satisfy the singleton’s dependencies. The lazy-initialized bean is injected into a singleton bean elsewhere that is not lazy-initialized.

但是,当一个延迟加载的bean是一个非延迟加载的单例bean的依赖项的时候,ApplicationContext 在启动的时候就会创建延迟加载的bean,因为他必须满足单例的依赖关系。这个延迟初始化的bean(知秋注:此时已经被容器初始化了)会被注入到其他未延迟初始化的单例bean中。

You can also control lazy-initialization at the container level by using the default-lazy-init attribute on the <beans/> element, a the following example shows:

你也可以通过使用 <beans/> 元素中 default-lazy-init 属性在容器级别控制bean的延迟初始化,如下示例所示:

<beans default-lazy-init="true">
    <!-- no beans will be pre-instantiated... -->
</beans>

自动装配协作者(对bean的相关依赖进行自动注入)(Autowiring Collaborators)

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知秋注:自动装配也就是我们常说的Spring自己的自动注入,不过这里使用自动装配更符合Spring下的场景动作设计。

The Spring container can autowire relationships between collaborating beans. You can let Spring resolve collaborators (other beans) automatically for your bean by inspecting the contents of the ApplicationContext. Autowiring has the following advantages:

Spring容器能自动装配协作bean之间的关系。你可以让Spring通过检查 ApplicationContext 的内容,自动地为你的bean解析它内部的协作者(即其所依赖的其他bean)。自动装配有以下优点:

  • Autowiring can significantly reduce the need to specify properties or constructor arguments. (Other mechanisms such as a bean template discussed elsewhere in this chapter are also valuable in this regard.)

  • Autowiring can update a configuration as your objects evolve. For example, if you need to add a dependency to a class, that dependency can be satisfied automatically without you needing to modify the configuration. Thus autowiring can be especially useful during development, without negating the option of switching to explicit wiring when the code base becomes more stable.

  • 自动装配可以大大减少所需要指定的属性或构造函数参数。(在此方面,其他机制(例如Bean模板 在本章其他地方讨论 )也很有价值。)

  • 随着你的对象的演变,自动装配可以随着对象的变化更新配置。例如,如果你需要为一个类添加依赖项,无需修改配置即可自动满足依赖关系。因此,自动装配在开发过程中非常有用,当代码库变得更稳定时,自动装配可以避免我们切换到使用显式的方式来写一些东西(即对之前的代码做修改)。(知秋注:可能根据场景我们会写一个全新的类来代替之前的默认选项,那么我们无须修改上层代码中配置,只需要自定义一下配置即可,如果是自己的代码,则完全可以将之前加入到容器中的bean通过@component注解修改为当前需要的bean,或者通过@Autowired进行bean name的全限定名的设定,不影响已经稳定的代码内部结构)

When using XML-based configuration metadata (see 依赖注入(Dependency Injection)), you can specify the autowire mode for a bean definition with the autowire attribute of the <bean/> element. The autowiring functionality has four modes. You specify autowiring per bean and can thus choose which ones to autowire. The following table describes the four autowiring modes:

当使用基于XML的配置元数据时(查阅 依赖注入(Dependency Injection) ),你可以使用 <bean/> 元素的 autowire 属性来为一个bean 定义指明自动装配的模式。自动装配功能有四种模式。你可以为每一个bean指定自动装配,因此可以选择自动装配哪些bean。下表描述了四种自动装配模式:

Table 2. Autowiring modes
Mode Explanation

no

(Default) No autowiring. Bean references must be defined by ref elements. Changing the default setting is not recommended for larger deployments, because specifying collaborators explicitly gives greater control and clarity. To some extent, it documents the structure of a system.

byName

Autowiring by property name. Spring looks for a bean with the same name as the property that needs to be autowired. For example, if a bean definition is set to autowire by name and it contains a master property (that is, it has a setMaster(..) method), Spring looks for a bean definition named master and uses it to set the property.

byType

Lets a property be autowired if exactly one bean of the property type exists in the container. If more than one exists, a fatal exception is thrown, which indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens (the property is not set).

constructor

Analogous to byType but applies to constructor arguments. If there is not exactly one bean of the constructor argument type in the container, a fatal error is raised.

Table 3. 自动装配模式
模式 解释

no

(默认) 无自动装配。Bean的引用必须由 ref 元素定义。对于大规模的部署,不建议修改默认配置,因为显式地指定协作者可以提供更好的控制和清晰度。在某种程度上,它记录了一个系统的结构。

byName

按属性名自动装配。Spring会查找与需要自动装配的属性同名的bean。例如,如果将一个bean设置为按属性名自动装配,并且它包含一个 master 属性(也就是说,它有一个 setMaster(..) 方法),Spring会查找一个名为 master 的bean,并使用它来设置属性。

byType

如果容器中恰好存在一个该属性类型的bean,则自动装配该属性。如果存在多个,则抛出一个fatal 异常,表明你不能使用 byType 来自动装配该bean。如果没有匹配的bean,则什么都不会发生(属性未被设置)。

constructor

byType 类似,但适用于构造函数参数。如果容器中没有一个构造函数参数类型的bean(知秋注:如果没有该bean,则构造器无法初始化),将引发fatal 错误。

With byType or constructor autowiring mode, you can wire arrays and typed collections. In such cases, all autowire candidates within the container that match the expected type are provided to satisfy the dependency. You can autowire strongly-typed Map instances if the expected key type is String. An autowired Map instance’s values consist of all bean instances that match the expected type, and the Map instance’s keys contain the corresponding bean names.

使用 byTypeconstructor 自动装配模式,你可以装配数组和类型集合。在这种情况下,将提供容器中与预期类型匹配的所有自动装配的候选对象,以满足依赖性。如果预期key类型是 String,你可以自动装配强类型的 Map 实例。一个自动装配的 Map 实例的values包含所有与预期类型匹配的bean实例,并且 Map 实例的keys包含相应的bean名称。

自动装配的局限性和缺点(Limitations and Disadvantages of Autowiring)

在整个项目中统一使用自动装配时,效果最好。如果项目中基本就没用过自动装配,那可能让开发者对使用自动装配对仅配置一两个bean definition这种行为感到困惑。

Consider the limitations and disadvantages of autowiring:

思考自动装配的局限性和缺点:

  • Explicit dependencies in property and constructor-arg settings always override autowiring. You cannot autowire simple properties such as primitives, Strings, and Classes (and arrays of such simple properties). This limitation is by-design.

  • 通过 propertyconstructor-arg 进行依赖设定,这种显式的设定始终会覆盖自动装配。你不能自动装配简单的属性,例如基本类型,Strings ,和 Classes (以及这些简单属性的数组)。这种限制是由设计造成的。

  • Autowiring is less exact than explicit wiring. Although, as noted in the earlier table, Spring is careful to avoid guessing in case of ambiguity that might have unexpected results. The relationships between your Spring-managed objects are no longer documented explicitly.

  • 自动装配不如显示装配精确。如前表所述,尽量小心让Spring避免去猜测模棱两可的事情(知秋注:比如两个相同类型的bean,也没有其他特殊指定,Spring不知道该选择哪个来进行注入操作),以免产生意想不到的结果。如果你不仔细,你的Spring所管理对象之间的关系不再明确。

  • Wiring information may not be available to tools that may generate documentation from a Spring container.

  • 装配信息可能对从Spring容器中生成文档的工具不可用。

  • Multiple bean definitions within the container may match the type specified by the setter method or constructor argument to be autowired. For arrays, collections, or Map instances, this is not necessarily a problem. However, for dependencies that expect a single value, this ambiguity is not arbitrarily resolved. If no unique bean definition is available, an exception is thrown.

  • 容器内的多个bean definition可能与要自动装配的setter方法或构造函数参数指明的类型匹配。对于数组,集合,或者 Map 实例,这就不是什么问题了。然而,对于期望单值的依赖项来说,这种歧义性不能随意的解决。如果没有唯一的bean可用,将抛出一个异常。

In the latter scenario, you have several options:

在后一种情况下,你有几种选择:

  • Abandon autowiring in favor of explicit wiring.

  • 放弃自动装配,转而使用显式装配,明确指定。

  • Avoid autowiring for a bean definition by setting its autowire-candidate attributes to false, as described in the next section.

  • 通过将bean definition 的 autowire-candidate 属性设置为 false ,来避免自动装配,如 下一节 所述。

  • Designate a single bean definition as the primary candidate by setting the primary attribute of its <bean/> element to true.

  • 通过将其 <bean/> 元素的 primary 属性设置为 true,将单个bean definition 指定为主要的候选对象。

  • Implement the more fine-grained control available with annotation-based configuration, as described in Annotation-based Container Configuration.

  • Annotation-based Container Configuration 所述,通过基于注解的配置来实现更细粒度的控制。

从自动装配中排除bean(Excluding a Bean from Autowiring)

On a per-bean basis, you can exclude a bean from autowiring. In Spring’s XML format, set the autowire-candidate attribute of the <bean/> element to false. The container makes that specific bean definition unavailable to the autowiring infrastructure (including annotation style configurations such as @Autowired).

在每个bean的基础上,你可以将一个bean排除在自动装配之外。在Spring的XML格式中,将 <bean/> 元素的 autowire-candidate 属性设置为 false 。容器将使该特定的bean不适用于自动装配的基础结构(包括注解样式的配置,例如 @Autowired )。

The autowire-candidate attribute is designed to only affect type-based autowiring. It does not affect explicit references by name, which get resolved even if the specified bean is not marked as an autowire candidate. As a consequence, autowiring by name nevertheless injects a bean if the name matches.
autowire-candidate 属性被设计用来只影响基于类型的自动装配。它不影响按名称显式指定的引用,即使指定的bean没有标记为自动装配的候选bean,也会解析这些引用。因此,如果名称匹配的话,自动装配仍然会按名称注入bean。

You can also limit autowire candidates based on pattern-matching against bean names. The top-level <beans/> element accepts one or more patterns within its default-autowire-candidates attribute. For example, to limit autowire candidate status to any bean whose name ends with Repository, provide a value of *Repository. To provide multiple patterns, define them in a comma-separated list. An explicit value of true or false for a bean definition’s autowire-candidate attribute always takes precedence. For such beans, the pattern matching rules do not apply.

你还可以根据bean名称的模式匹配来限制自动装配哪些候选bean。顶级 <beans/> 元素在其 default-autowire-candidates 属性中接受一个或多个模式(知秋注:所谓的模式就是一个表达式规则)。例如,要将自动装配候选状态限制为名称以 Repository 结尾的任何bean,意思就是说请提供一个值为 *Repository 的bean来进行注入 。要提供多种模式,请在逗号分割的列表中定义它们。对于bean definition中的 autowire-candidate 属性,若显式设定为 true 或者 false ,则始终优先。对于此类bean,模式匹配规则不适用。(译者哺乳类注:autowire-candidate 属性的默认值是 default 。)

These techniques are useful for beans that you never want to be injected into other beans by autowiring. It does not mean that an excluded bean cannot itself be configured by using autowiring. Rather, the bean itself is not a candidate for autowiring other beans.

这些技术对于那些你不想通过自动装配将其注入到其他bean中的bean非常有用。但这并不意味着被排除的这个bean本身不能使用自动装配来配置。而是说,这个bean自身不能作为自动装配到其他bean的候选对象。

方法注入(Method Injection)

In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean or a non-singleton bean needs to collaborate with another non-singleton bean, you typically handle the dependency by defining one bean as a property of the other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use non-singleton (prototype) bean B, perhaps on each method invocation on A. The container creates the singleton bean A only once, and thus only gets one opportunity to set the properties. The container cannot provide bean A with a new instance of bean B every time one is needed.

在大多数应用场景中,容器中大部分bean都是singletons。当一个单例bean需要与另一个单例bean协作或一个非单例bean需要与另一个非单例bean协作时,通常是将一个bean定义为另一个bean的属性来处理依赖关系。当bean的生命周期不同时,就会出现问题。可能是在A的每个方法调用上,假设单例bean A需要使用非单例(原型)bean B。容器只会创建单例bean A一次,因此只有一次机会来设置属性。容器无法在每次进行A方法调用时,都为bean A提供一个全新的bean B实例。(知秋注:也就是多例可以调用单例,但是单例无法调用多例)

A solution is to forego some inversion of control. You can make bean A aware of the container by implementing the ApplicationContextAware interface, and by making a getBean("B") call to the container ask for (a typically new) bean B instance every time bean A needs it. The following example shows this approach:

解决方案是放弃一些控制翻转。通过实现 ApplicationContextAware 接口,从而使bean A能够拿到容器的上下文 ,并在每次bean A 需要bean B时,通过调用容器上下文的 getBean("B") 方法来请求得到(通常是新创建的)bean B实例。以下示例展示了此方法:

Java
// a class that uses a stateful Command-style class to perform some processing
package fiona.apple;

// Spring-API imports
import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;

public class CommandManager implements ApplicationContextAware {

    private ApplicationContext applicationContext;

    public Object process(Map commandState) {
        // grab a new instance of the appropriate Command
        Command command = createCommand();
        // set the state on the (hopefully brand new) Command instance
        command.setState(commandState);
        return command.execute();
    }

    protected Command createCommand() {
        // notice the Spring API dependency!
        return this.applicationContext.getBean("command", Command.class);
    }

    public void setApplicationContext(
            ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
}
Kotlin
// a class that uses a stateful Command-style class to perform some processing
package fiona.apple

// Spring-API imports
import org.springframework.context.ApplicationContext
import org.springframework.context.ApplicationContextAware

class CommandManager : ApplicationContextAware {

    private lateinit var applicationContext: ApplicationContext

    fun process(commandState: Map<*, *>): Any {
        // grab a new instance of the appropriate Command
        val command = createCommand()
        // set the state on the (hopefully brand new) Command instance
        command.state = commandState
        return command.execute()
    }

    // notice the Spring API dependency!
    protected fun createCommand() =
            applicationContext.getBean("command", Command::class.java)

    override fun setApplicationContext(applicationContext: ApplicationContext) {
        this.applicationContext = applicationContext
    }
}

The preceding is not desirable, because the business code is aware of and coupled to the Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC container, lets you handle this use case cleanly.

以上方法并不可取,因为业务代码会使用Spring内部的上下文,也就是会和Spring Framework耦合到一起。方法注入是Spring IoC容器的一项比较高级的特性,能使你干净利落地处理这个点。

You can read more about the motivation for Method Injection in this blog entry.

你可以在 这篇博客中了解更多有关方法注入的内容。

lookup-method 注入(Lookup Method Injection)

Lookup method injection is the ability of the container to override methods on container-managed beans and return the lookup result for another named bean in the container. The lookup typically involves a prototype bean, as in the scenario described in the preceding section. The Spring Framework implements this method injection by using bytecode generation from the CGLIB library to dynamically generate a subclass that overrides the method.

通过lookup-method 注入可以使得容器有能力对所管理bean上的方法进行覆盖,针对该方法所要的返回值,查找并返回容器中另一个所命名bean。lookup-method通常用于一个原型(prototype) bean,如 上一节的场景描述所示。Spring框架通过使用CGLIB库中的字节码生成来动态生成覆盖该方法的子类来实现该方法的注入。

  • For this dynamic subclassing to work, the class that the Spring bean container subclasses cannot be final, and the method to be overridden cannot be final, either.

  • Unit-testing a class that has an abstract method requires you to subclass the class yourself and to supply a stub implementation of the abstract method.

  • Concrete methods are also necessary for component scanning, which requires concrete classes to pick up.

  • A further key limitation is that lookup methods do not work with factory methods and in particular not with @Bean methods in configuration classes, since, in that case, the container is not in charge of creating the instance and therefore cannot create a runtime-generated subclass on the fly.

  • 为了使这动态子类起作用,Spring容器对所要进行重写操作的class不能用 final 修饰,且重写的方法也不能用 final 修饰。

  • 对一个具有 abstract 方法的类进行单元测试,就需要你提供一个对该 abstract 方法的实现来动态生成一个子类bean(通过lookup-method的方式)。

  • 具体的方法需要通过组件扫描,来找到具体类来进行选择(知秋注:如果按类型查找的话,可能会有多个bean,此时可以用权限定名来做,如果只有一个该类型的bean,可以不标注)

  • 进一步的关键限制是lookup-method不适用于工厂方法,尤其不适用于配置类中的 @Bean 方法,因为在这种情况下,容器不负责创建实例,所以不能动态地创建运行时生成的子类。

In the case of the CommandManager class in the previous code snippet, the Spring container dynamically overrides the implementation of the createCommand() method. The CommandManager class does not have any Spring dependencies, as the reworked example shows:

对于前面代码片段中的 CommandManager 类,Spring容器动态地覆盖了其 createCommand() 方法的实现。如重写的例子所示,CommandManager 类没有任何Spring的依赖:

Java
package fiona.apple;

// no more Spring imports!

public abstract class CommandManager {

    public Object process(Object commandState) {
        // grab a new instance of the appropriate Command interface
        Command command = createCommand();
        // set the state on the (hopefully brand new) Command instance
        command.setState(commandState);
        return command.execute();
    }

    // okay... but where is the implementation of this method?
    protected abstract Command createCommand();
}
Kotlin
package fiona.apple

// no more Spring imports!

abstract class CommandManager {

    fun process(commandState: Any): Any {
        // grab a new instance of the appropriate Command interface
        val command = createCommand()
        // set the state on the (hopefully brand new) Command instance
        command.state = commandState
        return command.execute()
    }

    // okay... but where is the implementation of this method?
    protected abstract fun createCommand(): Command
}

In the client class that contains the method to be injected (the CommandManager in this case), the method to be injected requires a signature of the following form:

在这个包含需要注入的方法的客户端class中(例子中的 CommandManager ),要求注入的方法具有以下签名:

<public|protected> [abstract] <return-type> theMethodName(no-arguments);

If the method is abstract, the dynamically-generated subclass implements the method. Otherwise, the dynamically-generated subclass overrides the concrete method defined in the original class. Consider the following example:

如果方法是 abstract 的,则动态生成的子类将实现该方法。否则,动态生成的子类将覆盖原始类中定义的具体方法。

知秋注:

  • public|protected:要求方法必须是可以被子类重写和调用的;

  • abstract可选,如果是抽象方法,CGLIB的动态代理类就会实现这个方法,如果不是抽象方法,就会覆盖这个方法,所以没什么影响;

  • return-type如果是non-singleton-bean的类型才有使用的意义,否则没必要使用

  • no-arguments 不允许有参数。 思考以下例子:

<!-- a stateful bean deployed as a prototype (non-singleton) -->
<bean id="myCommand" class="fiona.apple.AsyncCommand" scope="prototype">
    <!-- inject dependencies here as required -->
</bean>

<!-- commandProcessor uses statefulCommandHelper -->
<bean id="commandManager" class="fiona.apple.CommandManager">
    <lookup-method name="createCommand" bean="myCommand"/>
</bean>

The bean identified as commandManager calls its own createCommand() method whenever it needs a new instance of the myCommand bean. You must be careful to deploy the myCommand bean as a prototype if that is actually what is needed. If it is a singleton, the same instance of the myCommand bean is returned each time.

每当需要新的 myCommand bean实例时,id 为 commandManager 的bean将调用它自己的 createCommand() 方法。如果真的需要的话(知秋注:即得到一个全新的bean实例),你必须很仔细的将 myCommand bean配置成prototype。如果它是singleton,那每次将返回同一个 myCommand bean实例。

Alternatively, within the annotation-based component model, you can declare a lookup method through the @Lookup annotation, as the following example shows:

另外,在基于注解的组件类中,你可以通过 @Lookup 注解来声明一个lookup-method,如以下示例所示:

Java
public abstract class CommandManager {

    public Object process(Object commandState) {
        Command command = createCommand();
        command.setState(commandState);
        return command.execute();
    }

    @Lookup("myCommand")
    protected abstract Command createCommand();
}
Kotlin
abstract class CommandManager {

    fun process(commandState: Any): Any {
        val command = createCommand()
        command.state = commandState
        return command.execute()
    }

    @Lookup("myCommand")
    protected abstract fun createCommand(): Command
}

Or, more idiomatically, you can rely on the target bean getting resolved against the declared return type of the lookup method:

或者,更常用的,你可以根据声明的lookup-method的返回类型来解析目标bean。

Java
public abstract class CommandManager {

    public Object process(Object commandState) {
        MyCommand command = createCommand();
        command.setState(commandState);
        return command.execute();
    }

    @Lookup
    protected abstract MyCommand createCommand();
}
Kotlin
abstract class CommandManager {

    fun process(commandState: Any): Any {
        val command = createCommand()
        command.state = commandState
        return command.execute()
    }

    @Lookup
    protected abstract fun createCommand(): Command
}

Note that you should typically declare such annotated lookup methods with a concrete stub implementation, in order for them to be compatible with Spring’s component scanning rules where abstract classes get ignored by default. This limitation does not apply to explicitly registered or explicitly imported bean classes.

请注意,通常情况下,这种lookup-method的注解往往声明在一个具体的方法实现上,为了使它们与Spring的组件扫描规则兼容,在默认情况下,抽象类将被忽略。此限制不适用于显式注册或显式导入的Bean classes。

Another way of accessing differently scoped target beans is an ObjectFactory/ Provider injection point. See Scoped Beans as Dependencies.

You may also find the ServiceLocatorFactoryBean (in the org.springframework.beans.factory.config package) to be useful.

访问不同作用域的目标bean的另一种方式是 ObjectFactory/Provider 注入点。请查阅 Scoped Beans as Dependencies

你可能还会发现 ServiceLocatorFactoryBean (在 org.springframework.beans.factory.config 包中)很有用。

任意方法替换(Arbitrary Method Replacement)

A less useful form of method injection than lookup method injection is the ability to replace arbitrary methods in a managed bean with another method implementation. You can safely skip the rest of this section until you actually need this functionality.

与lookup-method注入相比,一种不太常用的方法注入形式是使用另一种方法实现来替换容器所管理bean中的任意方法。你可以安心的跳过本节的剩余部分,直到你真正需要此功能为止。

With XML-based configuration metadata, you can use the replaced-method element to replace an existing method implementation with another, for a deployed bean. Consider the following class, which has a method called computeValue that we want to override:

对于基于XML的配置元数据,你可以使用 replaced-method 元素将已部署bean中现有的方法实现替换为其他方法实现。思考下面这个类,我们想要覆盖它的 computeValue 方法:

Java
public class MyValueCalculator {

    public String computeValue(String input) {
        // some real code...
    }

    // some other methods...
}
Kotlin
class MyValueCalculator {

    fun computeValue(input: String): String {
        // some real code...
    }

    // some other methods...
}

A class that implements the org.springframework.beans.factory.support.MethodReplacer interface provides the new method definition, as the following example shows:

一个类可以通过实现 org.springframework.beans.factory.support.MethodReplacer 接口来提供了新的方法定义,如下示例所示::

Java
/**
 * meant to be used to override the existing computeValue(String)
 * implementation in MyValueCalculator
 */
public class ReplacementComputeValue implements MethodReplacer {

    public Object reimplement(Object o, Method m, Object[] args) throws Throwable {
        // get the input value, work with it, and return a computed result
        String input = (String) args[0];
        ...
        return ...;
    }
}
Kotlin
/**
* meant to be used to override the existing computeValue(String)
* implementation in MyValueCalculator
*/
class ReplacementComputeValue : MethodReplacer {

    override fun reimplement(obj: Any, method: Method, args: Array<out Any>): Any {
        // get the input value, work with it, and return a computed result
        val input = args[0] as String;
        ...
        return ...;
    }
}

The bean definition to deploy the original class and specify the method override would resemble the following example:

对用于部署的原始类的bean definition 指定要覆盖的方法,操作类似于下面的示例:

<bean id="myValueCalculator" class="x.y.z.MyValueCalculator">
    <!-- arbitrary method replacement -->
    <replaced-method name="computeValue" replacer="replacementComputeValue">
        <arg-type>String</arg-type>
    </replaced-method>
</bean>

<bean id="replacementComputeValue" class="a.b.c.ReplacementComputeValue"/>

You can use one or more <arg-type/> elements within the <replaced-method/> element to indicate the method signature of the method being overridden. The signature for the arguments is necessary only if the method is overloaded and multiple variants exist within the class. For convenience, the type string for an argument may be a substring of the fully qualified type name. For example, the following all match java.lang.String:

你可以在 <replaced-method/> 元素中使用一个或多个 <arg-type/> 元素来指明被覆盖方法的方法签名。仅当这个类中要重载的方法存在多个参数时,才有必要配置这些参数签名。为了方便起见,代表参数类型的字符串可以是该参数完整类型name的一段字符串。例如,以下都匹配 java.lang.String

java.lang.String
String
Str

Because the number of arguments is often enough to distinguish between each possible choice, this shortcut can save a lot of typing, by letting you type only the shortest string that matches an argument type.

因为参数的数量通常足以区分每种可能的选择,所以这种快捷方式允许只输入与参数类型匹配的最短字符串,从而节省了大量的输入操作。

Bean作用域(Bean Scopes)

本节翻译人员

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梦小七

GitHub

Bean作用域、Singleton 作用域、Prototype 作用域

知秋

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翻译与审校

虚生花

掘金

文档整理

When you create a bean definition, you create a recipe for creating actual instances of the class defined by that bean definition. The idea that a bean definition is a recipe is important, because it means that, as with a class, you can create many object instances from a single recipe.

当创建一个Bean definition时, 实际上你通过该bean definition来为对应的class创建了一个"配方(recipe)",可用于创建具体类实例。 把Bean定义当成“配方”的想法是很有意义的, 因为这意味着你可以类似于class一样, 根据一个“配方”创建多个实例.

You can control not only the various dependencies and configuration values that are to be plugged into an object that is created from a particular bean definition but also control the scope of the objects created from a particular bean definition. This approach is powerful and flexible, because you can choose the scope of the objects you create through configuration instead of having to bake in the scope of an object at the Java class level. Beans can be defined to be deployed in one of a number of scopes. The Spring Framework supports six scopes, four of which are available only if you use a web-aware ApplicationContext. You can also create a custom scope.

根据一个特定的Bean definition 来创建对象,你不仅可以控制注入到对象中的各种依赖和配置值,还可以控制该对象的作用域。这种方式是非常强大和灵活的,因为你可以选择所要创建对象的作用域,而不必在Java class级别定义作用域。Spring Framework支持6种作用域,其中四种只能用在基于web的 ApplicationContext,当然你也可以创建 一个自定义作用域 ,Bean可以定义为其中的作用域之一。

The following table describes the supported scopes:

下面的表格描述了所支持的作用域:

Table 4. Bean scopes
Scope Description

singleton

(Default) Scopes a single bean definition to a single object instance for each Spring IoC container.

prototype

Scopes a single bean definition to any number of object instances.

request

Scopes a single bean definition to the lifecycle of a single HTTP request. That is, each HTTP request has its own instance of a bean created off the back of a single bean definition. Only valid in the context of a web-aware Spring ApplicationContext.

session

Scopes a single bean definition to the lifecycle of an HTTP Session. Only valid in the context of a web-aware Spring ApplicationContext.

application

Scopes a single bean definition to the lifecycle of a ServletContext. Only valid in the context of a web-aware Spring ApplicationContext.

websocket

Scopes a single bean definition to the lifecycle of a WebSocket. Only valid in the context of a web-aware Spring ApplicationContext.

Table 5. Bean 作用域
作用域 描述

singleton

(默认情况下),对于每个Spring IoC容器,一个Bean definition 在该容器中只有一个对应的实例对象。

prototype

一个Bean definition有多个对应实例对象。

request

Bean definition 的作用域为一次HTTP请求生命周期,即每个HTTP请求都有根据某个Bean definition 创建的属于自己的实例,只能在基于web的 ApplicationContext 可用。

session

Bean definition 的作用域为一个HTTP Session 生命周期,只能在基于web的 ApplicationContext 可用。

application

Bean definition 的作用域为 ServletContext 生命周期,只能在基于web的 ApplicationContext 可用。

websocket

Bean definition 的作用域为 WebSocket 生命周期,只能在基于web的 ApplicationContext 可用。

As of Spring 3.0, a thread scope is available but is not registered by default. For more information, see the documentation for SimpleThreadScope. For instructions on how to register this or any other custom scope, see Using a Custom Scope.
从Spring 3.0开始,引入了线程作用域,但默认是不注册的。更多的信息请看 SimpleThreadScope文档。关于怎么注册线程作用域或者自定义其它作用域的介绍,请看Using a Custom Scope

Singleton作用域(The Singleton Scope)

Only one shared instance of a singleton bean is managed, and all requests for beans with an ID or IDs that match that bean definition result in that one specific bean instance being returned by the Spring container.

对于singleton作用域的Bean, Spring容器中仅管理着一个共享的Bean实例,并且在获取Bean时,只要ID或者IDS与该bean definition相匹配,那么返回的是同一个Bean实例。

To put it another way, when you define a bean definition and it is scoped as a singleton, the Spring IoC container creates exactly one instance of the object defined by that bean definition. This single instance is stored in a cache of such singleton beans, and all subsequent requests and references for that named bean return the cached object. The following image shows how the singleton scope works:

换句话讲,当你定义Bean的作用域为singleton时,Spring IoC容器根据该 bean definition 仅会创建一个Bean的对象实例。这个对象实例存储在singleton beans的缓存中,后续对该Bean的请求和引用都会返回该对象实例。下图展示了singleton作用域如何工作:

singleton

Spring’s concept of a singleton bean differs from the singleton pattern as defined in the Gang of Four (GoF) patterns book. The GoF singleton hard-codes the scope of an object such that one and only one instance of a particular class is created per ClassLoader. The scope of the Spring singleton is best described as being per-container and per-bean. This means that, if you define one bean for a particular class in a single Spring container, the Spring container creates one and only one instance of the class defined by that bean definition. The singleton scope is the default scope in Spring. To define a bean as a singleton in XML, you can define a bean as shown in the following example:

Spring中的singleton Bean与“四人组(GoF)”书中所定义的singleton模式的概念有所不同。GoF中硬编码了对象的作用域,一个ClassLoader只会创建一个特定的类实例。而Spring的singleton则被定义为一个IoC容器对应一个Bean,这意味着,如果你在Spring IoC容器中为特定class定义了一个Bean,那么Spring容器仅会根据该Bean definition创建一个Bean实例。要在XML中定义Bean的作用域为singleton,你可以参照下面的示例:

<bean id="accountService" class="com.something.DefaultAccountService"/>

<!-- the following is equivalent, though redundant (singleton scope is the default) -->
<bean id="accountService" class="com.something.DefaultAccountService" scope="singleton"/>

Prototype 作用域(The Prototype Scope)

The non-singleton prototype scope of bean deployment results in the creation of a new bean instance every time a request for that specific bean is made. That is, the bean is injected into another bean or you request it through a getBean() method call on the container. As a rule, you should use the prototype scope for all stateful beans and the singleton scope for stateless beans.

对于非singletion的prototype作用域来说,它会导致在每次请求该Bean(将其注入到另一个Bean或者在容器中调用 getBean() 方法)时,都会创建一个新的Bean实例。通常,对于有状态的Bean,你应该使用prototype作用域,而对于无状态的Bean使用singleton作用域。

The following diagram illustrates the Spring prototype scope:

下图阐述了Spring prototype作用域:

prototype

(A data access object (DAO) is not typically configured as a prototype, because a typical DAO does not hold any conversational state. It was easier for us to reuse the core of the singleton diagram.)

(通常情况下,数据访问对象(DAO)不会配置为prototype的,因为一个典型的DAO是不会有任何会话状态的。因为相对比较简单,此处借用了singleton图解核心部分。)

The following example defines a bean as a prototype in XML:

下面的例子是在XML中定义一个prototype作用域:

<bean id="accountService" class="com.something.DefaultAccountService" scope="prototype"/>

In contrast to the other scopes, Spring does not manage the complete lifecycle of a prototype bean. The container instantiates, configures, and otherwise assembles a prototype object and hands it to the client, with no further record of that prototype instance. Thus, although initialization lifecycle callback methods are called on all objects regardless of scope, in the case of prototypes, configured destruction lifecycle callbacks are not called. The client code must clean up prototype-scoped objects and release expensive resources that the prototype beans hold. To get the Spring container to release resources held by prototype-scoped beans, try using a custom bean post-processor, which holds a reference to beans that need to be cleaned up.

与其他作用域相比,Spring不会管理prototype作用域Bean的整个生命周期。在完成容器的初始化,配置以及装配一个prototype bean,返回给请求Bean的客户端之后,就不会再管理这个Bean实例了。因此,不论这个bean所属作用域如何,这些bean都会有初始化生命周期方法的回调,尽管如此,对于prototype作用域的bean来讲,它所配置的生命周期中的销毁方法并不会被调用。客户端代码中必须清理prototype作用域实例对象并释放掉prototype Bean占用的昂贵的资源。为了让Spring容器释放掉prototype作用域Bean占用的资源,请尝试使用自定义 bean后置处理器,该处理器包含对需要清理的bean的引用。

In some respects, the Spring container’s role in regard to a prototype-scoped bean is a replacement for the Java new operator. All lifecycle management past that point must be handled by the client. (For details on the lifecycle of a bean in the Spring container, see Lifecycle Callbacks.)

从某些方面来讲,Spring容器对prototype作用域Bean的定位其实就是代替了Java的 new 操作。此后的所有生命周期管理必须由客户端自己处理。(关于Bean在Spring容器的生命周期的详情,请看 Lifecycle Callbacks。)

具有原型bean依赖关系的单例bean(Singleton Beans with Prototype-bean Dependencies)

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When you use singleton-scoped beans with dependencies on prototype beans, be aware that dependencies are resolved at instantiation time. Thus, if you dependency-inject a prototype-scoped bean into a singleton-scoped bean, a new prototype bean is instantiated and then dependency-injected into the singleton bean. The prototype instance is the sole instance that is ever supplied to the singleton-scoped bean.

当你使用作用域为单例的Bean对原型bean有产生依赖时,请注意,依赖关系在实例化时已解决。 因此,如果你在依赖注入一个原型作用域的bean到一个单例作用域的bean时,一个新的原型 bean会被实例化然后依赖注入到这个单例bean中。这个原型实例对于该单例作用域bean来讲会一直依赖下去,不会发生改变。

However, suppose you want the singleton-scoped bean to acquire a new instance of the prototype-scoped bean repeatedly at runtime. You cannot dependency-inject a prototype-scoped bean into your singleton bean, because that injection occurs only once, when the Spring container instantiates the singleton bean and resolves and injects its dependencies. If you need a new instance of a prototype bean at runtime more than once, see 方法注入(Method Injection)

然而,假设你想要这个单例作用域bean在运行时每次获取的这个原型bean依赖都是一个全新的实例。那么你就不能将一个原型作用域的bean依赖注入到你的单例bean中。因为当Spring容器实例化单例bean并解决注入依赖时,注入只发生一次。如果你在运行时需要不止一个原型bean对象时,请参见 方法注入(Method Injection)

Request, Session, Application, 和 WebSocket 作用域(Request, Session, Application, and WebSocket Scopes)

The request, session, application, and websocket scopes are available only if you use a web-aware Spring ApplicationContext implementation (such as XmlWebApplicationContext). If you use these scopes with regular Spring IoC containers, such as the ClassPathXmlApplicationContext, an IllegalStateException that complains about an unknown bean scope is thrown.

request , session , application , 和 websocket 作用域只能用在web相关的Spring ApplicationContext 实现中(比如:XmlWebApplicationContext )。如果你将这些作用域用于常规的Spring IoC容器,就比如 ClassPathXmlApplicationContext,会抛出一个未知bean作用域的 IllegalStateException 异常。

初始化Web配置(Initial Web Configuration)

To support the scoping of beans at the request, session, application, and websocket levels (web-scoped beans), some minor initial configuration is required before you define your beans. (This initial setup is not required for the standard scopes: singleton and prototype.)

为了让bean支持 request , session , application , 和 websocket 这些级别的作用域(web作用域下的bean), 在定义bean之前,需要一些小的初始配置。 (标准的scope:singleton and prototype 不需要此初始设置)

How you accomplish this initial setup depends on your particular Servlet environment.

如何完成此初始设置取决于你的特定Servlet环境。

If you access scoped beans within Spring Web MVC, in effect, within a request that is processed by the Spring DispatcherServlet, no special setup is necessary. DispatcherServlet already exposes all relevant state.

如果你访问Spring Web MVC作用域中的Bean,事实上,request 请求会交由DispatcherServlet处理,不需要特殊的设置。 DispatcherServlet已经暴露了所有相关状态。

If you use a Servlet 2.5 web container, with requests processed outside of Spring’s DispatcherServlet (for example, when using JSF or Struts), you need to register the org.springframework.web.context.request.RequestContextListener ServletRequestListener. For Servlet 3.0+, this can be done programmatically by using the WebApplicationInitializer interface. Alternatively, or for older containers, add the following declaration to your web application’s web.xml file:

如果你使用Servlet 2.5 Web容器,并且在Spring的 DispatcherServlet 之外处理请求(例如,使用JSF或Struts时),则需要注册 org.springframework.web.context.request.RequestContextListener ServletRequestListener 。 对于Servlet 3.0+,这可以在编程时通过使用 WebApplicationInitializer 接口的方式来完成。 另外,或者对于较旧的容器,将以下声明添加到Web应用程序的 web.xml 文件中:

<web-app>
    ...
    <listener>
        <listener-class>
            org.springframework.web.context.request.RequestContextListener
        </listener-class>
    </listener>
    ...
</web-app>

Alternatively, if there are issues with your listener setup, consider using Spring’s RequestContextFilter. The filter mapping depends on the surrounding web application configuration, so you have to change it as appropriate. The following listing shows the filter part of a web application:

另外,如果你的listener设置存在问题,请考虑使用Spring的 RequestContextFilter 。 过滤器映射取决于周围的Web application 配置,因此你必须适当地对其进行更改。 以下配置显示了Web application 的过滤器部分:

<web-app>
    ...
    <filter>
        <filter-name>requestContextFilter</filter-name>
        <filter-class>org.springframework.web.filter.RequestContextFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>requestContextFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    ...
</web-app>

DispatcherServlet, RequestContextListener, and RequestContextFilter all do exactly the same thing, namely bind the HTTP request object to the Thread that is servicing that request. This makes beans that are request- and session-scoped available further down the call chain.

DispatcherServletRequestContextListenerRequestContextFilter 都在做完全相同的事情,即将HTTP请求对象绑定到为该请求提供服务的线程上。 这使得在request和session作用域内的bean可以在调用链的下游使用。

Request作用域(Request scope)

Consider the following XML configuration for a bean definition:

请考虑以下bean definition的XML配置:

<bean id="loginAction" class="com.something.LoginAction" scope="request"/>

The Spring container creates a new instance of the LoginAction bean by using the loginAction bean definition for each and every HTTP request. That is, the loginAction bean is scoped at the HTTP request level. You can change the internal state of the instance that is created as much as you want, because other instances created from the same loginAction bean definition do not see these changes in state. They are particular to an individual request. When the request completes processing, the bean that is scoped to the request is discarded.

对于每个HTTP请求,Spring都会通过使用 loginAction bean的定义在其容器中创建一个新的 LoginAction bean的实例。那就是说,这个 loginAction bean的作用域是HTTP请求级别的。你可以根据需要改变所创建的实例的内部状态,因为你改变的状态不影响其他由 loginAction bean definition 所创建的实例。这些bean实例都在各自的请求作用域内。当请求处理完成,该请求作用域中的bean也会被销毁。

When using annotation-driven components or Java configuration, the @RequestScope annotation can be used to assign a component to the request scope. The following example shows how to do so:

当使用注解驱动的组件或者Java configuration时(知秋注:@Service @Controller这些注解所代表的组件和configuration注解配置类是分开的,所以会这么说),通过使用 @RequestScope 注解可以将一个组件的作用域分配为 request,具体参考下面这个示例:

Java
@RequestScope
@Component
public class LoginAction {
    // ...
}
Kotlin
@RequestScope
@Component
class LoginAction {
    // ...
}
Session作用域(Session Scope)

Consider the following XML configuration for a bean definition:

请思考下面这个XML配置中的bean definition:

<bean id="userPreferences" class="com.something.UserPreferences" scope="session"/>

The Spring container creates a new instance of the UserPreferences bean by using the userPreferences bean definition for the lifetime of a single HTTP Session. In other words, the userPreferences bean is effectively scoped at the HTTP Session level. As with request-scoped beans, you can change the internal state of the instance that is created as much as you want, knowing that other HTTP Session instances that are also using instances created from the same userPreferences bean definition do not see these changes in state, because they are particular to an individual HTTP Session. When the HTTP Session is eventually discarded, the bean that is scoped to that particular HTTP Session is also discarded.

Spring容器在一个HTTP Session 的生命周期中使用 userPreferences bean definition 来创建一个新的 UserPreferences bean。也就是说,userPreferences bean的有效作用域是HTTP Session 级别的。和request作用域中的beans一样,你可以根据需要改变所创建的实例的内部状态,在其他HTTP Session 中根据相同 userpreferences bean definition 所创建实例是看不到该状态改变的。因为这些实例只属于它自己特定那个HTTP Session。 如果这个HTTP Session 被销毁,这个特定作用域下的bean也会被销毁。

When using annotation-driven components or Java configuration, you can use the @SessionScope annotation to assign a component to the session scope.

当使用注解驱动的组件或者Java configuration时,你可以使用 @SessionScope 注解将一个组件的作用域分配为 session

Java
@SessionScope
@Component
public class UserPreferences {
    // ...
}
Kotlin
@SessionScope
@Component
class UserPreferences {
    // ...
}
Application作用域(Application Scope)

Consider the following XML configuration for a bean definition:

请思考下面这个XML配置中的bean definition:

<bean id="appPreferences" class="com.something.AppPreferences" scope="application"/>

The Spring container creates a new instance of the AppPreferences bean by using the appPreferences bean definition once for the entire web application. That is, the appPreferences bean is scoped at the ServletContext level and stored as a regular ServletContext attribute. This is somewhat similar to a Spring singleton bean but differs in two important ways: It is a singleton per ServletContext, not per Spring 'ApplicationContext' (for which there may be several in any given web application), and it is actually exposed and therefore visible as a ServletContext attribute.

在整个web application中通过使用一次 appPreferences bean definition来为Spring容器创建一个新的 AppPreferences bean实例。 也就是说,这个 appPreferences bean 的作用域是 ServletContext 级别的并作为一个常规的 ServletContext 属性来存储。这点和Spring 单例 bean有些相似,,但是有两个重要的方面不同:它之于每个 ServletContext 来说是单例的,但对于Spring ApplicationContext 就不是了(在web application中可能会有多个)。实际上它是被暴露在外的,因此它被作为一个可见的 ServletContext 属性。

When using annotation-driven components or Java configuration, you can use the @ApplicationScope annotation to assign a component to the application scope. The following example shows how to do so:

当使用注解驱动的组件或者Java configuration时,你可以使用 @ApplicationScope 注解将一个组件的作用域分配为 application 。参考下面这个例子:

Java
@ApplicationScope
@Component
public class AppPreferences {
    // ...
}
Kotlin
@ApplicationScope
@Component
class AppPreferences {
    // ...
}
Scoped Beans as Dependencies

The Spring IoC container manages not only the instantiation of your objects (beans), but also the wiring up of collaborators (or dependencies). If you want to inject (for example) an HTTP request-scoped bean into another bean of a longer-lived scope, you may choose to inject an AOP proxy in place of the scoped bean. That is, you need to inject a proxy object that exposes the same public interface as the scoped object but that can also retrieve the real target object from the relevant scope (such as an HTTP request) and delegate method calls onto the real object.

You may also use <aop:scoped-proxy/> between beans that are scoped as singleton, with the reference then going through an intermediate proxy that is serializable and therefore able to re-obtain the target singleton bean on deserialization.

When declaring <aop:scoped-proxy/> against a bean of scope prototype, every method call on the shared proxy leads to the creation of a new target instance to which the call is then being forwarded.

Also, scoped proxies are not the only way to access beans from shorter scopes in a lifecycle-safe fashion. You may also declare your injection point (that is, the constructor or setter argument or autowired field) as ObjectFactory<MyTargetBean>, allowing for a getObject() call to retrieve the current instance on demand every time it is needed — without holding on to the instance or storing it separately.

As an extended variant, you may declare ObjectProvider<MyTargetBean>, which delivers several additional access variants, including getIfAvailable and getIfUnique.

The JSR-330 variant of this is called Provider and is used with a Provider<MyTargetBean> declaration and a corresponding get() call for every retrieval attempt. See here for more details on JSR-330 overall.

The configuration in the following example is only one line, but it is important to understand the “why” as well as the “how” behind it:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/aop
        https://www.springframework.org/schema/aop/spring-aop.xsd">

    <!-- an HTTP Session-scoped bean exposed as a proxy -->
    <bean id="userPreferences" class="com.something.UserPreferences" scope="session">
        <!-- instructs the container to proxy the surrounding bean -->
        <aop:scoped-proxy/> (1)
    </bean>

    <!-- a singleton-scoped bean injected with a proxy to the above bean -->
    <bean id="userService" class="com.something.SimpleUserService">
        <!-- a reference to the proxied userPreferences bean -->
        <property name="userPreferences" ref="userPreferences"/>
    </bean>
</beans>
1 The line that defines the proxy.

To create such a proxy, you insert a child <aop:scoped-proxy/> element into a scoped bean definition (see Choosing the Type of Proxy to Create and XML Schema-based configuration). Why do definitions of beans scoped at the request, session and custom-scope levels require the <aop:scoped-proxy/> element? Consider the following singleton bean definition and contrast it with what you need to define for the aforementioned scopes (note that the following userPreferences bean definition as it stands is incomplete):

<bean id="userPreferences" class="com.something.UserPreferences" scope="session"/>

<bean id="userManager" class="com.something.UserManager">
    <property name="userPreferences" ref="userPreferences"/>
</bean>

In the preceding example, the singleton bean (userManager) is injected with a reference to the HTTP Session-scoped bean (userPreferences). The salient point here is that the userManager bean is a singleton: it is instantiated exactly once per container, and its dependencies (in this case only one, the userPreferences bean) are also injected only once. This means that the userManager bean operates only on the exact same userPreferences object (that is, the one with which it was originally injected.

This is not the behavior you want when injecting a shorter-lived scoped bean into a longer-lived scoped bean (for example, injecting an HTTP Session-scoped collaborating bean as a dependency into singleton bean). Rather, you need a single userManager object, and, for the lifetime of an HTTP Session, you need a userPreferences object that is specific to the HTTP Session. Thus, the container creates an object that exposes the exact same public interface as the UserPreferences class (ideally an object that is a UserPreferences instance), which can fetch the real UserPreferences object from the scoping mechanism (HTTP request, Session, and so forth). The container injects this proxy object into the userManager bean, which is unaware that this UserPreferences reference is a proxy. In this example, when a UserManager instance invokes a method on the dependency-injected UserPreferences object, it is actually invoking a method on the proxy. The proxy then fetches the real UserPreferences object from (in this case) the HTTP Session and delegates the method invocation onto the retrieved real UserPreferences object.

Thus, you need the following (correct and complete) configuration when injecting request- and session-scoped beans into collaborating objects, as the following example shows:

<bean id="userPreferences" class="com.something.UserPreferences" scope="session">
    <aop:scoped-proxy/>
</bean>

<bean id="userManager" class="com.something.UserManager">
    <property name="userPreferences" ref="userPreferences"/>
</bean>
Choosing the Type of Proxy to Create

By default, when the Spring container creates a proxy for a bean that is marked up with the <aop:scoped-proxy/> element, a CGLIB-based class proxy is created.

CGLIB proxies intercept only public method calls! Do not call non-public methods on such a proxy. They are not delegated to the actual scoped target object.

Alternatively, you can configure the Spring container to create standard JDK interface-based proxies for such scoped beans, by specifying false for the value of the proxy-target-class attribute of the <aop:scoped-proxy/> element. Using JDK interface-based proxies means that you do not need additional libraries in your application classpath to affect such proxying. However, it also means that the class of the scoped bean must implement at least one interface and that all collaborators into which the scoped bean is injected must reference the bean through one of its interfaces. The following example shows a proxy based on an interface:

<!-- DefaultUserPreferences implements the UserPreferences interface -->
<bean id="userPreferences" class="com.stuff.DefaultUserPreferences" scope="session">
    <aop:scoped-proxy proxy-target-class="false"/>
</bean>

<bean id="userManager" class="com.stuff.UserManager">
    <property name="userPreferences" ref="userPreferences"/>
</bean>

For more detailed information about choosing class-based or interface-based proxying, see Proxying Mechanisms.

Custom Scopes

The bean scoping mechanism is extensible. You can define your own scopes or even redefine existing scopes, although the latter is considered bad practice and you cannot override the built-in singleton and prototype scopes.

Creating a Custom Scope

To integrate your custom scopes into the Spring container, you need to implement the org.springframework.beans.factory.config.Scope interface, which is described in this section. For an idea of how to implement your own scopes, see the Scope implementations that are supplied with the Spring Framework itself and the Scope javadoc, which explains the methods you need to implement in more detail.

The Scope interface has four methods to get objects from the scope, remove them from the scope, and let them be destroyed.

The session scope implementation, for example, returns the session-scoped bean (if it does not exist, the method returns a new instance of the bean, after having bound it to the session for future reference). The following method returns the object from the underlying scope:

Java
Object get(String name, ObjectFactory<?> objectFactory)
Kotlin
fun get(name: String, objectFactory: ObjectFactory<*>): Any

The session scope implementation, for example, removes the session-scoped bean from the underlying session. The object should be returned, but you can return null if the object with the specified name is not found. The following method removes the object from the underlying scope:

Java
Object remove(String name)
Kotlin
fun remove(name: String): Any

The following method registers the callbacks the scope should execute when it is destroyed or when the specified object in the scope is destroyed:

Java
void registerDestructionCallback(String name, Runnable destructionCallback)
Kotlin
fun registerDestructionCallback(name: String, destructionCallback: Runnable)

See the javadoc or a Spring scope implementation for more information on destruction callbacks.

The following method obtains the conversation identifier for the underlying scope:

Java
String getConversationId()
Kotlin
fun getConversationId(): String

This identifier is different for each scope. For a session scoped implementation, this identifier can be the session identifier.

Using a Custom Scope

After you write and test one or more custom Scope implementations, you need to make the Spring container aware of your new scopes. The following method is the central method to register a new Scope with the Spring container:

Java
void registerScope(String scopeName, Scope scope);
Kotlin
fun registerScope(scopeName: String, scope: Scope)

This method is declared on the ConfigurableBeanFactory interface, which is available through the BeanFactory property on most of the concrete ApplicationContext implementations that ship with Spring.

The first argument to the registerScope(..) method is the unique name associated with a scope. Examples of such names in the Spring container itself are singleton and prototype. The second argument to the registerScope(..) method is an actual instance of the custom Scope implementation that you wish to register and use.

Suppose that you write your custom Scope implementation, and then register it as shown in the next example.

The next example uses SimpleThreadScope, which is included with Spring but is not registered by default. The instructions would be the same for your own custom Scope implementations.
Java
Scope threadScope = new SimpleThreadScope();
beanFactory.registerScope("thread", threadScope);
Kotlin
val threadScope = SimpleThreadScope()
beanFactory.registerScope("thread", threadScope)

You can then create bean definitions that adhere to the scoping rules of your custom Scope, as follows:

<bean id="..." class="..." scope="thread">

With a custom Scope implementation, you are not limited to programmatic registration of the scope. You can also do the Scope registration declaratively, by using the CustomScopeConfigurer class, as the following example shows:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/aop
        https://www.springframework.org/schema/aop/spring-aop.xsd">

    <bean class="org.springframework.beans.factory.config.CustomScopeConfigurer">
        <property name="scopes">
            <map>
                <entry key="thread">
                    <bean class="org.springframework.context.support.SimpleThreadScope"/>
                </entry>
            </map>
        </property>
    </bean>

    <bean id="thing2" class="x.y.Thing2" scope="thread">
        <property name="name" value="Rick"/>
        <aop:scoped-proxy/>
    </bean>

    <bean id="thing1" class="x.y.Thing1">
        <property name="thing2" ref="thing2"/>
    </bean>

</beans>
When you place <aop:scoped-proxy/> in a FactoryBean implementation, it is the factory bean itself that is scoped, not the object returned from getObject().

Customizing the Nature of a Bean

The Spring Framework provides a number of interfaces you can use to customize the nature of a bean. This section groups them as follows:

Lifecycle Callbacks

To interact with the container’s management of the bean lifecycle, you can implement the Spring InitializingBean and DisposableBean interfaces. The container calls afterPropertiesSet() for the former and destroy() for the latter to let the bean perform certain actions upon initialization and destruction of your beans.

The JSR-250 @PostConstruct and @PreDestroy annotations are generally considered best practice for receiving lifecycle callbacks in a modern Spring application. Using these annotations means that your beans are not coupled to Spring-specific interfaces. For details, see Using @PostConstruct and @PreDestroy.

If you do not want to use the JSR-250 annotations but you still want to remove coupling, consider init-method and destroy-method bean definition metadata.

Internally, the Spring Framework uses BeanPostProcessor implementations to process any callback interfaces it can find and call the appropriate methods. If you need custom features or other lifecycle behavior Spring does not by default offer, you can implement a BeanPostProcessor yourself. For more information, see Container Extension Points.

In addition to the initialization and destruction callbacks, Spring-managed objects may also implement the Lifecycle interface so that those objects can participate in the startup and shutdown process, as driven by the container’s own lifecycle.

The lifecycle callback interfaces are described in this section.

Initialization Callbacks

The org.springframework.beans.factory.InitializingBean interface lets a bean perform initialization work after the container has set all necessary properties on the bean. The InitializingBean interface specifies a single method:

Java
void afterPropertiesSet() throws Exception;
Kotlin
fun afterPropertiesSet()

We recommend that you do not use the InitializingBean interface, because it unnecessarily couples the code to Spring. Alternatively, we suggest using the @PostConstruct annotation or specifying a POJO initialization method. In the case of XML-based configuration metadata, you can use the init-method attribute to specify the name of the method that has a void no-argument signature. With Java configuration, you can use the initMethod attribute of @Bean. See Receiving Lifecycle Callbacks. Consider the following example:

<bean id="exampleInitBean" class="examples.ExampleBean" init-method="init"/>
Java
public class ExampleBean {

    public void init() {
        // do some initialization work
    }
}
Kotlin
class ExampleBean {

    fun init() {
        // do some initialization work
    }
}

The preceding example has almost exactly the same effect as the following example (which consists of two listings):

<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
Java
public class AnotherExampleBean implements InitializingBean {

    @Override
    public void afterPropertiesSet() {
        // do some initialization work
    }
}
Kotlin
class AnotherExampleBean : InitializingBean {

    override fun afterPropertiesSet() {
        // do some initialization work
    }
}

However, the first of the two preceding examples does not couple the code to Spring.

Destruction Callbacks

Implementing the org.springframework.beans.factory.DisposableBean interface lets a bean get a callback when the container that contains it is destroyed. The DisposableBean interface specifies a single method:

Java
void destroy() throws Exception;
Kotlin
fun destroy()

We recommend that you do not use the DisposableBean callback interface, because it unnecessarily couples the code to Spring. Alternatively, we suggest using the @PreDestroy annotation or specifying a generic method that is supported by bean definitions. With XML-based configuration metadata, you can use the destroy-method attribute on the <bean/>. With Java configuration, you can use the destroyMethod attribute of @Bean. See Receiving Lifecycle Callbacks. Consider the following definition:

<bean id="exampleInitBean" class="examples.ExampleBean" destroy-method="cleanup"/>
Java
public class ExampleBean {

    public void cleanup() {
        // do some destruction work (like releasing pooled connections)
    }
}
Kotlin
class ExampleBean {

    fun cleanup() {
        // do some destruction work (like releasing pooled connections)
    }
}

The preceding definition has almost exactly the same effect as the following definition:

<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
Java
public class AnotherExampleBean implements DisposableBean {

    @Override
    public void destroy() {
        // do some destruction work (like releasing pooled connections)
    }
}
Kotlin
class AnotherExampleBean : DisposableBean {

    override fun destroy() {
        // do some destruction work (like releasing pooled connections)
    }
}

However, the first of the two preceding definitions does not couple the code to Spring.

You can assign the destroy-method attribute of a <bean> element a special (inferred) value, which instructs Spring to automatically detect a public close or shutdown method on the specific bean class. (Any class that implements java.lang.AutoCloseable or java.io.Closeable would therefore match.) You can also set this special (inferred) value on the default-destroy-method attribute of a <beans> element to apply this behavior to an entire set of beans (see Default Initialization and Destroy Methods). Note that this is the default behavior with Java configuration.
Default Initialization and Destroy Methods

When you write initialization and destroy method callbacks that do not use the Spring-specific InitializingBean and DisposableBean callback interfaces, you typically write methods with names such as init(), initialize(), dispose(), and so on. Ideally, the names of such lifecycle callback methods are standardized across a project so that all developers use the same method names and ensure consistency.

You can configure the Spring container to “look” for named initialization and destroy callback method names on every bean. This means that you, as an application developer, can write your application classes and use an initialization callback called init(), without having to configure an init-method="init" attribute with each bean definition. The Spring IoC container calls that method when the bean is created (and in accordance with the standard lifecycle callback contract described previously). This feature also enforces a consistent naming convention for initialization and destroy method callbacks.

Suppose that your initialization callback methods are named init() and your destroy callback methods are named destroy(). Your class then resembles the class in the following example:

Java
public class DefaultBlogService implements BlogService {

    private BlogDao blogDao;

    public void setBlogDao(BlogDao blogDao) {
        this.blogDao = blogDao;
    }

    // this is (unsurprisingly) the initialization callback method
    public void init() {
        if (this.blogDao == null) {
            throw new IllegalStateException("The [blogDao] property must be set.");
        }
    }
}
Kotlin
class DefaultBlogService : BlogService {

    private var blogDao: BlogDao? = null

    // this is (unsurprisingly) the initialization callback method
    fun init() {
        if (blogDao == null) {
            throw IllegalStateException("The [blogDao] property must be set.")
        }
    }
}

You could then use that class in a bean resembling the following:

<beans default-init-method="init">

    <bean id="blogService" class="com.something.DefaultBlogService">
        <property name="blogDao" ref="blogDao" />
    </bean>

</beans>

The presence of the default-init-method attribute on the top-level <beans/> element attribute causes the Spring IoC container to recognize a method called init on the bean class as the initialization method callback. When a bean is created and assembled, if the bean class has such a method, it is invoked at the appropriate time.

You can configure destroy method callbacks similarly (in XML, that is) by using the default-destroy-method attribute on the top-level <beans/> element.

Where existing bean classes already have callback methods that are named at variance with the convention, you can override the default by specifying (in XML, that is) the method name by using the init-method and destroy-method attributes of the <bean/> itself.

The Spring container guarantees that a configured initialization callback is called immediately after a bean is supplied with all dependencies. Thus, the initialization callback is called on the raw bean reference, which means that AOP interceptors and so forth are not yet applied to the bean. A target bean is fully created first and then an AOP proxy (for example) with its interceptor chain is applied. If the target bean and the proxy are defined separately, your code can even interact with the raw target bean, bypassing the proxy. Hence, it would be inconsistent to apply the interceptors to the init method, because doing so would couple the lifecycle of the target bean to its proxy or interceptors and leave strange semantics when your code interacts directly with the raw target bean.

Combining Lifecycle Mechanisms

As of Spring 2.5, you have three options for controlling bean lifecycle behavior:

If multiple lifecycle mechanisms are configured for a bean and each mechanism is configured with a different method name, then each configured method is executed in the order listed after this note. However, if the same method name is configured — for example, init() for an initialization method — for more than one of these lifecycle mechanisms, that method is executed once, as explained in the preceding section.

Multiple lifecycle mechanisms configured for the same bean, with different initialization methods, are called as follows:

  1. Methods annotated with @PostConstruct

  2. afterPropertiesSet() as defined by the InitializingBean callback interface

  3. A custom configured init() method

Destroy methods are called in the same order:

  1. Methods annotated with @PreDestroy

  2. destroy() as defined by the DisposableBean callback interface

  3. A custom configured destroy() method

Startup and Shutdown Callbacks

The Lifecycle interface defines the essential methods for any object that has its own lifecycle requirements (such as starting and stopping some background process):

Java
public interface Lifecycle {

    void start();

    void stop();

    boolean isRunning();
}
Kotlin
interface Lifecycle {

    fun start()

    fun stop()

    val isRunning: Boolean
}

Any Spring-managed object may implement the Lifecycle interface. Then, when the ApplicationContext itself receives start and stop signals (for example, for a stop/restart scenario at runtime), it cascades those calls to all Lifecycle implementations defined within that context. It does this by delegating to a LifecycleProcessor, shown in the following listing:

Java
public interface LifecycleProcessor extends Lifecycle {

    void onRefresh();

    void onClose();
}
Kotlin
interface LifecycleProcessor : Lifecycle {

    fun onRefresh()

    fun onClose()
}

Notice that the LifecycleProcessor is itself an extension of the Lifecycle interface. It also adds two other methods for reacting to the context being refreshed and closed.

Note that the regular org.springframework.context.Lifecycle interface is a plain contract for explicit start and stop notifications and does not imply auto-startup at context refresh time. For fine-grained control over auto-startup of a specific bean (including startup phases), consider implementing org.springframework.context.SmartLifecycle instead.

Also, please note that stop notifications are not guaranteed to come before destruction. On regular shutdown, all Lifecycle beans first receive a stop notification before the general destruction callbacks are being propagated. However, on hot refresh during a context’s lifetime or on aborted refresh attempts, only destroy methods are called.

The order of startup and shutdown invocations can be important. If a “depends-on” relationship exists between any two objects, the dependent side starts after its dependency, and it stops before its dependency. However, at times, the direct dependencies are unknown. You may only know that objects of a certain type should start prior to objects of another type. In those cases, the SmartLifecycle interface defines another option, namely the getPhase() method as defined on its super-interface, Phased. The following listing shows the definition of the Phased interface:

Java
public interface Phased {

    int getPhase();
}
Kotlin
interface Phased {

    val phase: Int
}

The following listing shows the definition of the SmartLifecycle interface:

Java
public interface SmartLifecycle extends Lifecycle, Phased {

    boolean isAutoStartup();

    void stop(Runnable callback);
}
Kotlin
interface SmartLifecycle : Lifecycle, Phased {

    val isAutoStartup: Boolean

    fun stop(callback: Runnable)
}

When starting, the objects with the lowest phase start first. When stopping, the reverse order is followed. Therefore, an object that implements SmartLifecycle and whose getPhase() method returns Integer.MIN_VALUE would be among the first to start and the last to stop. At the other end of the spectrum, a phase value of Integer.MAX_VALUE would indicate that the object should be started last and stopped first (likely because it depends on other processes to be running). When considering the phase value, it is also important to know that the default phase for any “normal” Lifecycle object that does not implement SmartLifecycle is 0. Therefore, any negative phase value indicates that an object should start before those standard components (and stop after them). The reverse is true for any positive phase value.

The stop method defined by SmartLifecycle accepts a callback. Any implementation must invoke that callback’s run() method after that implementation’s shutdown process is complete. That enables asynchronous shutdown where necessary, since the default implementation of the LifecycleProcessor interface, DefaultLifecycleProcessor, waits up to its timeout value for the group of objects within each phase to invoke that callback. The default per-phase timeout is 30 seconds. You can override the default lifecycle processor instance by defining a bean named lifecycleProcessor within the context. If you want only to modify the timeout, defining the following would suffice:

<bean id="lifecycleProcessor" class="org.springframework.context.support.DefaultLifecycleProcessor">
    <!-- timeout value in milliseconds -->
    <property name="timeoutPerShutdownPhase" value="10000"/>
</bean>

As mentioned earlier, the LifecycleProcessor interface defines callback methods for the refreshing and closing of the context as well. The latter drives the shutdown process as if stop() had been called explicitly, but it happens when the context is closing. The 'refresh' callback, on the other hand, enables another feature of SmartLifecycle beans. When the context is refreshed (after all objects have been instantiated and initialized), that callback is invoked. At that point, the default lifecycle processor checks the boolean value returned by each SmartLifecycle object’s isAutoStartup() method. If true, that object is started at that point rather than waiting for an explicit invocation of the context’s or its own start() method (unlike the context refresh, the context start does not happen automatically for a standard context implementation). The phase value and any “depends-on” relationships determine the startup order as described earlier.

Shutting Down the Spring IoC Container Gracefully in Non-Web Applications

This section applies only to non-web applications. Spring’s web-based ApplicationContext implementations already have code in place to gracefully shut down the Spring IoC container when the relevant web application is shut down.

If you use Spring’s IoC container in a non-web application environment (for example, in a rich client desktop environment), register a shutdown hook with the JVM. Doing so ensures a graceful shutdown and calls the relevant destroy methods on your singleton beans so that all resources are released. You must still configure and implement these destroy callbacks correctly.

To register a shutdown hook, call the registerShutdownHook() method that is declared on the ConfigurableApplicationContext interface, as the following example shows:

Java
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public final class Boot {

    public static void main(final String[] args) throws Exception {
        ConfigurableApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");

        // add a shutdown hook for the above context...
        ctx.registerShutdownHook();

        // app runs here...

        // main method exits, hook is called prior to the app shutting down...
    }
}
Kotlin
import org.springframework.context.support.ClassPathXmlApplicationContext

fun main() {
    val ctx = ClassPathXmlApplicationContext("beans.xml")

    // add a shutdown hook for the above context...
    ctx.registerShutdownHook()

    // app runs here...

    // main method exits, hook is called prior to the app shutting down...
}

ApplicationContextAware and BeanNameAware

When an ApplicationContext creates an object instance that implements the org.springframework.context.ApplicationContextAware interface, the instance is provided with a reference to that ApplicationContext. The following listing shows the definition of the ApplicationContextAware interface:

Java
public interface ApplicationContextAware {

    void setApplicationContext(ApplicationContext applicationContext) throws BeansException;
}
Kotlin
interface ApplicationContextAware {

    @Throws(BeansException::class)
    fun setApplicationContext(applicationContext: ApplicationContext)
}

Thus, beans can programmatically manipulate the ApplicationContext that created them, through the ApplicationContext interface or by casting the reference to a known subclass of this interface (such as ConfigurableApplicationContext, which exposes additional functionality). One use would be the programmatic retrieval of other beans. Sometimes this capability is useful. However, in general, you should avoid it, because it couples the code to Spring and does not follow the Inversion of Control style, where collaborators are provided to beans as properties. Other methods of the ApplicationContext provide access to file resources, publishing application events, and accessing a MessageSource. These additional features are described in Additional Capabilities of the ApplicationContext.

Autowiring is another alternative to obtain a reference to the ApplicationContext. The traditional constructor and byType autowiring modes (as described in 自动装配协作者(对bean的相关依赖进行自动注入)(Autowiring Collaborators)) can provide a dependency of type ApplicationContext for a constructor argument or a setter method parameter, respectively. For more flexibility, including the ability to autowire fields and multiple parameter methods, use the annotation-based autowiring features. If you do, the ApplicationContext is autowired into a field, constructor argument, or method parameter that expects the ApplicationContext type if the field, constructor, or method in question carries the @Autowired annotation. For more information, see Using @Autowired.

When an ApplicationContext creates a class that implements the org.springframework.beans.factory.BeanNameAware interface, the class is provided with a reference to the name defined in its associated object definition. The following listing shows the definition of the BeanNameAware interface:

Java
public interface BeanNameAware {

    void setBeanName(String name) throws BeansException;
}
Kotlin
interface BeanNameAware {

    @Throws(BeansException::class)
    fun setBeanName(name: String)
}

The callback is invoked after population of normal bean properties but before an initialization callback such as InitializingBean, afterPropertiesSet, or a custom init-method.

Other Aware Interfaces

Besides ApplicationContextAware and BeanNameAware (discussed earlier), Spring offers a wide range of Aware callback interfaces that let beans indicate to the container that they require a certain infrastructure dependency. As a general rule, the name indicates the dependency type. The following table summarizes the most important Aware interfaces:

Table 6. Aware interfaces
Name Injected Dependency Explained in…​

ApplicationContextAware

Declaring ApplicationContext.

ApplicationContextAware and BeanNameAware

ApplicationEventPublisherAware

Event publisher of the enclosing ApplicationContext.

Additional Capabilities of the ApplicationContext

BeanClassLoaderAware

Class loader used to load the bean classes.

初始化Beans(Instantiating Beans)

BeanFactoryAware

Declaring BeanFactory.

ApplicationContextAware and BeanNameAware

BeanNameAware

Name of the declaring bean.

ApplicationContextAware and BeanNameAware

BootstrapContextAware

Resource adapter BootstrapContext the container runs in. Typically available only in JCA-aware ApplicationContext instances.

JCA CCI

LoadTimeWeaverAware

Defined weaver for processing class definition at load time.

Load-time Weaving with AspectJ in the Spring Framework

MessageSourceAware

Configured strategy for resolving messages (with support for parametrization and internationalization).

Additional Capabilities of the ApplicationContext

NotificationPublisherAware

Spring JMX notification publisher.

Notifications

ResourceLoaderAware

Configured loader for low-level access to resources.

Resources

ServletConfigAware

Current ServletConfig the container runs in. Valid only in a web-aware Spring ApplicationContext.

Spring MVC

ServletContextAware

Current ServletContext the container runs in. Valid only in a web-aware Spring ApplicationContext.

Spring MVC

Note again that using these interfaces ties your code to the Spring API and does not follow the Inversion of Control style. As a result, we recommend them for infrastructure beans that require programmatic access to the container.

Bean Definition Inheritance

A bean definition can contain a lot of configuration information, including constructor arguments, property values, and container-specific information, such as the initialization method, a static factory method name, and so on. A child bean definition inherits configuration data from a parent definition. The child definition can override some values or add others as needed. Using parent and child bean definitions can save a lot of typing. Effectively, this is a form of templating.

If you work with an ApplicationContext interface programmatically, child bean definitions are represented by the ChildBeanDefinition class. Most users do not work with them on this level. Instead, they configure bean definitions declaratively in a class such as the ClassPathXmlApplicationContext. When you use XML-based configuration metadata, you can indicate a child bean definition by using the parent attribute, specifying the parent bean as the value of this attribute. The following example shows how to do so:

<bean id="inheritedTestBean" abstract="true"
        class="org.springframework.beans.TestBean">
    <property name="name" value="parent"/>
    <property name="age" value="1"/>
</bean>

<bean id="inheritsWithDifferentClass"
        class="org.springframework.beans.DerivedTestBean"
        parent="inheritedTestBean" init-method="initialize">  (1)
    <property name="name" value="override"/>
    <!-- the age property value of 1 will be inherited from parent -->
</bean>
1 Note the parent attribute.

A child bean definition uses the bean class from the parent definition if none is specified but can also override it. In the latter case, the child bean class must be compatible with the parent (that is, it must accept the parent’s property values).

A child bean definition inherits scope, constructor argument values, property values, and method overrides from the parent, with the option to add new values. Any scope, initialization method, destroy method, or static factory method settings that you specify override the corresponding parent settings.

The remaining settings are always taken from the child definition: depends on, autowire mode, dependency check, singleton, and lazy init.

The preceding example explicitly marks the parent bean definition as abstract by using the abstract attribute. If the parent definition does not specify a class, explicitly marking the parent bean definition as abstract is required, as the following example shows:

<bean id="inheritedTestBeanWithoutClass" abstract="true">
    <property name="name" value="parent"/>
    <property name="age" value="1"/>
</bean>

<bean id="inheritsWithClass" class="org.springframework.beans.DerivedTestBean"
        parent="inheritedTestBeanWithoutClass" init-method="initialize">
    <property name="name" value="override"/>
    <!-- age will inherit the value of 1 from the parent bean definition-->
</bean>

The parent bean cannot be instantiated on its own because it is incomplete, and it is also explicitly marked as abstract. When a definition is abstract, it is usable only as a pure template bean definition that serves as a parent definition for child definitions. Trying to use such an abstract parent bean on its own, by referring to it as a ref property of another bean or doing an explicit getBean() call with the parent bean ID returns an error. Similarly, the container’s internal preInstantiateSingletons() method ignores bean definitions that are defined as abstract.

ApplicationContext pre-instantiates all singletons by default. Therefore, it is important (at least for singleton beans) that if you have a (parent) bean definition which you intend to use only as a template, and this definition specifies a class, you must make sure to set the abstract attribute to true, otherwise the application context will actually (attempt to) pre-instantiate the abstract bean.

Container Extension Points

Typically, an application developer does not need to subclass ApplicationContext implementation classes. Instead, the Spring IoC container can be extended by plugging in implementations of special integration interfaces. The next few sections describe these integration interfaces.

Customizing Beans by Using a BeanPostProcessor

The BeanPostProcessor interface defines callback methods that you can implement to provide your own (or override the container’s default) instantiation logic, dependency resolution logic, and so forth. If you want to implement some custom logic after the Spring container finishes instantiating, configuring, and initializing a bean, you can plug in one or more custom BeanPostProcessor implementations.

You can configure multiple BeanPostProcessor instances, and you can control the order in which these BeanPostProcessor instances execute by setting the order property. You can set this property only if the BeanPostProcessor implements the Ordered interface. If you write your own BeanPostProcessor, you should consider implementing the Ordered interface, too. For further details, see the javadoc of the BeanPostProcessor and Ordered interfaces. See also the note on programmatic registration of BeanPostProcessor instances.

BeanPostProcessor instances operate on bean (or object) instances. That is, the Spring IoC container instantiates a bean instance and then BeanPostProcessor instances do their work.

BeanPostProcessor instances are scoped per-container. This is relevant only if you use container hierarchies. If you define a BeanPostProcessor in one container, it post-processes only the beans in that container. In other words, beans that are defined in one container are not post-processed by a BeanPostProcessor defined in another container, even if both containers are part of the same hierarchy.

To change the actual bean definition (that is, the blueprint that defines the bean), you instead need to use a BeanFactoryPostProcessor, as described in Customizing Configuration Metadata with a BeanFactoryPostProcessor.

The org.springframework.beans.factory.config.BeanPostProcessor interface consists of exactly two callback methods. When such a class is registered as a post-processor with the container, for each bean instance that is created by the container, the post-processor gets a callback from the container both before container initialization methods (such as InitializingBean.afterPropertiesSet() or any declared init method) are called, and after any bean initialization callbacks. The post-processor can take any action with the bean instance, including ignoring the callback completely. A bean post-processor typically checks for callback interfaces, or it may wrap a bean with a proxy. Some Spring AOP infrastructure classes are implemented as bean post-processors in order to provide proxy-wrapping logic.

An ApplicationContext automatically detects any beans that are defined in the configuration metadata that implements the BeanPostProcessor interface. The ApplicationContext registers these beans as post-processors so that they can be called later, upon bean creation. Bean post-processors can be deployed in the container in the same fashion as any other beans.

Note that, when declaring a BeanPostProcessor by using an @Bean factory method on a configuration class, the return type of the factory method should be the implementation class itself or at least the org.springframework.beans.factory.config.BeanPostProcessor interface, clearly indicating the post-processor nature of that bean. Otherwise, the ApplicationContext cannot autodetect it by type before fully creating it. Since a BeanPostProcessor needs to be instantiated early in order to apply to the initialization of other beans in the context, this early type detection is critical.

Programmatically registering BeanPostProcessor instances
While the recommended approach for BeanPostProcessor registration is through ApplicationContext auto-detection (as described earlier), you can register them programmatically against a ConfigurableBeanFactory by using the addBeanPostProcessor method. This can be useful when you need to evaluate conditional logic before registration or even for copying bean post processors across contexts in a hierarchy. Note, however, that BeanPostProcessor instances added programmatically do not respect the Ordered interface. Here, it is the order of registration that dictates the order of execution. Note also that BeanPostProcessor instances registered programmatically are always processed before those registered through auto-detection, regardless of any explicit ordering.
BeanPostProcessor instances and AOP auto-proxying

Classes that implement the BeanPostProcessor interface are special and are treated differently by the container. All BeanPostProcessor instances and beans that they directly reference are instantiated on startup, as part of the special startup phase of the ApplicationContext. Next, all BeanPostProcessor instances are registered in a sorted fashion and applied to all further beans in the container. Because AOP auto-proxying is implemented as a BeanPostProcessor itself, neither BeanPostProcessor instances nor the beans they directly reference are eligible for auto-proxying and, thus, do not have aspects woven into them.

For any such bean, you should see an informational log message: Bean someBean is not eligible for getting processed by all BeanPostProcessor interfaces (for example: not eligible for auto-proxying).

If you have beans wired into your BeanPostProcessor by using autowiring or @Resource (which may fall back to autowiring), Spring might access unexpected beans when searching for type-matching dependency candidates and, therefore, make them ineligible for auto-proxying or other kinds of bean post-processing. For example, if you have a dependency annotated with @Resource where the field or setter name does not directly correspond to the declared name of a bean and no name attribute is used, Spring accesses other beans for matching them by type.

The following examples show how to write, register, and use BeanPostProcessor instances in an ApplicationContext.

Example: Hello World, BeanPostProcessor-style

This first example illustrates basic usage. The example shows a custom BeanPostProcessor implementation that invokes the toString() method of each bean as it is created by the container and prints the resulting string to the system console.

The following listing shows the custom BeanPostProcessor implementation class definition:

Java
package scripting;

import org.springframework.beans.factory.config.BeanPostProcessor;

public class InstantiationTracingBeanPostProcessor implements BeanPostProcessor {

    // simply return the instantiated bean as-is
    public Object postProcessBeforeInitialization(Object bean, String beanName) {
        return bean; // we could potentially return any object reference here...
    }

    public Object postProcessAfterInitialization(Object bean, String beanName) {
        System.out.println("Bean '" + beanName + "' created : " + bean.toString());
        return bean;
    }
}
Kotlin
import org.springframework.beans.factory.config.BeanPostProcessor

class InstantiationTracingBeanPostProcessor : BeanPostProcessor {

    // simply return the instantiated bean as-is
    override fun postProcessBeforeInitialization(bean: Any, beanName: String): Any? {
        return bean // we could potentially return any object reference here...
    }

    override fun postProcessAfterInitialization(bean: Any, beanName: String): Any? {
        println("Bean '$beanName' created : $bean")
        return bean
    }
}

The following beans element uses the InstantiationTracingBeanPostProcessor:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:lang="http://www.springframework.org/schema/lang"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/lang
        https://www.springframework.org/schema/lang/spring-lang.xsd">

    <lang:groovy id="messenger"
            script-source="classpath:org/springframework/scripting/groovy/Messenger.groovy">
        <lang:property name="message" value="Fiona Apple Is Just So Dreamy."/>
    </lang:groovy>

    <!--
    when the above bean (messenger) is instantiated, this custom
    BeanPostProcessor implementation will output the fact to the system console
    -->
    <bean class="scripting.InstantiationTracingBeanPostProcessor"/>

</beans>

Notice how the InstantiationTracingBeanPostProcessor is merely defined. It does not even have a name, and, because it is a bean, it can be dependency-injected as you would any other bean. (The preceding configuration also defines a bean that is backed by a Groovy script. The Spring dynamic language support is detailed in the chapter entitled Dynamic Language Support.)

The following Java application runs the preceding code and configuration:

Java
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.scripting.Messenger;

public final class Boot {

    public static void main(final String[] args) throws Exception {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("scripting/beans.xml");
        Messenger messenger = ctx.getBean("messenger", Messenger.class);
        System.out.println(messenger);
    }

}
Kotlin
import org.springframework.beans.factory.getBean

fun main() {
    val ctx = ClassPathXmlApplicationContext("scripting/beans.xml")
    val messenger = ctx.getBean<Messenger>("messenger")
    println(messenger)
}

The output of the preceding application resembles the following:

Bean 'messenger' created : [email protected]
[email protected]
Example: The RequiredAnnotationBeanPostProcessor

Using callback interfaces or annotations in conjunction with a custom BeanPostProcessor implementation is a common means of extending the Spring IoC container. An example is Spring’s RequiredAnnotationBeanPostProcessor — a BeanPostProcessor implementation that ships with the Spring distribution and that ensures that JavaBean properties on beans that are marked with an (arbitrary) annotation are actually (configured to be) dependency-injected with a value.

Customizing Configuration Metadata with a BeanFactoryPostProcessor

The next extension point that we look at is the org.springframework.beans.factory.config.BeanFactoryPostProcessor. The semantics of this interface are similar to those of the BeanPostProcessor, with one major difference: BeanFactoryPostProcessor operates on the bean configuration metadata. That is, the Spring IoC container lets a BeanFactoryPostProcessor read the configuration metadata and potentially change it before the container instantiates any beans other than BeanFactoryPostProcessor instances.

You can configure multiple BeanFactoryPostProcessor instances, and you can control the order in which these BeanFactoryPostProcessor instances run by setting the order property. However, you can only set this property if the BeanFactoryPostProcessor implements the Ordered interface. If you write your own BeanFactoryPostProcessor, you should consider implementing the Ordered interface, too. See the javadoc of the BeanFactoryPostProcessor and Ordered interfaces for more details.

If you want to change the actual bean instances (that is, the objects that are created from the configuration metadata), then you instead need to use a BeanPostProcessor (described earlier in Customizing Beans by Using a BeanPostProcessor). While it is technically possible to work with bean instances within a BeanFactoryPostProcessor (for example, by using BeanFactory.getBean()), doing so causes premature bean instantiation, violating the standard container lifecycle. This may cause negative side effects, such as bypassing bean post processing.

Also, BeanFactoryPostProcessor instances are scoped per-container. This is only relevant if you use container hierarchies. If you define a BeanFactoryPostProcessor in one container, it is applied only to the bean definitions in that container. Bean definitions in one container are not post-processed by BeanFactoryPostProcessor instances in another container, even if both containers are part of the same hierarchy.

A bean factory post-processor is automatically executed when it is declared inside an ApplicationContext, in order to apply changes to the configuration metadata that define the container. Spring includes a number of predefined bean factory post-processors, such as PropertyOverrideConfigurer and PropertySourcesPlaceholderConfigurer. You can also use a custom BeanFactoryPostProcessor — for example, to register custom property editors.

An ApplicationContext automatically detects any beans that are deployed into it that implement the BeanFactoryPostProcessor interface. It uses these beans as bean factory post-processors, at the appropriate time. You can deploy these post-processor beans as you would any other bean.

As with BeanPostProcessors , you typically do not want to configure BeanFactoryPostProcessors for lazy initialization. If no other bean references a Bean(Factory)PostProcessor, that post-processor will not get instantiated at all. Thus, marking it for lazy initialization will be ignored, and the Bean(Factory)PostProcessor will be instantiated eagerly even if you set the default-lazy-init attribute to true on the declaration of your <beans /> element.
Example: The Class Name Substitution PropertySourcesPlaceholderConfigurer

You can use the PropertySourcesPlaceholderConfigurer to externalize property values from a bean definition in a separate file by using the standard Java Properties format. Doing so enables the person deploying an application to customize environment-specific properties, such as database URLs and passwords, without the complexity or risk of modifying the main XML definition file or files for the container.

Consider the following XML-based configuration metadata fragment, where a DataSource with placeholder values is defined:

<bean class="org.springframework.context.support.PropertySourcesPlaceholderConfigurer">
    <property name="locations" value="classpath:com/something/jdbc.properties"/>
</bean>

<bean id="dataSource" destroy-method="close"
        class="org.apache.commons.dbcp.BasicDataSource">
    <property name="driverClassName" value="${jdbc.driverClassName}"/>
    <property name="url" value="${jdbc.url}"/>
    <property name="username" value="${jdbc.username}"/>
    <property name="password" value="${jdbc.password}"/>
</bean>

The example shows properties configured from an external Properties file. At runtime, a PropertySourcesPlaceholderConfigurer is applied to the metadata that replaces some properties of the DataSource. The values to replace are specified as placeholders of the form ${property-name}, which follows the Ant and log4j and JSP EL style.

The actual values come from another file in the standard Java Properties format:

jdbc.driverClassName=org.hsqldb.jdbcDriver
jdbc.url=jdbc:hsqldb:hsql://production:9002
jdbc.username=sa
jdbc.password=root

Therefore, the ${jdbc.username} string is replaced at runtime with the value, 'sa', and the same applies for other placeholder values that match keys in the properties file. The PropertySourcesPlaceholderConfigurer checks for placeholders in most properties and attributes of a bean definition. Furthermore, you can customize the placeholder prefix and suffix.

With the context namespace introduced in Spring 2.5, you can configure property placeholders with a dedicated configuration element. You can provide one or more locations as a comma-separated list in the location attribute, as the following example shows:

<context:property-placeholder location="classpath:com/something/jdbc.properties"/>

The PropertySourcesPlaceholderConfigurer not only looks for properties in the Properties file you specify. By default, if it cannot find a property in the specified properties files, it checks against Spring Environment properties and regular Java System properties.

You can use the PropertySourcesPlaceholderConfigurer to substitute class names, which is sometimes useful when you have to pick a particular implementation class at runtime. The following example shows how to do so:

<bean class="org.springframework.beans.factory.config.PropertySourcesPlaceholderConfigurer">
    <property name="locations">
        <value>classpath:com/something/strategy.properties</value>
    </property>
    <property name="properties">
        <value>custom.strategy.class=com.something.DefaultStrategy</value>
    </property>
</bean>

<bean id="serviceStrategy" class="${custom.strategy.class}"/>

If the class cannot be resolved at runtime to a valid class, resolution of the bean fails when it is about to be created, which is during the preInstantiateSingletons() phase of an ApplicationContext for a non-lazy-init bean.

Example: The PropertyOverrideConfigurer

The PropertyOverrideConfigurer, another bean factory post-processor, resembles the PropertySourcesPlaceholderConfigurer, but unlike the latter, the original definitions can have default values or no values at all for bean properties. If an overriding Properties file does not have an entry for a certain bean property, the default context definition is used.

Note that the bean definition is not aware of being overridden, so it is not immediately obvious from the XML definition file that the override configurer is being used. In case of multiple PropertyOverrideConfigurer instances that define different values for the same bean property, the last one wins, due to the overriding mechanism.

Properties file configuration lines take the following format:

beanName.property=value

The following listing shows an example of the format:

dataSource.driverClassName=com.mysql.jdbc.Driver
dataSource.url=jdbc:mysql:mydb

This example file can be used with a container definition that contains a bean called dataSource that has driver and url properties.

Compound property names are also supported, as long as every component of the path except the final property being overridden is already non-null (presumably initialized by the constructors). In the following example, the sammy property of the bob property of the fred property of the tom bean is set to the scalar value 123:

tom.fred.bob.sammy=123
Specified override values are always literal values. They are not translated into bean references. This convention also applies when the original value in the XML bean definition specifies a bean reference.

With the context namespace introduced in Spring 2.5, it is possible to configure property overriding with a dedicated configuration element, as the following example shows:

<context:property-override location="classpath:override.properties"/>

Customizing Instantiation Logic with a FactoryBean

You can implement the org.springframework.beans.factory.FactoryBean interface for objects that are themselves factories.

The FactoryBean interface is a point of pluggability into the Spring IoC container’s instantiation logic. If you have complex initialization code that is better expressed in Java as opposed to a (potentially) verbose amount of XML, you can create your own FactoryBean, write the complex initialization inside that class, and then plug your custom FactoryBean into the container.

The FactoryBean interface provides three methods:

  • Object getObject(): Returns an instance of the object this factory creates. The instance can possibly be shared, depending on whether this factory returns singletons or prototypes.

  • boolean isSingleton(): Returns true if this FactoryBean returns singletons or false otherwise.

  • Class getObjectType(): Returns the object type returned by the getObject() method or null if the type is not known in advance.

The FactoryBean concept and interface is used in a number of places within the Spring Framework. More than 50 implementations of the FactoryBean interface ship with Spring itself.

When you need to ask a container for an actual FactoryBean instance itself instead of the bean it produces, preface the bean’s id with the ampersand symbol (&) when calling the getBean() method of the ApplicationContext. So, for a given FactoryBean with an id of myBean, invoking getBean("myBean") on the container returns the product of the FactoryBean, whereas invoking getBean("&myBean") returns the FactoryBean instance itself.

Annotation-based Container Configuration

Are annotations better than XML for configuring Spring?

The introduction of annotation-based configuration raised the question of whether this approach is “better” than XML. The short answer is “it depends.” The long answer is that each approach has its pros and cons, and, usually, it is up to the developer to decide which strategy suits them better. Due to the way they are defined, annotations provide a lot of context in their declaration, leading to shorter and more concise configuration. However, XML excels at wiring up components without touching their source code or recompiling them. Some developers prefer having the wiring close to the source while others argue that annotated classes are no longer POJOs and, furthermore, that the configuration becomes decentralized and harder to control.

No matter the choice, Spring can accommodate both styles and even mix them together. It is worth pointing out that through its JavaConfig option, Spring lets annotations be used in a non-invasive way, without touching the target components source code and that, in terms of tooling, all configuration styles are supported by the Spring Tool Suite.

An alternative to XML setup is provided by annotation-based configuration, which relies on the bytecode metadata for wiring up components instead of angle-bracket declarations. Instead of using XML to describe a bean wiring, the developer moves the configuration into the component class itself by using annotations on the relevant class, method, or field declaration. As mentioned in Example: The RequiredAnnotationBeanPostProcessor, using a BeanPostProcessor in conjunction with annotations is a common means of extending the Spring IoC container. For example, Spring 2.0 introduced the possibility of enforcing required properties with the @Required annotation. Spring 2.5 made it possible to follow that same general approach to drive Spring’s dependency injection. Essentially, the @Autowired annotation provides the same capabilities as described in 自动装配协作者(对bean的相关依赖进行自动注入)(Autowiring Collaborators) but with more fine-grained control and wider applicability. Spring 2.5 also added support for JSR-250 annotations, such as @PostConstruct and @PreDestroy. Spring 3.0 added support for JSR-330 (Dependency Injection for Java) annotations contained in the javax.inject package such as @Inject and @Named. Details about those annotations can be found in the relevant section.

Annotation injection is performed before XML injection. Thus, the XML configuration overrides the annotations for properties wired through both approaches.

As always, you can register them as individual bean definitions, but they can also be implicitly registered by including the following tag in an XML-based Spring configuration (notice the inclusion of the context namespace):

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd">

    <context:annotation-config/>

</beans>

<context:annotation-config/> only looks for annotations on beans in the same application context in which it is defined. This means that, if you put <context:annotation-config/> in a WebApplicationContext for a DispatcherServlet, it only checks for @Autowired beans in your controllers, and not your services. See The DispatcherServlet for more information.

@Required

The @Required annotation applies to bean property setter methods, as in the following example:

Java
public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Required
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}
Kotlin
    class SimpleMovieLister {

    @Required
    lateinit var movieFinder: MovieFinder

    // ...
}

This annotation indicates that the affected bean property must be populated at configuration time, through an explicit property value in a bean definition or through autowiring. The container throws an exception if the affected bean property has not been populated. This allows for eager and explicit failure, avoiding NullPointerException instances or the like later on. We still recommend that you put assertions into the bean class itself (for example, into an init method). Doing so enforces those required references and values even when you use the class outside of a container.

The @Required annotation is formally deprecated as of Spring Framework 5.1, in favor of using constructor injection for required settings (or a custom implementation of InitializingBean.afterPropertiesSet() along with bean property setter methods).

Using @Autowired

JSR 330’s @Inject annotation can be used in place of Spring’s @Autowired annotation in the examples included in this section. See here for more details.

You can apply the @Autowired annotation to constructors, as the following example shows:

Java
public class MovieRecommender {

    private final CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}
Kotlin
class MovieRecommender @Autowired constructor(
    private val customerPreferenceDao: CustomerPreferenceDao)

As of Spring Framework 4.3, an @Autowired annotation on such a constructor is no longer necessary if the target bean defines only one constructor to begin with. However, if several constructors are available, at least one must be annotated with @Autowired in order to instruct the container which one to use.

You can also apply the @Autowired annotation to traditional setter methods, as the following example shows:

Java
public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}
Kotlin
class SimpleMovieLister {

    @Autowired
    lateinit var movieFinder: MovieFinder

    // ...

}

You can also apply the annotation to methods with arbitrary names and multiple arguments, as the following example shows:

Java
public class MovieRecommender {

    private MovieCatalog movieCatalog;

    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(MovieCatalog movieCatalog,
            CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}
Kotlin
class MovieRecommender {

    private lateinit var movieCatalog: MovieCatalog

    private lateinit var customerPreferenceDao: CustomerPreferenceDao

    @Autowired
    fun prepare(movieCatalog: MovieCatalog,
                customerPreferenceDao: CustomerPreferenceDao) {
        this.movieCatalog = movieCatalog
        this.customerPreferenceDao = customerPreferenceDao
    }

    // ...
}

You can apply @Autowired to fields as well and even mix it with constructors, as the following example shows:

Java
public class MovieRecommender {

    private final CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    private MovieCatalog movieCatalog;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}
Kotlin
class MovieRecommender @Autowired constructor(
    private val customerPreferenceDao: CustomerPreferenceDao) {

    @Autowired
    private lateinit var movieCatalog: MovieCatalog

    // ...
}

Make sure that your target components (for example, MovieCatalog or CustomerPreferenceDao) are consistently declared by the type that you use for your @Autowired-annotated injection points. Otherwise, injection may fail due to a "no type match found" error at runtime.

For XML-defined beans or component classes found via classpath scanning, the container usually knows the concrete type up front. However, for @Bean factory methods, you need to make sure that the declared return type is sufficiently expressive. For components that implement several interfaces or for components potentially referred to by their implementation type, consider declaring the most specific return type on your factory method (at least as specific as required by the injection points referring to your bean).

You can also instruct Spring to provide all beans of a particular type from the ApplicationContext by adding the @Autowired annotation to a field or method that expects an array of that type, as the following example shows:

Java
public class MovieRecommender {

    @Autowired
    private MovieCatalog[] movieCatalogs;

    // ...
}
Kotlin
class MovieRecommender {

    @Autowired
    private lateinit var movieCatalogs: Array<MovieCatalog>

    // ...
}

The same applies for typed collections, as the following example shows:

Java
public class MovieRecommender {

    private Set<MovieCatalog> movieCatalogs;

    @Autowired
    public void setMovieCatalogs(Set<MovieCatalog> movieCatalogs) {
        this.movieCatalogs = movieCatalogs;
    }

    // ...
}
Kotlin
class MovieRecommender {

    @Autowired
    lateinit var movieCatalogs: Set<MovieCatalog>

    // ...
}

Your target beans can implement the org.springframework.core.Ordered interface or use the @Order or standard @Priority annotation if you want items in the array or list to be sorted in a specific order. Otherwise, their order follows the registration order of the corresponding target bean definitions in the container.

You can declare the @Order annotation at the target class level and on @Bean methods, potentially for individual bean definitions (in case of multiple definitions that use the same bean class). @Order values may influence priorities at injection points, but be aware that they do not influence singleton startup order, which is an orthogonal concern determined by dependency relationships and @DependsOn declarations.

Note that the standard javax.annotation.Priority annotation is not available at the @Bean level, since it cannot be declared on methods. Its semantics can be modeled through @Order values in combination with @Primary on a single bean for each type.

Even typed Map instances can be autowired as long as the expected key type is String. The map values contain all beans of the expected type, and the keys contain the corresponding bean names, as the following example shows:

Java
public class MovieRecommender {

    private Map<String, MovieCatalog> movieCatalogs;

    @Autowired
    public void setMovieCatalogs(Map<String, MovieCatalog> movieCatalogs) {
        this.movieCatalogs = movieCatalogs;
    }

    // ...
}
Kotlin
class MovieRecommender {

    @Autowired
    lateinit var movieCatalogs: Map<String, MovieCatalog>

    // ...
}

By default, autowiring fails when no matching candidate beans are available for a given injection point. In the case of a declared array, collection, or map, at least one matching element is expected.

The default behavior is to treat annotated methods and fields as indicating required dependencies. You can change this behavior as demonstrated in the following example, enabling the framework to skip a non-satisfiable injection point through marking it as non-required (i.e., by setting the required attribute in @Autowired to false):

Java
public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired(required = false)
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}
Kotlin
class SimpleMovieLister {

    @Autowired(required = false)
    var movieFinder: MovieFinder? = null

    // ...
}

A non-required method will not be called at all if its dependency (or one of its dependencies, in case of multiple arguments) is not available. A non-required field will not get populated at all in such case, leaving its default value in place.

Injected constructor and factory method arguments are a special case since the required attribute in @Autowired has a somewhat different meaning due to Spring’s constructor resolution algorithm that may potentially deal with multiple constructors. Constructor and factory method arguments are effectively required by default but with a few special rules in a single-constructor scenario, such as multi-element injection points (arrays, collections, maps) resolving to empty instances if no matching beans are available. This allows for a common implementation pattern where all dependencies can be declared in a unique multi-argument constructor — for example, declared as a single public constructor without an @Autowired annotation.

Only one constructor of any given bean class may declare @Autowired with the required attribute set to true, indicating the constructor to autowire when used as a Spring bean. Furthermore, if the required attribute is set to true, only a single constructor may be annotated with @Autowired. If multiple non-required constructors declare the annotation, they will be considered as candidates for autowiring. The constructor with the greatest number of dependencies that can be satisfied by matching beans in the Spring container will be chosen. If none of the candidates can be satisfied, then a primary/default constructor (if present) will be used. If a class only declares a single constructor to begin with, it will always be used, even if not annotated. An annotated constructor does not have to be public.

The required attribute of @Autowired is recommended over the deprecated @Required annotation on setter methods. Setting the required attribute to false indicates that the property is not required for autowiring purposes, and the property is ignored if it cannot be autowired. @Required, on the other hand, is stronger in that it enforces the property to be set by any means supported by the container, and if no value is defined, a corresponding exception is raised.

Alternatively, you can express the non-required nature of a particular dependency through Java 8’s java.util.Optional, as the following example shows:

public class SimpleMovieLister {

    @Autowired
    public void setMovieFinder(Optional<MovieFinder> movieFinder) {
        ...
    }
}

As of Spring Framework 5.0, you can also use a @Nullable annotation (of any kind in any package — for example, javax.annotation.Nullable from JSR-305) or just leverage Kotlin builtin null-safety support:

Java
public class SimpleMovieLister {

    @Autowired
    public void setMovieFinder(@Nullable MovieFinder movieFinder) {
        ...
    }
}
Kotlin
class SimpleMovieLister {

    @Autowired
    var movieFinder: MovieFinder? = null

    // ...
}

You can also use @Autowired for interfaces that are well-known resolvable dependencies: BeanFactory, ApplicationContext, Environment, ResourceLoader, ApplicationEventPublisher, and MessageSource. These interfaces and their extended interfaces, such as ConfigurableApplicationContext or ResourcePatternResolver, are automatically resolved, with no special setup necessary. The following example autowires an ApplicationContext object:

Java
public class MovieRecommender {

    @Autowired
    private ApplicationContext context;

    public MovieRecommender() {
    }

    // ...
}
Kotlin
class MovieRecommender {

    @Autowired
    lateinit var context: ApplicationContext

    // ...
}

The @Autowired, @Inject, @Value, and @Resource annotations are handled by Spring BeanPostProcessor implementations. This means that you cannot apply these annotations within your own BeanPostProcessor or BeanFactoryPostProcessor types (if any). These types must be 'wired up' explicitly by using XML or a Spring @Bean method.

Fine-tuning Annotation-based Autowiring with @Primary

Because autowiring by type may lead to multiple candidates, it is often necessary to have more control over the selection process. One way to accomplish this is with Spring’s @Primary annotation. @Primary indicates that a particular bean should be given preference when multiple beans are candidates to be autowired to a single-valued dependency. If exactly one primary bean exists among the candidates, it becomes the autowired value.

Consider the following configuration that defines firstMovieCatalog as the primary MovieCatalog:

Java
@Configuration
public class MovieConfiguration {

    @Bean
    @Primary
    public MovieCatalog firstMovieCatalog() { ... }

    @Bean
    public MovieCatalog secondMovieCatalog() { ... }

    // ...
}
Kotlin
@Configuration
class MovieConfiguration {

    @Bean
    @Primary
    fun firstMovieCatalog(): MovieCatalog { ... }

    @Bean
    fun secondMovieCatalog(): MovieCatalog { ... }

    // ...
}

With the preceding configuration, the following MovieRecommender is autowired with the firstMovieCatalog:

Java
public class MovieRecommender {

    @Autowired
    private MovieCatalog movieCatalog;

    // ...
}
Kotlin
class MovieRecommender {

    @Autowired
    private lateinit var movieCatalog: MovieCatalog

    // ...
}

The corresponding bean definitions follow:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd">

    <context:annotation-config/>

    <bean class="example.SimpleMovieCatalog" primary="true">
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean id="movieRecommender" class="example.MovieRecommender"/>

</beans>

Fine-tuning Annotation-based Autowiring with Qualifiers

@Primary is an effective way to use autowiring by type with several instances when one primary candidate can be determined. When you need more control over the selection process, you can use Spring’s @Qualifier annotation. You can associate qualifier values with specific arguments, narrowing the set of type matches so that a specific bean is chosen for each argument. In the simplest case, this can be a plain descriptive value, as shown in the following example:

Java
public class MovieRecommender {

    @Autowired
    @Qualifier("main")
    private MovieCatalog movieCatalog;

    // ...
}
Kotlin
class MovieRecommender {

    @Autowired
    @Qualifier("main")
    private lateinit var movieCatalog: MovieCatalog

    // ...
}

You can also specify the @Qualifier annotation on individual constructor arguments or method parameters, as shown in the following example:

Java
public class MovieRecommender {

    private MovieCatalog movieCatalog;

    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(@Qualifier("main") MovieCatalog movieCatalog,
            CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}
Kotlin
class MovieRecommender {

    private lateinit var movieCatalog: MovieCatalog

    private lateinit var customerPreferenceDao: CustomerPreferenceDao

    @Autowired
    fun prepare(@Qualifier("main") movieCatalog: MovieCatalog,
                customerPreferenceDao: CustomerPreferenceDao) {
        this.movieCatalog = movieCatalog
        this.customerPreferenceDao = customerPreferenceDao
    }

    // ...
}

The following example shows corresponding bean definitions.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd">

    <context:annotation-config/>

    <bean class="example.SimpleMovieCatalog">
        <qualifier value="main"/> (1)

        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <qualifier value="action"/> (2)

        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean id="movieRecommender" class="example.MovieRecommender"/>

</beans>
1 The bean with the main qualifier value is wired with the constructor argument that is qualified with the same value.
2 The bean with the action qualifier value is wired with the constructor argument that is qualified with the same value.

For a fallback match, the bean name is considered a default qualifier value. Thus, you can define the bean with an id of main instead of the nested qualifier element, leading to the same matching result. However, although you can use this convention to refer to specific beans by name, @Autowired is fundamentally about type-driven injection with optional semantic qualifiers. This means that qualifier values, even with the bean name fallback, always have narrowing semantics within the set of type matches. They do not semantically express a reference to a unique bean id. Good qualifier values are main or EMEA or persistent, expressing characteristics of a specific component that are independent from the bean id, which may be auto-generated in case of an anonymous bean definition such as the one in the preceding example.

Qualifiers also apply to typed collections, as discussed earlier — for example, to Set<MovieCatalog>. In this case, all matching beans, according to the declared qualifiers, are injected as a collection. This implies that qualifiers do not have to be unique. Rather, they constitute filtering criteria. For example, you can define multiple MovieCatalog beans with the same qualifier value “action”, all of which are injected into a Set<MovieCatalog> annotated with @Qualifier("action").

Letting qualifier values select against target bean names, within the type-matching candidates, does not require a @Qualifier annotation at the injection point. If there is no other resolution indicator (such as a qualifier or a primary marker), for a non-unique dependency situation, Spring matches the injection point name (that is, the field name or parameter name) against the target bean names and choose the same-named candidate, if any.

That said, if you intend to express annotation-driven injection by name, do not primarily use @Autowired, even if it is capable of selecting by bean name among type-matching candidates. Instead, use the JSR-250 @Resource annotation, which is semantically defined to identify a specific target component by its unique name, with the declared type being irrelevant for the matching process. @Autowired has rather different semantics: After selecting candidate beans by type, the specified String qualifier value is considered within those type-selected candidates only (for example, matching an account qualifier against beans marked with the same qualifier label).

For beans that are themselves defined as a collection, Map, or array type, @Resource is a fine solution, referring to the specific collection or array bean by unique name. That said, as of 4.3, collection, you can match Map, and array types through Spring’s @Autowired type matching algorithm as well, as long as the element type information is preserved in @Bean return type signatures or collection inheritance hierarchies. In this case, you can use qualifier values to select among same-typed collections, as outlined in the previous paragraph.

As of 4.3, @Autowired also considers self references for injection (that is, references back to the bean that is currently injected). Note that self injection is a fallback. Regular dependencies on other components always have precedence. In that sense, self references do not participate in regular candidate selection and are therefore in particular never primary. On the contrary, they always end up as lowest precedence. In practice, you should use self references as a last resort only (for example, for calling other methods on the same instance through the bean’s transactional proxy). Consider factoring out the effected methods to a separate delegate bean in such a scenario. Alternatively, you can use @Resource, which may obtain a proxy back to the current bean by its unique name.

Trying to inject the results from @Bean methods on the same configuration class is effectively a self-reference scenario as well. Either lazily resolve such references in the method signature where it is actually needed (as opposed to an autowired field in the configuration class) or declare the affected @Bean methods as static, decoupling them from the containing configuration class instance and its lifecycle. Otherwise, such beans are only considered in the fallback phase, with matching beans on other configuration classes selected as primary candidates instead (if available).

@Autowired applies to fields, constructors, and multi-argument methods, allowing for narrowing through qualifier annotations at the parameter level. In contrast, @Resource is supported only for fields and bean property setter methods with a single argument. As a consequence, you should stick with qualifiers if your injection target is a constructor or a multi-argument method.

You can create your own custom qualifier annotations. To do so, define an annotation and provide the @Qualifier annotation within your definition, as the following example shows:

Java
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface Genre {

    String value();
}
Kotlin
@Target(AnnotationTarget.FIELD, AnnotationTarget.VALUE_PARAMETER)
@Retention(AnnotationRetention.RUNTIME)
@Qualifier
annotation class Genre(val value: String)

Then you can provide the custom qualifier on autowired fields and parameters, as the following example shows:

Java
public class MovieRecommender {

    @Autowired
    @Genre("Action")
    private MovieCatalog actionCatalog;

    private MovieCatalog comedyCatalog;

    @Autowired
    public void setComedyCatalog(@Genre("Comedy") MovieCatalog comedyCatalog) {
        this.comedyCatalog = comedyCatalog;
    }

    // ...
}
Kotlin
class MovieRecommender {

    @Autowired
    @Genre("Action")
    private lateinit var actionCatalog: MovieCatalog

    private lateinit var comedyCatalog: MovieCatalog

    @Autowired
    fun setComedyCatalog(@Genre("Comedy") comedyCatalog: MovieCatalog) {
        this.comedyCatalog = comedyCatalog
    }

    // ...
}

Next, you can provide the information for the candidate bean definitions. You can add <qualifier/> tags as sub-elements of the <bean/> tag and then specify the type and value to match your custom qualifier annotations. The type is matched against the fully-qualified class name of the annotation. Alternately, as a convenience if no risk of conflicting names exists, you can use the short class name. The following example demonstrates both approaches:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd">

    <context:annotation-config/>

    <bean class="example.SimpleMovieCatalog">
        <qualifier type="Genre" value="Action"/>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <qualifier type="example.Genre" value="Comedy"/>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean id="movieRecommender" class="example.MovieRecommender"/>

</beans>

In Classpath Scanning and Managed Components, you can see an annotation-based alternative to providing the qualifier metadata in XML. Specifically, see Providing Qualifier Metadata with Annotations.

In some cases, using an annotation without a value may suffice. This can be useful when the annotation serves a more generic purpose and can be applied across several different types of dependencies. For example, you may provide an offline catalog that can be searched when no Internet connection is available. First, define the simple annotation, as the following example shows:

Java
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface Offline {

}
Kotlin
@Target(AnnotationTarget.FIELD, AnnotationTarget.VALUE_PARAMETER)
@Retention(AnnotationRetention.RUNTIME)
@Qualifier
annotation class Offline

Then add the annotation to the field or property to be autowired, as shown in the following example:

Java
public class MovieRecommender {

    @Autowired
    @Offline (1)
    private MovieCatalog offlineCatalog;

    // ...
}
1 This line adds the @Offline annotation.
Kotlin
class MovieRecommender {

    @Autowired
    @Offline (1)
    private lateinit var offlineCatalog: MovieCatalog

    // ...
}
1 This line adds the @Offline annotation.

Now the bean definition only needs a qualifier type, as shown in the following example:

<bean class="example.SimpleMovieCatalog">
    <qualifier type="Offline"/> (1)
    <!-- inject any dependencies required by this bean -->
</bean>
1 This element specifies the qualifier.

You can also define custom qualifier annotations that accept named attributes in addition to or instead of the simple value attribute. If multiple attribute values are then specified on a field or parameter to be autowired, a bean definition must match all such attribute values to be considered an autowire candidate. As an example, consider the following annotation definition:

Java
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface MovieQualifier {

    String genre();

    Format format();
}
Kotlin
@Target(AnnotationTarget.FIELD, AnnotationTarget.VALUE_PARAMETER)
@Retention(AnnotationRetention.RUNTIME)
@Qualifier
annotation class MovieQualifier(val genre: String, val format: Format)

In this case Format is an enum, defined as follows:

Java
public enum Format {
    VHS, DVD, BLURAY
}
Kotlin
enum class Format {
    VHS, DVD, BLURAY
}

The fields to be autowired are annotated with the custom qualifier and include values for both attributes: genre and format, as the following example shows:

Java
public class MovieRecommender {

    @Autowired
    @MovieQualifier(format=Format.VHS, genre="Action")
    private MovieCatalog actionVhsCatalog;

    @Autowired
    @MovieQualifier(format=Format.VHS, genre="Comedy")
    private MovieCatalog comedyVhsCatalog;

    @Autowired
    @MovieQualifier(format=Format.DVD, genre="Action")
    private MovieCatalog actionDvdCatalog;

    @Autowired
    @MovieQualifier(format=Format.BLURAY, genre="Comedy")
    private MovieCatalog comedyBluRayCatalog;

    // ...
}
Kotlin
class MovieRecommender {

    @Autowired
    @MovieQualifier(format = Format.VHS, genre = "Action")
    private lateinit var actionVhsCatalog: MovieCatalog

    @Autowired
    @MovieQualifier(format = Format.VHS, genre = "Comedy")
    private lateinit var comedyVhsCatalog: MovieCatalog

    @Autowired
    @MovieQualifier(format = Format.DVD, genre = "Action")
    private lateinit var actionDvdCatalog: MovieCatalog

    @Autowired
    @MovieQualifier(format = Format.BLURAY, genre = "Comedy")
    private lateinit var comedyBluRayCatalog: MovieCatalog

    // ...
}

Finally, the bean definitions should contain matching qualifier values. This example also demonstrates that you can use bean meta attributes instead of the <qualifier/> elements. If available, the <qualifier/> element and its attributes take precedence, but the autowiring mechanism falls back on the values provided within the <meta/> tags if no such qualifier is present, as in the last two bean definitions in the following example:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd">

    <context:annotation-config/>

    <bean class="example.SimpleMovieCatalog">
        <qualifier type="MovieQualifier">
            <attribute key="format" value="VHS"/>
            <attribute key="genre" value="Action"/>
        </qualifier>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <qualifier type="MovieQualifier">
            <attribute key="format" value="VHS"/>
            <attribute key="genre" value="Comedy"/>
        </qualifier>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <meta key="format" value="DVD"/>
        <meta key="genre" value="Action"/>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <meta key="format" value="BLURAY"/>
        <meta key="genre" value="Comedy"/>
        <!-- inject any dependencies required by this bean -->
    </bean>

</beans>

Using Generics as Autowiring Qualifiers

In addition to the @Qualifier annotation, you can use Java generic types as an implicit form of qualification. For example, suppose you have the following configuration:

Java
@Configuration
public class MyConfiguration {

    @Bean
    public StringStore stringStore() {
        return new StringStore();
    }

    @Bean
    public IntegerStore integerStore() {
        return new IntegerStore();
    }
}
Kotlin
@Configuration
class MyConfiguration {

    @Bean
    fun stringStore() = StringStore()

    @Bean
    fun integerStore() = IntegerStore()
}

Assuming that the preceding beans implement a generic interface, (that is, Store<String> and Store<Integer>), you can @Autowire the Store interface and the generic is used as a qualifier, as the following example shows:

Java
@Autowired
private Store<String> s1; // <String> qualifier, injects the stringStore bean

@Autowired
private Store<Integer> s2; // <Integer> qualifier, injects the integerStore bean
Kotlin
@Autowired
private lateinit var s1: Store<String> // <String> qualifier, injects the stringStore bean

@Autowired
private lateinit var s2: Store<Integer> // <Integer> qualifier, injects the integerStore bean

Generic qualifiers also apply when autowiring lists, Map instances and arrays. The following example autowires a generic List:

Java
// Inject all Store beans as long as they have an <Integer> generic
// Store<String> beans will not appear in this list
@Autowired
private List<Store<Integer>> s;
Kotlin
// Inject all Store beans as long as they have an <Integer> generic
// Store<String> beans will not appear in this list
@Autowired
private lateinit var s: List<Store<Integer>>

Using CustomAutowireConfigurer

CustomAutowireConfigurer is a BeanFactoryPostProcessor that lets you register your own custom qualifier annotation types, even if they are not annotated with Spring’s @Qualifier annotation. The following example shows how to use CustomAutowireConfigurer:

<bean id="customAutowireConfigurer"
        class="org.springframework.beans.factory.annotation.CustomAutowireConfigurer">
    <property name="customQualifierTypes">
        <set>
            <value>example.CustomQualifier</value>
        </set>
    </property>
</bean>

The AutowireCandidateResolver determines autowire candidates by:

  • The autowire-candidate value of each bean definition

  • Any default-autowire-candidates patterns available on the <beans/> element

  • The presence of @Qualifier annotations and any custom annotations registered with the CustomAutowireConfigurer

When multiple beans qualify as autowire candidates, the determination of a “primary” is as follows: If exactly one bean definition among the candidates has a primary attribute set to true, it is selected.

Injection with @Resource

Spring also supports injection by using the JSR-250 @Resource annotation (javax.annotation.Resource) on fields or bean property setter methods. This is a common pattern in Java EE: for example, in JSF-managed beans and JAX-WS endpoints. Spring supports this pattern for Spring-managed objects as well.

@Resource takes a name attribute. By default, Spring interprets that value as the bean name to be injected. In other words, it follows by-name semantics, as demonstrated in the following example:

Java
public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Resource(name="myMovieFinder") (1)
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
}
1 This line injects a @Resource.
Kotlin
class SimpleMovieLister {

    @Resource(name="myMovieFinder") (1)
    private lateinit var movieFinder:MovieFinder
}
1 This line injects a @Resource.

If no name is explicitly specified, the default name is derived from the field name or setter method. In case of a field, it takes the field name. In case of a setter method, it takes the bean property name. The following example is going to have the bean named movieFinder injected into its setter method:

Java
public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Resource
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
}
Kotlin
class SimpleMovieLister {

    @Resource
    private lateinit var movieFinder: MovieFinder

}
The name provided with the annotation is resolved as a bean name by the ApplicationContext of which the CommonAnnotationBeanPostProcessor is aware. The names can be resolved through JNDI if you configure Spring’s SimpleJndiBeanFactory explicitly. However, we recommend that you rely on the default behavior and use Spring’s JNDI lookup capabilities to preserve the level of indirection.

In the exclusive case of @Resource usage with no explicit name specified, and similar to @Autowired, @Resource finds a primary type match instead of a specific named bean and resolves well known resolvable dependencies: the BeanFactory, ApplicationContext, ResourceLoader, ApplicationEventPublisher, and MessageSource interfaces.

Thus, in the following example, the customerPreferenceDao field first looks for a bean named "customerPreferenceDao" and then falls back to a primary type match for the type CustomerPreferenceDao:

Java
public class MovieRecommender {

    @Resource
    private CustomerPreferenceDao customerPreferenceDao;

    @Resource
    private ApplicationContext context; (1)

    public MovieRecommender() {
    }

    // ...
}
1 The context field is injected based on the known resolvable dependency type: ApplicationContext.
Kotlin
class MovieRecommender {

    @Resource
    private lateinit var customerPreferenceDao: CustomerPreferenceDao


    @Resource
    private lateinit var context: ApplicationContext (1)

    // ...
}
1 The context field is injected based on the known resolvable dependency type: ApplicationContext.

Using @Value

@Value is typically used to inject externalized properties:

Java
@Component
public class MovieRecommender {

    private final String catalog;

    public MovieRecommender(@Value("${catalog.name}") String catalog) {
        this.catalog = catalog;
    }
}
Kotlin
@Component
class MovieRecommender(@Value("\${catalog.name}") private val catalog: String)

With the following configuration:

Java
@Configuration
@PropertySource("classpath:application.properties")
public class AppConfig { }
Kotlin
@Configuration
@PropertySource("classpath:application.properties")
class AppConfig

And the following application.properties file:

catalog.name=MovieCatalog

In that case, the catalog parameter and field will be equal to the MovieCatalog value.

A default lenient embedded value resolver is provided by Spring. It will try to resolve the property value and if it cannot be resolved, the property name (for example ${catalog.name}) will be injected as the value. If you want to maintain strict control over nonexistent values, you should declare a PropertySourcesPlaceholderConfigurer bean, as the following example shows:

Java
@Configuration
public class AppConfig {

     @Bean
     public static PropertySourcesPlaceholderConfigurer propertyPlaceholderConfigurer() {
           return new PropertySourcesPlaceholderConfigurer();
     }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun propertyPlaceholderConfigurer() = PropertySourcesPlaceholderConfigurer()
}
When configuring a PropertySourcesPlaceholderConfigurer using JavaConfig, the @Bean method must be static.

Using the above configuration ensures Spring initialization failure if any ${} placeholder could not be resolved. It is also possible to use methods like setPlaceholderPrefix, setPlaceholderSuffix, or setValueSeparator to customize placeholders.

Spring Boot configures by default a PropertySourcesPlaceholderConfigurer bean that will get properties from application.properties and application.yml files.

Built-in converter support provided by Spring allows simple type conversion (to Integer or int for example) to be automatically handled. Multiple comma-separated values can be automatically converted to String array without extra effort.

It is possible to provide a default value as following:

Java
@Component
public class MovieRecommender {

    private final String catalog;

    public MovieRecommender(@Value("${catalog.name:defaultCatalog}") String catalog) {
        this.catalog = catalog;
    }
}
Kotlin
@Component
class MovieRecommender(@Value("\${catalog.name:defaultCatalog}") private val catalog: String)

A Spring BeanPostProcessor uses a ConversionService behind the scene to handle the process for converting the String value in @Value to the target type. If you want to provide conversion support for your own custom type, you can provide your own ConversionService bean instance as the following example shows:

Java
@Configuration
public class AppConfig {

    @Bean
    public ConversionService conversionService() {
        DefaultFormattingConversionService conversionService = new DefaultFormattingConversionService();
        conversionService.addConverter(new MyCustomConverter());
        return conversionService;
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun conversionService(): ConversionService {
            return DefaultFormattingConversionService().apply {
            addConverter(MyCustomConverter())
        }
    }
}

When @Value contains a SpEL expression the value will be dynamically computed at runtime as the following example shows:

Java
@Component
public class MovieRecommender {

    private final String catalog;

    public MovieRecommender(@Value("#{systemProperties['user.catalog'] + 'Catalog' }") String catalog) {
        this.catalog = catalog;
    }
}
Kotlin
@Component
class MovieRecommender(
    @Value("#{systemProperties['user.catalog'] + 'Catalog' }") private val catalog: String)

SpEL also enables the use of more complex data structures:

Java
@Component
public class MovieRecommender {

    private final Map<String, Integer> countOfMoviesPerCatalog;

    public MovieRecommender(
            @Value("#{{'Thriller': 100, 'Comedy': 300}}") Map<String, Integer> countOfMoviesPerCatalog) {
        this.countOfMoviesPerCatalog = countOfMoviesPerCatalog;
    }
}
Kotlin
@Component
class MovieRecommender(
    @Value("#{{'Thriller': 100, 'Comedy': 300}}") private val countOfMoviesPerCatalog: Map<String, Int>)

Using @PostConstruct and @PreDestroy

The CommonAnnotationBeanPostProcessor not only recognizes the @Resource annotation but also the JSR-250 lifecycle annotations: javax.annotation.PostConstruct and javax.annotation.PreDestroy. Introduced in Spring 2.5, the support for these annotations offers an alternative to the lifecycle callback mechanism described in initialization callbacks and destruction callbacks. Provided that the CommonAnnotationBeanPostProcessor is registered within the Spring ApplicationContext, a method carrying one of these annotations is invoked at the same point in the lifecycle as the corresponding Spring lifecycle interface method or explicitly declared callback method. In the following example, the cache is pre-populated upon initialization and cleared upon destruction:

Java
public class CachingMovieLister {

    @PostConstruct
    public void populateMovieCache() {
        // populates the movie cache upon initialization...
    }

    @PreDestroy
    public void clearMovieCache() {
        // clears the movie cache upon destruction...
    }
}
Kotlin
class CachingMovieLister {

    @PostConstruct
    fun populateMovieCache() {
        // populates the movie cache upon initialization...
    }

    @PreDestroy
    fun clearMovieCache() {
        // clears the movie cache upon destruction...
    }
}

For details about the effects of combining various lifecycle mechanisms, see Combining Lifecycle Mechanisms.

Like @Resource, the @PostConstruct and @PreDestroy annotation types were a part of the standard Java libraries from JDK 6 to 8. However, the entire javax.annotation package got separated from the core Java modules in JDK 9 and eventually removed in JDK 11. If needed, the javax.annotation-api artifact needs to be obtained via Maven Central now, simply to be added to the application’s classpath like any other library.

Classpath Scanning and Managed Components

Most examples in this chapter use XML to specify the configuration metadata that produces each BeanDefinition within the Spring container. The previous section (Annotation-based Container Configuration) demonstrates how to provide a lot of the configuration metadata through source-level annotations. Even in those examples, however, the “base” bean definitions are explicitly defined in the XML file, while the annotations drive only the dependency injection. This section describes an option for implicitly detecting the candidate components by scanning the classpath. Candidate components are classes that match against a filter criteria and have a corresponding bean definition registered with the container. This removes the need to use XML to perform bean registration. Instead, you can use annotations (for example, @Component), AspectJ type expressions, or your own custom filter criteria to select which classes have bean definitions registered with the container.

Starting with Spring 3.0, many features provided by the Spring JavaConfig project are part of the core Spring Framework. This allows you to define beans using Java rather than using the traditional XML files. Take a look at the @Configuration, @Bean, @Import, and @DependsOn annotations for examples of how to use these new features.

@Component and Further Stereotype Annotations

The @Repository annotation is a marker for any class that fulfills the role or stereotype of a repository (also known as Data Access Object or DAO). Among the uses of this marker is the automatic translation of exceptions, as described in Exception Translation.

Spring provides further stereotype annotations: @Component, @Service, and @Controller. @Component is a generic stereotype for any Spring-managed component. @Repository, @Service, and @Controller are specializations of @Component for more specific use cases (in the persistence, service, and presentation layers, respectively). Therefore, you can annotate your component classes with @Component, but, by annotating them with @Repository, @Service, or @Controller instead, your classes are more properly suited for processing by tools or associating with aspects. For example, these stereotype annotations make ideal targets for pointcuts. @Repository, @Service, and @Controller can also carry additional semantics in future releases of the Spring Framework. Thus, if you are choosing between using @Component or @Service for your service layer, @Service is clearly the better choice. Similarly, as stated earlier, @Repository is already supported as a marker for automatic exception translation in your persistence layer.

Using Meta-annotations and Composed Annotations

Many of the annotations provided by Spring can be used as meta-annotations in your own code. A meta-annotation is an annotation that can be applied to another annotation. For example, the @Service annotation mentioned earlier is meta-annotated with @Component, as the following example shows:

Java
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component (1)
public @interface Service {

    // ...
}
1 The Component causes @Service to be treated in the same way as @Component.
Kotlin
@Target(AnnotationTarget.TYPE)
@Retention(AnnotationRetention.RUNTIME)
@MustBeDocumented
@Component (1)
annotation class Service {

    // ...
}
1 The Component causes @Service to be treated in the same way as @Component.

You can also combine meta-annotations to create “composed annotations”. For example, the @RestController annotation from Spring MVC is composed of @Controller and @ResponseBody.

In addition, composed annotations can optionally redeclare attributes from meta-annotations to allow customization. This can be particularly useful when you want to only expose a subset of the meta-annotation’s attributes. For example, Spring’s @SessionScope annotation hardcodes the scope name to session but still allows customization of the proxyMode. The following listing shows the definition of the SessionScope annotation:

Java
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Scope(WebApplicationContext.SCOPE_SESSION)
public @interface SessionScope {

    /**
     * Alias for {@link Scope#proxyMode}.
     * <p>Defaults to {@link ScopedProxyMode#TARGET_CLASS}.
     */
    @AliasFor(annotation = Scope.class)
    ScopedProxyMode proxyMode() default ScopedProxyMode.TARGET_CLASS;

}
Kotlin
@Target(AnnotationTarget.TYPE, AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
@MustBeDocumented
@Scope(WebApplicationContext.SCOPE_SESSION)
annotation class SessionScope(
        @get:AliasFor(annotation = Scope::class)
        val proxyMode: ScopedProxyMode = ScopedProxyMode.TARGET_CLASS
)

You can then use @SessionScope without declaring the proxyMode as follows:

Java
@Service
@SessionScope
public class SessionScopedService {
    // ...
}
Kotlin
@Service
@SessionScope
class SessionScopedService {
    // ...
}

You can also override the value for the proxyMode, as the following example shows:

Java
@Service
@SessionScope(proxyMode = ScopedProxyMode.INTERFACES)
public class SessionScopedUserService implements UserService {
    // ...
}
Kotlin
@Service
@SessionScope(proxyMode = ScopedProxyMode.INTERFACES)
class SessionScopedUserService : UserService {
    // ...
}

For further details, see the Spring Annotation Programming Model wiki page.

Automatically Detecting Classes and Registering Bean Definitions

Spring can automatically detect stereotyped classes and register corresponding BeanDefinition instances with the ApplicationContext. For example, the following two classes are eligible for such autodetection:

Java
@Service
public class SimpleMovieLister {

    private MovieFinder movieFinder;

    public SimpleMovieLister(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
}
Kotlin
@Service
class SimpleMovieLister(private val movieFinder: MovieFinder)
Java
@Repository
public class JpaMovieFinder implements MovieFinder {
    // implementation elided for clarity
}
Kotlin
@Repository
class JpaMovieFinder : MovieFinder {
    // implementation elided for clarity
}

To autodetect these classes and register the corresponding beans, you need to add @ComponentScan to your @Configuration class, where the basePackages attribute is a common parent package for the two classes. (Alternatively, you can specify a comma- or semicolon- or space-separated list that includes the parent package of each class.)

Java
@Configuration
@ComponentScan(basePackages = "org.example")
public class AppConfig  {
    // ...
}
Kotlin
@Configuration
@ComponentScan(basePackages = ["org.example"])
class AppConfig  {
    // ...
}
For brevity, the preceding example could have used the value attribute of the annotation (that is, @ComponentScan("org.example")).

The following alternative uses XML:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd">

    <context:component-scan base-package="org.example"/>

</beans>
The use of <context:component-scan> implicitly enables the functionality of <context:annotation-config>. There is usually no need to include the <context:annotation-config> element when using <context:component-scan>.

The scanning of classpath packages requires the presence of corresponding directory entries in the classpath. When you build JARs with Ant, make sure that you do not activate the files-only switch of the JAR task. Also, classpath directories may not be exposed based on security policies in some environments — for example, standalone apps on JDK 1.7.0_45 and higher (which requires 'Trusted-Library' setup in your manifests — see https://stackoverflow.com/questions/19394570/java-jre-7u45-breaks-classloader-getresources).

On JDK 9’s module path (Jigsaw), Spring’s classpath scanning generally works as expected. However, make sure that your component classes are exported in your module-info descriptors. If you expect Spring to invoke non-public members of your classes, make sure that they are 'opened' (that is, that they use an opens declaration instead of an exports declaration in your module-info descriptor).

Furthermore, the AutowiredAnnotationBeanPostProcessor and CommonAnnotationBeanPostProcessor are both implicitly included when you use the component-scan element. That means that the two components are autodetected and wired together — all without any bean configuration metadata provided in XML.

You can disable the registration of AutowiredAnnotationBeanPostProcessor and CommonAnnotationBeanPostProcessor by including the annotation-config attribute with a value of false.

Using Filters to Customize Scanning

By default, classes annotated with @Component, @Repository, @Service, @Controller, @Configuration, or a custom annotation that itself is annotated with @Component are the only detected candidate components. However, you can modify and extend this behavior by applying custom filters. Add them as includeFilters or excludeFilters attributes of the @ComponentScan annotation (or as <context:include-filter /> or <context:exclude-filter /> child elements of the <context:component-scan> element in XML configuration). Each filter element requires the type and expression attributes. The following table describes the filtering options:

Table 7. Filter Types
Filter Type Example Expression Description

annotation (default)

org.example.SomeAnnotation

An annotation to be present or meta-present at the type level in target components.

assignable

org.example.SomeClass

A class (or interface) that the target components are assignable to (extend or implement).

aspectj

org.example..*Service+

An AspectJ type expression to be matched by the target components.

regex

org\.example\.Default.*

A regex expression to be matched by the target components' class names.

custom

org.example.MyTypeFilter

A custom implementation of the org.springframework.core.type.TypeFilter interface.

The following example shows the configuration ignoring all @Repository annotations and using “stub” repositories instead:

Java
@Configuration
@ComponentScan(basePackages = "org.example",
        includeFilters = @Filter(type = FilterType.REGEX, pattern = ".*Stub.*Repository"),
        excludeFilters = @Filter(Repository.class))
public class AppConfig {
    ...
}
Kotlin
@Configuration
@ComponentScan(basePackages = "org.example",
        includeFilters = [Filter(type = FilterType.REGEX, pattern = [".*Stub.*Repository"])],
        excludeFilters = [Filter(Repository::class)])
class AppConfig {
    // ...
}

The following listing shows the equivalent XML:

<beans>
    <context:component-scan base-package="org.example">
        <context:include-filter type="regex"
                expression=".*Stub.*Repository"/>
        <context:exclude-filter type="annotation"
                expression="org.springframework.stereotype.Repository"/>
    </context:component-scan>
</beans>
You can also disable the default filters by setting useDefaultFilters=false on the annotation or by providing use-default-filters="false" as an attribute of the <component-scan/> element. This effectively disables automatic detection of classes annotated or meta-annotated with @Component, @Repository, @Service, @Controller, @RestController, or @Configuration.

Defining Bean Metadata within Components

Spring components can also contribute bean definition metadata to the container. You can do this with the same @Bean annotation used to define bean metadata within @Configuration annotated classes. The following example shows how to do so:

Java
@Component
public class FactoryMethodComponent {

    @Bean
    @Qualifier("public")
    public TestBean publicInstance() {
        return new TestBean("publicInstance");
    }

    public void doWork() {
        // Component method implementation omitted
    }
}
Kotlin
@Component
class FactoryMethodComponent {

    @Bean
    @Qualifier("public")
    fun publicInstance() = TestBean("publicInstance")

    fun doWork() {
        // Component method implementation omitted
    }
}

The preceding class is a Spring component that has application-specific code in its doWork() method. However, it also contributes a bean definition that has a factory method referring to the method publicInstance(). The @Bean annotation identifies the factory method and other bean definition properties, such as a qualifier value through the @Qualifier annotation. Other method-level annotations that can be specified are @Scope, @Lazy, and custom qualifier annotations.

In addition to its role for component initialization, you can also place the @Lazy annotation on injection points marked with @Autowired or @Inject. In this context, it leads to the injection of a lazy-resolution proxy.

Autowired fields and methods are supported, as previously discussed, with additional support for autowiring of @Bean methods. The following example shows how to do so:

Java
@Component
public class FactoryMethodComponent {

    private static int i;

    @Bean
    @Qualifier("public")
    public TestBean publicInstance() {
        return new TestBean("publicInstance");
    }

    // use of a custom qualifier and autowiring of method parameters
    @Bean
    protected TestBean protectedInstance(
            @Qualifier("public") TestBean spouse,
            @Value("#{privateInstance.age}") String country) {
        TestBean tb = new TestBean("protectedInstance", 1);
        tb.setSpouse(spouse);
        tb.setCountry(country);
        return tb;
    }

    @Bean
    private TestBean privateInstance() {
        return new TestBean("privateInstance", i++);
    }

    @Bean
    @RequestScope
    public TestBean requestScopedInstance() {
        return new TestBean("requestScopedInstance", 3);
    }
}
Kotlin
@Component
class FactoryMethodComponent {

    companion object {
        private var i: Int = 0
    }

    @Bean
    @Qualifier("public")
    fun publicInstance() = TestBean("publicInstance")

    // use of a custom qualifier and autowiring of method parameters
    @Bean
    protected fun protectedInstance(
            @Qualifier("public") spouse: TestBean,
            @Value("#{privateInstance.age}") country: String) = TestBean("protectedInstance", 1).apply {
        this.spouse = spouse
        this.country = country
    }

    @Bean
    private fun privateInstance() = TestBean("privateInstance", i++)

    @Bean
    @RequestScope
    fun requestScopedInstance() = TestBean("requestScopedInstance", 3)
}

The example autowires the String method parameter country to the value of the age property on another bean named privateInstance. A Spring Expression Language element defines the value of the property through the notation #{ <expression> }. For @Value annotations, an expression resolver is preconfigured to look for bean names when resolving expression text.

As of Spring Framework 4.3, you may also declare a factory method parameter of type InjectionPoint (or its more specific subclass: DependencyDescriptor) to access the requesting injection point that triggers the creation of the current bean. Note that this applies only to the actual creation of bean instances, not to the injection of existing instances. As a consequence, this feature makes most sense for beans of prototype scope. For other scopes, the factory method only ever sees the injection point that triggered the creation of a new bean instance in the given scope (for example, the dependency that triggered the creation of a lazy singleton bean). You can use the provided injection point metadata with semantic care in such scenarios. The following example shows how to do use InjectionPoint:

Java
@Component
public class FactoryMethodComponent {

    @Bean @Scope("prototype")
    public TestBean prototypeInstance(InjectionPoint injectionPoint) {
        return new TestBean("prototypeInstance for " + injectionPoint.getMember());
    }
}
Kotlin
@Component
class FactoryMethodComponent {

    @Bean
    @Scope("prototype")
    fun prototypeInstance(injectionPoint: InjectionPoint) =
            TestBean("prototypeInstance for ${injectionPoint.member}")
}

The @Bean methods in a regular Spring component are processed differently than their counterparts inside a Spring @Configuration class. The difference is that @Component classes are not enhanced with CGLIB to intercept the invocation of methods and fields. CGLIB proxying is the means by which invoking methods or fields within @Bean methods in @Configuration classes creates bean metadata references to collaborating objects. Such methods are not invoked with normal Java semantics but rather go through the container in order to provide the usual lifecycle management and proxying of Spring beans, even when referring to other beans through programmatic calls to @Bean methods. In contrast, invoking a method or field in a @Bean method within a plain @Component class has standard Java semantics, with no special CGLIB processing or other constraints applying.

You may declare @Bean methods as static, allowing for them to be called without creating their containing configuration class as an instance. This makes particular sense when defining post-processor beans (for example, of type BeanFactoryPostProcessor or BeanPostProcessor), since such beans get initialized early in the container lifecycle and should avoid triggering other parts of the configuration at that point.

Calls to static @Bean methods never get intercepted by the container, not even within @Configuration classes (as described earlier in this section), due to technical limitations: CGLIB subclassing can override only non-static methods. As a consequence, a direct call to another @Bean method has standard Java semantics, resulting in an independent instance being returned straight from the factory method itself.

The Java language visibility of @Bean methods does not have an immediate impact on the resulting bean definition in Spring’s container. You can freely declare your factory methods as you see fit in non-@Configuration classes and also for static methods anywhere. However, regular @Bean methods in @Configuration classes need to be overridable — that is, they must not be declared as private or final.

@Bean methods are also discovered on base classes of a given component or configuration class, as well as on Java 8 default methods declared in interfaces implemented by the component or configuration class. This allows for a lot of flexibility in composing complex configuration arrangements, with even multiple inheritance being possible through Java 8 default methods as of Spring 4.2.

Finally, a single class may hold multiple @Bean methods for the same bean, as an arrangement of multiple factory methods to use depending on available dependencies at runtime. This is the same algorithm as for choosing the “greediest” constructor or factory method in other configuration scenarios: The variant with the largest number of satisfiable dependencies is picked at construction time, analogous to how the container selects between multiple @Autowired constructors.

Naming Autodetected Components

When a component is autodetected as part of the scanning process, its bean name is generated by the BeanNameGenerator strategy known to that scanner. By default, any Spring stereotype annotation (@Component, @Repository, @Service, and @Controller) that contains a name value thereby provides that name to the corresponding bean definition.

If such an annotation contains no name value or for any other detected component (such as those discovered by custom filters), the default bean name generator returns the uncapitalized non-qualified class name. For example, if the following component classes were detected, the names would be myMovieLister and movieFinderImpl:

Java
@Service("myMovieLister")
public class SimpleMovieLister {
    // ...
}
Kotlin
@Service("myMovieLister")
class SimpleMovieLister {
    // ...
}
Java
@Repository
public class MovieFinderImpl implements MovieFinder {
    // ...
}
Kotlin
@Repository
class MovieFinderImpl : MovieFinder {
    // ...
}
If you do not want to rely on the default bean-naming strategy, you can provide a custom bean-naming strategy. First, implement the BeanNameGenerator interface, and be sure to include a default no-arg constructor. Then, provide the fully qualified class name when configuring the scanner, as the following example annotation and bean definition show:
Java
@Configuration
@ComponentScan(basePackages = "org.example", nameGenerator = MyNameGenerator.class)
public class AppConfig {
    // ...
}
Kotlin
@Configuration
@ComponentScan(basePackages = ["org.example"], nameGenerator = MyNameGenerator::class)
class AppConfig {
    // ...
}
<beans>
    <context:component-scan base-package="org.example"
        name-generator="org.example.MyNameGenerator" />
</beans>

As a general rule, consider specifying the name with the annotation whenever other components may be making explicit references to it. On the other hand, the auto-generated names are adequate whenever the container is responsible for wiring.

Providing a Scope for Autodetected Components

As with Spring-managed components in general, the default and most common scope for autodetected components is singleton. However, sometimes you need a different scope that can be specified by the @Scope annotation. You can provide the name of the scope within the annotation, as the following example shows:

Java
@Scope("prototype")
@Repository
public class MovieFinderImpl implements MovieFinder {
    // ...
}
Kotlin
@Scope("prototype")
@Repository
class MovieFinderImpl : MovieFinder {
    // ...
}
@Scope annotations are only introspected on the concrete bean class (for annotated components) or the factory method (for @Bean methods). In contrast to XML bean definitions, there is no notion of bean definition inheritance, and inheritance hierarchies at the class level are irrelevant for metadata purposes.

For details on web-specific scopes such as “request” or “session” in a Spring context, see Request, Session, Application, 和 WebSocket 作用域(Request, Session, Application, and WebSocket Scopes). As with the pre-built annotations for those scopes, you may also compose your own scoping annotations by using Spring’s meta-annotation approach: for example, a custom annotation meta-annotated with @Scope("prototype"), possibly also declaring a custom scoped-proxy mode.

To provide a custom strategy for scope resolution rather than relying on the annotation-based approach, you can implement the ScopeMetadataResolver interface. Be sure to include a default no-arg constructor. Then you can provide the fully qualified class name when configuring the scanner, as the following example of both an annotation and a bean definition shows:
Java
@Configuration
@ComponentScan(basePackages = "org.example", scopeResolver = MyScopeResolver.class)
public class AppConfig {
    // ...
}
Kotlin
@Configuration
@ComponentScan(basePackages = ["org.example"], scopeResolver = MyScopeResolver::class)
class AppConfig {
    // ...
}
<beans>
    <context:component-scan base-package="org.example" scope-resolver="org.example.MyScopeResolver"/>
</beans>

When using certain non-singleton scopes, it may be necessary to generate proxies for the scoped objects. The reasoning is described in Scoped Beans as Dependencies. For this purpose, a scoped-proxy attribute is available on the component-scan element. The three possible values are: no, interfaces, and targetClass. For example, the following configuration results in standard JDK dynamic proxies:

Java
@Configuration
@ComponentScan(basePackages = "org.example", scopedProxy = ScopedProxyMode.INTERFACES)
public class AppConfig {
    // ...
}
Kotlin
@Configuration
@ComponentScan(basePackages = ["org.example"], scopedProxy = ScopedProxyMode.INTERFACES)
class AppConfig {
    // ...
}
<beans>
    <context:component-scan base-package="org.example" scoped-proxy="interfaces"/>
</beans>

Providing Qualifier Metadata with Annotations

The @Qualifier annotation is discussed in Fine-tuning Annotation-based Autowiring with Qualifiers. The examples in that section demonstrate the use of the @Qualifier annotation and custom qualifier annotations to provide fine-grained control when you resolve autowire candidates. Because those examples were based on XML bean definitions, the qualifier metadata was provided on the candidate bean definitions by using the qualifier or meta child elements of the bean element in the XML. When relying upon classpath scanning for auto-detection of components, you can provide the qualifier metadata with type-level annotations on the candidate class. The following three examples demonstrate this technique:

Java
@Component
@Qualifier("Action")
public class ActionMovieCatalog implements MovieCatalog {
    // ...
}
Kotlin
@Component
@Qualifier("Action")
class ActionMovieCatalog : MovieCatalog
Java
@Component
@Genre("Action")
public class ActionMovieCatalog implements MovieCatalog {
    // ...
}
Kotlin
@Component
@Genre("Action")
class ActionMovieCatalog : MovieCatalog {
    // ...
}
Java
@Component
@Offline
public class CachingMovieCatalog implements MovieCatalog {
    // ...
}
Kotlin
@Component
@Offline
class CachingMovieCatalog : MovieCatalog {
    // ...
}
As with most annotation-based alternatives, keep in mind that the annotation metadata is bound to the class definition itself, while the use of XML allows for multiple beans of the same type to provide variations in their qualifier metadata, because that metadata is provided per-instance rather than per-class.

Generating an Index of Candidate Components

While classpath scanning is very fast, it is possible to improve the startup performance of large applications by creating a static list of candidates at compilation time. In this mode, all modules that are target of component scan must use this mechanism.

Your existing @ComponentScan or <context:component-scan directives must stay as is to request the context to scan candidates in certain packages. When the ApplicationContext detects such an index, it automatically uses it rather than scanning the classpath.

To generate the index, add an additional dependency to each module that contains components that are targets for component scan directives. The following example shows how to do so with Maven:

<dependencies>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-context-indexer</artifactId>
        <version>{spring-version}</version>
        <optional>true</optional>
    </dependency>
</dependencies>

With Gradle 4.5 and earlier, the dependency should be declared in the compileOnly configuration, as shown in the following example:

dependencies {
    compileOnly "org.springframework:spring-context-indexer:{spring-version}"
}

With Gradle 4.6 and later, the dependency should be declared in the annotationProcessor configuration, as shown in the following example:

dependencies {
    annotationProcessor "org.springframework:spring-context-indexer:{spring-version}"
}

That process generates a META-INF/spring.components file that is included in the jar file.

When working with this mode in your IDE, the spring-context-indexer must be registered as an annotation processor to make sure the index is up-to-date when candidate components are updated.
The index is enabled automatically when a META-INF/spring.components is found on the classpath. If an index is partially available for some libraries (or use cases) but could not be built for the whole application, you can fallback to a regular classpath arrangement (as though no index was present at all) by setting spring.index.ignore to true, either as a system property or in a spring.properties file at the root of the classpath.

Using JSR 330 Standard Annotations

Starting with Spring 3.0, Spring offers support for JSR-330 standard annotations (Dependency Injection). Those annotations are scanned in the same way as the Spring annotations. To use them, you need to have the relevant jars in your classpath.

If you use Maven, the javax.inject artifact is available in the standard Maven repository ( https://repo1.maven.org/maven2/javax/inject/javax.inject/1/). You can add the following dependency to your file pom.xml:

<dependency>
    <groupId>javax.inject</groupId>
    <artifactId>javax.inject</artifactId>
    <version>1</version>
</dependency>

Dependency Injection with @Inject and @Named

Instead of @Autowired, you can use @javax.inject.Inject as follows:

Java
import javax.inject.Inject;

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Inject
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    public void listMovies() {
        this.movieFinder.findMovies(...);
        // ...
    }
}
Kotlin
import javax.inject.Inject

class SimpleMovieLister {

    @Inject
    lateinit var movieFinder: MovieFinder


    fun listMovies() {
        movieFinder.findMovies(...)
        // ...
    }
}

As with @Autowired, you can use @Inject at the field level, method level and constructor-argument level. Furthermore, you may declare your injection point as a Provider, allowing for on-demand access to beans of shorter scopes or lazy access to other beans through a Provider.get() call. The following example offers a variant of the preceding example:

Java
import javax.inject.Inject;
import javax.inject.Provider;

public class SimpleMovieLister {

    private Provider<MovieFinder> movieFinder;

    @Inject
    public void setMovieFinder(Provider<MovieFinder> movieFinder) {
        this.movieFinder = movieFinder;
    }

    public void listMovies() {
        this.movieFinder.get().findMovies(...);
        // ...
    }
}
Kotlin
import javax.inject.Inject

class SimpleMovieLister {

    @Inject
    lateinit var movieFinder: MovieFinder


    fun listMovies() {
        movieFinder.findMovies(...)
        // ...
    }
}

If you would like to use a qualified name for the dependency that should be injected, you should use the @Named annotation, as the following example shows:

Java
import javax.inject.Inject;
import javax.inject.Named;

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Inject
    public void setMovieFinder(@Named("main") MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}
Kotlin
import javax.inject.Inject
import javax.inject.Named

class SimpleMovieLister {

    private lateinit var movieFinder: MovieFinder

    @Inject
    fun setMovieFinder(@Named("main") movieFinder: MovieFinder) {
        this.movieFinder = movieFinder
    }

    // ...
}

As with @Autowired, @Inject can also be used with java.util.Optional or @Nullable. This is even more applicable here, since @Inject does not have a required attribute. The following pair of examples show how to use @Inject and @Nullable:

public class SimpleMovieLister {

    @Inject
    public void setMovieFinder(Optional<MovieFinder> movieFinder) {
        // ...
    }
}
Java
public class SimpleMovieLister {

    @Inject
    public void setMovieFinder(@Nullable MovieFinder movieFinder) {
        // ...
    }
}
Kotlin
class SimpleMovieLister {

    @Inject
    var movieFinder: MovieFinder? = null
}

@Named and @ManagedBean: Standard Equivalents to the @Component Annotation

Instead of @Component, you can use @javax.inject.Named or javax.annotation.ManagedBean, as the following example shows:

Java
import javax.inject.Inject;
import javax.inject.Named;

@Named("movieListener")  // @ManagedBean("movieListener") could be used as well
public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Inject
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}
Kotlin
import javax.inject.Inject
import javax.inject.Named

@Named("movieListener")  // @ManagedBean("movieListener") could be used as well
class SimpleMovieLister {

    @Inject
    lateinit var movieFinder: MovieFinder

    // ...
}

It is very common to use @Component without specifying a name for the component. @Named can be used in a similar fashion, as the following example shows:

Java
import javax.inject.Inject;
import javax.inject.Named;

@Named
public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Inject
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}
Kotlin
import javax.inject.Inject
import javax.inject.Named

@Named
class SimpleMovieLister {

    @Inject
    lateinit var movieFinder: MovieFinder

    // ...
}

When you use @Named or @ManagedBean, you can use component scanning in the exact same way as when you use Spring annotations, as the following example shows:

Java
@Configuration
@ComponentScan(basePackages = "org.example")
public class AppConfig  {
    // ...
}
Kotlin
@Configuration
@ComponentScan(basePackages = ["org.example"])
class AppConfig  {
    // ...
}
In contrast to @Component, the JSR-330 @Named and the JSR-250 ManagedBean annotations are not composable. You should use Spring’s stereotype model for building custom component annotations.

Limitations of JSR-330 Standard Annotations

When you work with standard annotations, you should know that some significant features are not available, as the following table shows:

Table 8. Spring component model elements versus JSR-330 variants
Spring javax.inject.* javax.inject restrictions / comments

@Autowired

@Inject

@Inject has no 'required' attribute. Can be used with Java 8’s Optional instead.

@Component

@Named / @ManagedBean

JSR-330 does not provide a composable model, only a way to identify named components.

@Scope("singleton")

@Singleton

The JSR-330 default scope is like Spring’s prototype. However, in order to keep it consistent with Spring’s general defaults, a JSR-330 bean declared in the Spring container is a singleton by default. In order to use a scope other than singleton, you should use Spring’s @Scope annotation. javax.inject also provides a @Scope annotation. Nevertheless, this one is only intended to be used for creating your own annotations.

@Qualifier

@Qualifier / @Named

javax.inject.Qualifier is just a meta-annotation for building custom qualifiers. Concrete String qualifiers (like Spring’s @Qualifier with a value) can be associated through javax.inject.Named.

@Value

-

no equivalent

@Required

-

no equivalent

@Lazy

-

no equivalent

ObjectFactory

Provider

javax.inject.Provider is a direct alternative to Spring’s ObjectFactory, only with a shorter get() method name. It can also be used in combination with Spring’s @Autowired or with non-annotated constructors and setter methods.

Java-based Container Configuration

This section covers how to use annotations in your Java code to configure the Spring container. It includes the following topics:

Basic Concepts: @Bean and @Configuration

The central artifacts in Spring’s new Java-configuration support are @Configuration-annotated classes and @Bean-annotated methods.

The @Bean annotation is used to indicate that a method instantiates, configures, and initializes a new object to be managed by the Spring IoC container. For those familiar with Spring’s <beans/> XML configuration, the @Bean annotation plays the same role as the <bean/> element. You can use @Bean-annotated methods with any Spring @Component. However, they are most often used with @Configuration beans.

Annotating a class with @Configuration indicates that its primary purpose is as a source of bean definitions. Furthermore, @Configuration classes let inter-bean dependencies be defined by calling other @Bean methods in the same class. The simplest possible @Configuration class reads as follows:

Java
@Configuration
public class AppConfig {

    @Bean
    public MyService myService() {
        return new MyServiceImpl();
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun myService(): MyService {
        return MyServiceImpl()
    }
}

The preceding AppConfig class is equivalent to the following Spring <beans/> XML:

<beans>
    <bean id="myService" class="com.acme.services.MyServiceImpl"/>
</beans>
Full @Configuration vs “lite” @Bean mode?

When @Bean methods are declared within classes that are not annotated with @Configuration, they are referred to as being processed in a “lite” mode. Bean methods declared in a @Component or even in a plain old class are considered to be “lite”, with a different primary purpose of the containing class and a @Bean method being a sort of bonus there. For example, service components may expose management views to the container through an additional @Bean method on each applicable component class. In such scenarios, @Bean methods are a general-purpose factory method mechanism.

Unlike full @Configuration, lite @Bean methods cannot declare inter-bean dependencies. Instead, they operate on their containing component’s internal state and, optionally, on arguments that they may declare. Such a @Bean method should therefore not invoke other @Bean methods. Each such method is literally only a factory method for a particular bean reference, without any special runtime semantics. The positive side-effect here is that no CGLIB subclassing has to be applied at runtime, so there are no limitations in terms of class design (that is, the containing class may be final and so forth).

In common scenarios, @Bean methods are to be declared within @Configuration classes, ensuring that “full” mode is always used and that cross-method references therefore get redirected to the container’s lifecycle management. This prevents the same @Bean method from accidentally being invoked through a regular Java call, which helps to reduce subtle bugs that can be hard to track down when operating in “lite” mode.

The @Bean and @Configuration annotations are discussed in depth in the following sections. First, however, we cover the various ways of creating a spring container using by Java-based configuration.

Instantiating the Spring Container by Using AnnotationConfigApplicationContext

The following sections document Spring’s AnnotationConfigApplicationContext, introduced in Spring 3.0. This versatile ApplicationContext implementation is capable of accepting not only @Configuration classes as input but also plain @Component classes and classes annotated with JSR-330 metadata.

When @Configuration classes are provided as input, the @Configuration class itself is registered as a bean definition and all declared @Bean methods within the class are also registered as bean definitions.

When @Component and JSR-330 classes are provided, they are registered as bean definitions, and it is assumed that DI metadata such as @Autowired or @Inject are used within those classes where necessary.

Simple Construction

In much the same way that Spring XML files are used as input when instantiating a ClassPathXmlApplicationContext, you can use @Configuration classes as input when instantiating an AnnotationConfigApplicationContext. This allows for completely XML-free usage of the Spring container, as the following example shows:

Java
public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
    MyService myService = ctx.getBean(MyService.class);
    myService.doStuff();
}
Kotlin
import org.springframework.beans.factory.getBean

fun main() {
    val ctx = AnnotationConfigApplicationContext(AppConfig::class.java)
    val myService = ctx.getBean<MyService>()
    myService.doStuff()
}

As mentioned earlier, AnnotationConfigApplicationContext is not limited to working only with @Configuration classes. Any @Component or JSR-330 annotated class may be supplied as input to the constructor, as the following example shows:

Java
public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext(MyServiceImpl.class, Dependency1.class, Dependency2.class);
    MyService myService = ctx.getBean(MyService.class);
    myService.doStuff();
}
Kotlin
import org.springframework.beans.factory.getBean

fun main() {
    val ctx = AnnotationConfigApplicationContext(MyServiceImpl::class.java, Dependency1::class.java, Dependency2::class.java)
    val myService = ctx.getBean<MyService>()
    myService.doStuff()
}

The preceding example assumes that MyServiceImpl, Dependency1, and Dependency2 use Spring dependency injection annotations such as @Autowired.

Building the Container Programmatically by Using register(Class<?>…​)

You can instantiate an AnnotationConfigApplicationContext by using a no-arg constructor and then configure it by using the register() method. This approach is particularly useful when programmatically building an AnnotationConfigApplicationContext. The following example shows how to do so:

Java
public static void main(String[] args) {
    AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
    ctx.register(AppConfig.class, OtherConfig.class);
    ctx.register(AdditionalConfig.class);
    ctx.refresh();
    MyService myService = ctx.getBean(MyService.class);
    myService.doStuff();
}
Kotlin
import org.springframework.beans.factory.getBean

fun main() {
    val ctx = AnnotationConfigApplicationContext()
    ctx.register(AppConfig::class.java, OtherConfig::class.java)
    ctx.register(AdditionalConfig::class.java)
    ctx.refresh()
    val myService = ctx.getBean<MyService>()
    myService.doStuff()
}
Enabling Component Scanning with scan(String…​)

To enable component scanning, you can annotate your @Configuration class as follows:

Java
@Configuration
@ComponentScan(basePackages = "com.acme") (1)
public class AppConfig  {
    ...
}
1 This annotation enables component scanning.
Kotlin
@Configuration
@ComponentScan(basePackages = ["com.acme"]) (1)
class AppConfig  {
    // ...
}
1 This annotation enables component scanning.

Experienced Spring users may be familiar with the XML declaration equivalent from Spring’s context: namespace, shown in the following example:

<beans>
    <context:component-scan base-package="com.acme"/>
</beans>

In the preceding example, the com.acme package is scanned to look for any @Component-annotated classes, and those classes are registered as Spring bean definitions within the container. AnnotationConfigApplicationContext exposes the scan(String…​) method to allow for the same component-scanning functionality, as the following example shows:

Java
public static void main(String[] args) {
    AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
    ctx.scan("com.acme");
    ctx.refresh();
    MyService myService = ctx.getBean(MyService.class);
}
Kotlin
fun main() {
    val ctx = AnnotationConfigApplicationContext()
    ctx.scan("com.acme")
    ctx.refresh()
    val myService = ctx.getBean<MyService>()
}
Remember that @Configuration classes are meta-annotated with @Component, so they are candidates for component-scanning. In the preceding example, assuming that AppConfig is declared within the com.acme package (or any package underneath), it is picked up during the call to scan(). Upon refresh(), all its @Bean methods are processed and registered as bean definitions within the container.
Support for Web Applications with AnnotationConfigWebApplicationContext

A WebApplicationContext variant of AnnotationConfigApplicationContext is available with AnnotationConfigWebApplicationContext. You can use this implementation when configuring the Spring ContextLoaderListener servlet listener, Spring MVC DispatcherServlet, and so forth. The following web.xml snippet configures a typical Spring MVC web application (note the use of the contextClass context-param and init-param):

<web-app>
    <!-- Configure ContextLoaderListener to use AnnotationConfigWebApplicationContext
        instead of the default XmlWebApplicationContext -->
    <context-param>
        <param-name>contextClass</param-name>
        <param-value>
            org.springframework.web.context.support.AnnotationConfigWebApplicationContext
        </param-value>
    </context-param>

    <!-- Configuration locations must consist of one or more comma- or space-delimited
        fully-qualified @Configuration classes. Fully-qualified packages may also be
        specified for component-scanning -->
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>com.acme.AppConfig</param-value>
    </context-param>

    <!-- Bootstrap the root application context as usual using ContextLoaderListener -->
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <!-- Declare a Spring MVC DispatcherServlet as usual -->
    <servlet>
        <servlet-name>dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <!-- Configure DispatcherServlet to use AnnotationConfigWebApplicationContext
            instead of the default XmlWebApplicationContext -->
        <init-param>
            <param-name>contextClass</param-name>
            <param-value>
                org.springframework.web.context.support.AnnotationConfigWebApplicationContext
            </param-value>
        </init-param>
        <!-- Again, config locations must consist of one or more comma- or space-delimited
            and fully-qualified @Configuration classes -->
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>com.acme.web.MvcConfig</param-value>
        </init-param>
    </servlet>

    <!-- map all requests for /app/* to the dispatcher servlet -->
    <servlet-mapping>
        <servlet-name>dispatcher</servlet-name>
        <url-pattern>/app/*</url-pattern>
    </servlet-mapping>
</web-app>

Using the @Bean Annotation

@Bean is a method-level annotation and a direct analog of the XML <bean/> element. The annotation supports some of the attributes offered by <bean/>, such as: * init-method * destroy-method * autowiring * name.

You can use the @Bean annotation in a @Configuration-annotated or in a @Component-annotated class.

Declaring a Bean

To declare a bean, you can annotate a method with the @Bean annotation. You use this method to register a bean definition within an ApplicationContext of the type specified as the method’s return value. By default, the bean name is the same as the method name. The following example shows a @Bean method declaration:

Java
@Configuration
public class AppConfig {

    @Bean
    public TransferServiceImpl transferService() {
        return new TransferServiceImpl();
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun transferService() = TransferServiceImpl()
}

The preceding configuration is exactly equivalent to the following Spring XML:

<beans>
    <bean id="transferService" class="com.acme.TransferServiceImpl"/>
</beans>

Both declarations make a bean named transferService available in the ApplicationContext, bound to an object instance of type TransferServiceImpl, as the following text image shows:

transferService -> com.acme.TransferServiceImpl

You can also declare your @Bean method with an interface (or base class) return type, as the following example shows:

Java
@Configuration
public class AppConfig {

    @Bean
    public TransferService transferService() {
        return new TransferServiceImpl();
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun transferService(): TransferService {
        return TransferServiceImpl()
    }
}

However, this limits the visibility for advance type prediction to the specified interface type (TransferService). Then, with the full type (TransferServiceImpl) known to the container only once, the affected singleton bean has been instantiated. Non-lazy singleton beans get instantiated according to their declaration order, so you may see different type matching results depending on when another component tries to match by a non-declared type (such as @Autowired TransferServiceImpl, which resolves only once the transferService bean has been instantiated).

If you consistently refer to your types by a declared service interface, your @Bean return types may safely join that design decision. However, for components that implement several interfaces or for components potentially referred to by their implementation type, it is safer to declare the most specific return type possible (at least as specific as required by the injection points that refer to your bean).
Bean Dependencies

A @Bean-annotated method can have an arbitrary number of parameters that describe the dependencies required to build that bean. For instance, if our TransferService requires an AccountRepository, we can materialize that dependency with a method parameter, as the following example shows:

Java
@Configuration
public class AppConfig {

    @Bean
    public TransferService transferService(AccountRepository accountRepository) {
        return new TransferServiceImpl(accountRepository);
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun transferService(accountRepository: AccountRepository): TransferService {
        return TransferServiceImpl(accountRepository)
    }
}

The resolution mechanism is pretty much identical to constructor-based dependency injection. See the relevant section for more details.

Receiving Lifecycle Callbacks

Any classes defined with the @Bean annotation support the regular lifecycle callbacks and can use the @PostConstruct and @PreDestroy annotations from JSR-250. See JSR-250 annotations for further details.

The regular Spring lifecycle callbacks are fully supported as well. If a bean implements InitializingBean, DisposableBean, or Lifecycle, their respective methods are called by the container.

The standard set of *Aware interfaces (such as BeanFactoryAware, BeanNameAware, MessageSourceAware, ApplicationContextAware, and so on) are also fully supported.

The @Bean annotation supports specifying arbitrary initialization and destruction callback methods, much like Spring XML’s init-method and destroy-method attributes on the bean element, as the following example shows:

Java
public class BeanOne {

    public void init() {
        // initialization logic
    }
}

public class BeanTwo {

    public void cleanup() {
        // destruction logic
    }
}

@Configuration
public class AppConfig {

    @Bean(initMethod = "init")
    public BeanOne beanOne() {
        return new BeanOne();
    }

    @Bean(destroyMethod = "cleanup")
    public BeanTwo beanTwo() {
        return new BeanTwo();
    }
}
Kotlin
class BeanOne {

    fun init() {
        // initialization logic
    }
}

class BeanTwo {

    fun cleanup() {
        // destruction logic
    }
}

@Configuration
class AppConfig {

    @Bean(initMethod = "init")
    fun beanOne() = BeanOne()

    @Bean(destroyMethod = "cleanup")
    fun beanTwo() = BeanTwo()
}

By default, beans defined with Java configuration that have a public close or shutdown method are automatically enlisted with a destruction callback. If you have a public close or shutdown method and you do not wish for it to be called when the container shuts down, you can add @Bean(destroyMethod="") to your bean definition to disable the default (inferred) mode.

You may want to do that by default for a resource that you acquire with JNDI, as its lifecycle is managed outside the application. In particular, make sure to always do it for a DataSource, as it is known to be problematic on Java EE application servers.

The following example shows how to prevent an automatic destruction callback for a DataSource:

Java
@Bean(destroyMethod="")
public DataSource dataSource() throws NamingException {
    return (DataSource) jndiTemplate.lookup("MyDS");
}
Kotlin
@Bean(destroyMethod = "")
fun dataSource(): DataSource {
    return jndiTemplate.lookup("MyDS") as DataSource
}

Also, with @Bean methods, you typically use programmatic JNDI lookups, either by using Spring’s JndiTemplate or JndiLocatorDelegate helpers or straight JNDI InitialContext usage but not the JndiObjectFactoryBean variant (which would force you to declare the return type as the FactoryBean type instead of the actual target type, making it harder to use for cross-reference calls in other @Bean methods that intend to refer to the provided resource here).

In the case of BeanOne from the example above the preceding note, it would be equally valid to call the init() method directly during construction, as the following example shows:

Java
@Configuration
public class AppConfig {

    @Bean
    public BeanOne beanOne() {
        BeanOne beanOne = new BeanOne();
        beanOne.init();
        return beanOne;
    }

    // ...
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun beanOne() = BeanOne().apply {
        init()
    }

    // ...
}
When you work directly in Java, you can do anything you like with your objects and do not always need to rely on the container lifecycle.
Specifying Bean Scope

Spring includes the @Scope annotation so that you can specify the scope of a bean.

Using the @Scope Annotation

You can specify that your beans defined with the @Bean annotation should have a specific scope. You can use any of the standard scopes specified in the Bean Scopes section.

The default scope is singleton, but you can override this with the @Scope annotation, as the following example shows:

Java
@Configuration
public class MyConfiguration {

    @Bean
    @Scope("prototype")
    public Encryptor encryptor() {
        // ...
    }
}
Kotlin
@Configuration
class MyConfiguration {

    @Bean
    @Scope("prototype")
    fun encryptor(): Encryptor {
        // ...
    }
}
@Scope and scoped-proxy

Spring offers a convenient way of working with scoped dependencies through scoped proxies. The easiest way to create such a proxy when using the XML configuration is the <aop:scoped-proxy/> element. Configuring your beans in Java with a @Scope annotation offers equivalent support with the proxyMode attribute. The default is no proxy (ScopedProxyMode.NO), but you can specify ScopedProxyMode.TARGET_CLASS or ScopedProxyMode.INTERFACES.

If you port the scoped proxy example from the XML reference documentation (see scoped proxies) to our @Bean using Java, it resembles the following:

Java
// an HTTP Session-scoped bean exposed as a proxy
@Bean
@SessionScope
public UserPreferences userPreferences() {
    return new UserPreferences();
}

@Bean
public Service userService() {
    UserService service = new SimpleUserService();
    // a reference to the proxied userPreferences bean
    service.setUserPreferences(userPreferences());
    return service;
}
Kotlin
// an HTTP Session-scoped bean exposed as a proxy
@Bean
@SessionScope
fun userPreferences() = UserPreferences()

@Bean
fun userService(): Service {
    return SimpleUserService().apply {
        // a reference to the proxied userPreferences bean
        setUserPreferences(userPreferences()
    }
}
Customizing Bean Naming

By default, configuration classes use a @Bean method’s name as the name of the resulting bean. This functionality can be overridden, however, with the name attribute, as the following example shows:

Java
@Configuration
public class AppConfig {

    @Bean(name = "myThing")
    public Thing thing() {
        return new Thing();
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean("myThing")
    fun thing() = Thing()
}
Bean Aliasing

As discussed in Bean的命名(Naming Beans), it is sometimes desirable to give a single bean multiple names, otherwise known as bean aliasing. The name attribute of the @Bean annotation accepts a String array for this purpose. The following example shows how to set a number of aliases for a bean:

Java
@Configuration
public class AppConfig {

    @Bean({"dataSource", "subsystemA-dataSource", "subsystemB-dataSource"})
    public DataSource dataSource() {
        // instantiate, configure and return DataSource bean...
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean("dataSource", "subsystemA-dataSource", "subsystemB-dataSource")
    fun dataSource(): DataSource {
        // instantiate, configure and return DataSource bean...
    }
}
Bean Description

Sometimes, it is helpful to provide a more detailed textual description of a bean. This can be particularly useful when beans are exposed (perhaps through JMX) for monitoring purposes.

To add a description to a @Bean, you can use the @Description annotation, as the following example shows:

Java
@Configuration
public class AppConfig {

    @Bean
    @Description("Provides a basic example of a bean")
    public Thing thing() {
        return new Thing();
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    @Description("Provides a basic example of a bean")
    fun thing() = Thing()
}

Using the @Configuration annotation

@Configuration is a class-level annotation indicating that an object is a source of bean definitions. @Configuration classes declare beans through public @Bean annotated methods. Calls to @Bean methods on @Configuration classes can also be used to define inter-bean dependencies. See Basic Concepts: @Bean and @Configuration for a general introduction.

Injecting Inter-bean Dependencies

When beans have dependencies on one another, expressing that dependency is as simple as having one bean method call another, as the following example shows:

Java
@Configuration
public class AppConfig {

    @Bean
    public BeanOne beanOne() {
        return new BeanOne(beanTwo());
    }

    @Bean
    public BeanTwo beanTwo() {
        return new BeanTwo();
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun beanOne() = BeanOne(beanTwo())

    @Bean
    fun beanTwo() = BeanTwo()
}

In the preceding example, beanOne receives a reference to beanTwo through constructor injection.

This method of declaring inter-bean dependencies works only when the @Bean method is declared within a @Configuration class. You cannot declare inter-bean dependencies by using plain @Component classes.
Lookup Method Injection

As noted earlier, lookup method injection is an advanced feature that you should use rarely. It is useful in cases where a singleton-scoped bean has a dependency on a prototype-scoped bean. Using Java for this type of configuration provides a natural means for implementing this pattern. The following example shows how to use lookup method injection:

Java
public abstract class CommandManager {
    public Object process(Object commandState) {
        // grab a new instance of the appropriate Command interface
        Command command = createCommand();
        // set the state on the (hopefully brand new) Command instance
        command.setState(commandState);
        return command.execute();
    }

    // okay... but where is the implementation of this method?
    protected abstract Command createCommand();
}
Kotlin
abstract class CommandManager {
    fun process(commandState: Any): Any {
        // grab a new instance of the appropriate Command interface
        val command = createCommand()
        // set the state on the (hopefully brand new) Command instance
        command.setState(commandState)
        return command.execute()
    }

    // okay... but where is the implementation of this method?
    protected abstract fun createCommand(): Command
}

By using Java configuration, you can create a subclass of CommandManager where the abstract createCommand() method is overridden in such a way that it looks up a new (prototype) command object. The following example shows how to do so:

Java
@Bean
@Scope("prototype")
public AsyncCommand asyncCommand() {
    AsyncCommand command = new AsyncCommand();
    // inject dependencies here as required
    return command;
}

@Bean
public CommandManager commandManager() {
    // return new anonymous implementation of CommandManager with createCommand()
    // overridden to return a new prototype Command object
    return new CommandManager() {
        protected Command createCommand() {
            return asyncCommand();
        }
    }
}
Kotlin
@Bean
@Scope("prototype")
fun asyncCommand(): AsyncCommand {
    val command = AsyncCommand()
    // inject dependencies here as required
    return command
}

@Bean
fun commandManager(): CommandManager {
    // return new anonymous implementation of CommandManager with createCommand()
    // overridden to return a new prototype Command object
    return object : CommandManager() {
        override fun createCommand(): Command {
            return asyncCommand()
        }
    }
}
Further Information About How Java-based Configuration Works Internally

Consider the following example, which shows a @Bean annotated method being called twice:

Java
@Configuration
public class AppConfig {

    @Bean
    public ClientService clientService1() {
        ClientServiceImpl clientService = new ClientServiceImpl();
        clientService.setClientDao(clientDao());
        return clientService;
    }

    @Bean
    public ClientService clientService2() {
        ClientServiceImpl clientService = new ClientServiceImpl();
        clientService.setClientDao(clientDao());
        return clientService;
    }

    @Bean
    public ClientDao clientDao() {
        return new ClientDaoImpl();
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun clientService1(): ClientService {
        return ClientServiceImpl().apply {
            clientDao = clientDao()
        }
    }

    @Bean
    fun clientService2(): ClientService {
        return ClientServiceImpl().apply {
            clientDao = clientDao()
        }
    }

    @Bean
    fun clientDao(): ClientDao {
        return ClientDaoImpl()
    }
}

clientDao() has been called once in clientService1() and once in clientService2(). Since this method creates a new instance of ClientDaoImpl and returns it, you would normally expect to have two instances (one for each service). That definitely would be problematic: In Spring, instantiated beans have a singleton scope by default. This is where the magic comes in: All @Configuration classes are subclassed at startup-time with CGLIB. In the subclass, the child method checks the container first for any cached (scoped) beans before it calls the parent method and creates a new instance.

The behavior could be different according to the scope of your bean. We are talking about singletons here.

As of Spring 3.2, it is no longer necessary to add CGLIB to your classpath because CGLIB classes have been repackaged under org.springframework.cglib and included directly within the spring-core JAR.

There are a few restrictions due to the fact that CGLIB dynamically adds features at startup-time. In particular, configuration classes must not be final. However, as of 4.3, any constructors are allowed on configuration classes, including the use of @Autowired or a single non-default constructor declaration for default injection.

If you prefer to avoid any CGLIB-imposed limitations, consider declaring your @Bean methods on non-@Configuration classes (for example, on plain @Component classes instead). Cross-method calls between @Bean methods are not then intercepted, so you have to exclusively rely on dependency injection at the constructor or method level there.

Composing Java-based Configurations

Spring’s Java-based configuration feature lets you compose annotations, which can reduce the complexity of your configuration.

Using the @Import Annotation

Much as the <import/> element is used within Spring XML files to aid in modularizing configurations, the @Import annotation allows for loading @Bean definitions from another configuration class, as the following example shows:

Java
@Configuration
public class ConfigA {

    @Bean
    public A a() {
        return new A();
    }
}

@Configuration
@Import(ConfigA.class)
public class ConfigB {

    @Bean
    public B b() {
        return new B();
    }
}
Kotlin
@Configuration
class ConfigA {

    @Bean
    fun a() = A()
}

@Configuration
@Import(ConfigA::class)
class ConfigB {

    @Bean
    fun b() = B()
}

Now, rather than needing to specify both ConfigA.class and ConfigB.class when instantiating the context, only ConfigB needs to be supplied explicitly, as the following example shows:

Java
public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext(ConfigB.class);

    // now both beans A and B will be available...
    A a = ctx.getBean(A.class);
    B b = ctx.getBean(B.class);
}
Kotlin
import org.springframework.beans.factory.getBean

fun main() {
    val ctx = AnnotationConfigApplicationContext(ConfigB::class.java)

    // now both beans A and B will be available...
    val a = ctx.getBean<A>()
    val b = ctx.getBean<B>()
}

This approach simplifies container instantiation, as only one class needs to be dealt with, rather than requiring you to remember a potentially large number of @Configuration classes during construction.

As of Spring Framework 4.2, @Import also supports references to regular component classes, analogous to the AnnotationConfigApplicationContext.register method. This is particularly useful if you want to avoid component scanning, by using a few configuration classes as entry points to explicitly define all your components.
Injecting Dependencies on Imported @Bean Definitions

The preceding example works but is simplistic. In most practical scenarios, beans have dependencies on one another across configuration classes. When using XML, this is not an issue, because no compiler is involved, and you can declare ref="someBean" and trust Spring to work it out during container initialization. When using @Configuration classes, the Java compiler places constraints on the configuration model, in that references to other beans must be valid Java syntax.

Fortunately, solving this problem is simple. As we already discussed, a @Bean method can have an arbitrary number of parameters that describe the bean dependencies. Consider the following more real-world scenario with several @Configuration classes, each depending on beans declared in the others:

Java
@Configuration
public class ServiceConfig {

    @Bean
    public TransferService transferService(AccountRepository accountRepository) {
        return new TransferServiceImpl(accountRepository);
    }
}

@Configuration
public class RepositoryConfig {

    @Bean
    public AccountRepository accountRepository(DataSource dataSource) {
        return new JdbcAccountRepository(dataSource);
    }
}

@Configuration
@Import({ServiceConfig.class, RepositoryConfig.class})
public class SystemTestConfig {

    @Bean
    public DataSource dataSource() {
        // return new DataSource
    }
}

public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
    // everything wires up across configuration classes...
    TransferService transferService = ctx.getBean(TransferService.class);
    transferService.transfer(100.00, "A123", "C456");
}
Kotlin
import org.springframework.beans.factory.getBean

@Configuration
class ServiceConfig {

    @Bean
    fun transferService(accountRepository: AccountRepository): TransferService {
        return TransferServiceImpl(accountRepository)
    }
}

@Configuration
class RepositoryConfig {

    @Bean
    fun accountRepository(dataSource: DataSource): AccountRepository {
        return JdbcAccountRepository(dataSource)
    }
}

@Configuration
@Import(ServiceConfig::class, RepositoryConfig::class)
class SystemTestConfig {

    @Bean
    fun dataSource(): DataSource {
        // return new DataSource
    }
}


fun main() {
    val ctx = AnnotationConfigApplicationContext(SystemTestConfig::class.java)
    // everything wires up across configuration classes...
    val transferService = ctx.getBean<TransferService>()
    transferService.transfer(100.00, "A123", "C456")
}

There is another way to achieve the same result. Remember that @Configuration classes are ultimately only another bean in the container: This means that they can take advantage of @Autowired and @Value injection and other features the same as any other bean.

Make sure that the dependencies you inject that way are of the simplest kind only. @Configuration classes are processed quite early during the initialization of the context, and forcing a dependency to be injected this way may lead to unexpected early initialization. Whenever possible, resort to parameter-based injection, as in the preceding example.

Also, be particularly careful with BeanPostProcessor and BeanFactoryPostProcessor definitions through @Bean. Those should usually be declared as static @Bean methods, not triggering the instantiation of their containing configuration class. Otherwise, @Autowired and @Value do not work on the configuration class itself, since it is being created as a bean instance too early.

The following example shows how one bean can be autowired to another bean:

Java
@Configuration
public class ServiceConfig {

    @Autowired
    private AccountRepository accountRepository;

    @Bean
    public TransferService transferService() {
        return new TransferServiceImpl(accountRepository);
    }
}

@Configuration
public class RepositoryConfig {

    private final DataSource dataSource;

    public RepositoryConfig(DataSource dataSource) {
        this.dataSource = dataSource;
    }

    @Bean
    public AccountRepository accountRepository() {
        return new JdbcAccountRepository(dataSource);
    }
}

@Configuration
@Import({ServiceConfig.class, RepositoryConfig.class})
public class SystemTestConfig {

    @Bean
    public DataSource dataSource() {
        // return new DataSource
    }
}

public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
    // everything wires up across configuration classes...
    TransferService transferService = ctx.getBean(TransferService.class);
    transferService.transfer(100.00, "A123", "C456");
}
Kotlin
import org.springframework.beans.factory.getBean

@Configuration
class ServiceConfig {

    @Autowired
    lateinit var accountRepository: AccountRepository

    @Bean
    fun transferService(): TransferService {
        return TransferServiceImpl(accountRepository)
    }
}

@Configuration
class RepositoryConfig(private val dataSource: DataSource) {

    @Bean
    fun accountRepository(): AccountRepository {
        return JdbcAccountRepository(dataSource)
    }
}

@Configuration
@Import(ServiceConfig::class, RepositoryConfig::class)
class SystemTestConfig {

    @Bean
    fun dataSource(): DataSource {
        // return new DataSource
    }
}

fun main() {
    val ctx = AnnotationConfigApplicationContext(SystemTestConfig::class.java)
    // everything wires up across configuration classes...
    val transferService = ctx.getBean<TransferService>()
    transferService.transfer(100.00, "A123", "C456")
}
Constructor injection in @Configuration classes is only supported as of Spring Framework 4.3. Note also that there is no need to specify @Autowired if the target bean defines only one constructor.
Fully-qualifying imported beans for ease of navigation

In the preceding scenario, using @Autowired works well and provides the desired modularity, but determining exactly where the autowired bean definitions are declared is still somewhat ambiguous. For example, as a developer looking at ServiceConfig, how do you know exactly where the @Autowired AccountRepository bean is declared? It is not explicit in the code, and this may be just fine. Remember that the Spring Tool Suite provides tooling that can render graphs showing how everything is wired, which may be all you need. Also, your Java IDE can easily find all declarations and uses of the AccountRepository type and quickly show you the location of @Bean methods that return that type.

In cases where this ambiguity is not acceptable and you wish to have direct navigation from within your IDE from one @Configuration class to another, consider autowiring the configuration classes themselves. The following example shows how to do so:

Java
@Configuration
public class ServiceConfig {

    @Autowired
    private RepositoryConfig repositoryConfig;

    @Bean
    public TransferService transferService() {
        // navigate 'through' the config class to the @Bean method!
        return new TransferServiceImpl(repositoryConfig.accountRepository());
    }
}
Kotlin
@Configuration
class ServiceConfig {

    @Autowired
    private lateinit var repositoryConfig: RepositoryConfig

    @Bean
    fun transferService(): TransferService {
        // navigate 'through' the config class to the @Bean method!
        return TransferServiceImpl(repositoryConfig.accountRepository())
    }
}

In the preceding situation, where AccountRepository is defined is completely explicit. However, ServiceConfig is now tightly coupled to RepositoryConfig. That is the tradeoff. This tight coupling can be somewhat mitigated by using interface-based or abstract class-based @Configuration classes. Consider the following example:

Java
@Configuration
public class ServiceConfig {

    @Autowired
    private RepositoryConfig repositoryConfig;

    @Bean
    public TransferService transferService() {
        return new TransferServiceImpl(repositoryConfig.accountRepository());
    }
}

@Configuration
public interface RepositoryConfig {

    @Bean
    AccountRepository accountRepository();
}

@Configuration
public class DefaultRepositoryConfig implements RepositoryConfig {

    @Bean
    public AccountRepository accountRepository() {
        return new JdbcAccountRepository(...);
    }
}

@Configuration
@Import({ServiceConfig.class, DefaultRepositoryConfig.class})  // import the concrete config!
public class SystemTestConfig {

    @Bean
    public DataSource dataSource() {
        // return DataSource
    }

}

public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
    TransferService transferService = ctx.getBean(TransferService.class);
    transferService.transfer(100.00, "A123", "C456");
}
Kotlin
import org.springframework.beans.factory.getBean

@Configuration
class ServiceConfig {

    @Autowired
    private lateinit var repositoryConfig: RepositoryConfig

    @Bean
    fun transferService(): TransferService {
        return TransferServiceImpl(repositoryConfig.accountRepository())
    }
}

@Configuration
interface RepositoryConfig {

    @Bean
    fun accountRepository(): AccountRepository
}

@Configuration
class DefaultRepositoryConfig : RepositoryConfig {

    @Bean
    fun accountRepository(): AccountRepository {
        return JdbcAccountRepository(...)
    }
}

@Configuration
@Import(ServiceConfig::class, DefaultRepositoryConfig::class)  // import the concrete config!
class SystemTestConfig {

    @Bean
    fun dataSource(): DataSource {
        // return DataSource
    }

}

fun main() {
    val ctx = AnnotationConfigApplicationContext(SystemTestConfig::class.java)
    val transferService = ctx.getBean<TransferService>()
    transferService.transfer(100.00, "A123", "C456")
}

Now ServiceConfig is loosely coupled with respect to the concrete DefaultRepositoryConfig, and built-in IDE tooling is still useful: You can easily get a type hierarchy of RepositoryConfig implementations. In this way, navigating @Configuration classes and their dependencies becomes no different than the usual process of navigating interface-based code.

If you want to influence the startup creation order of certain beans, consider declaring some of them as @Lazy (for creation on first access instead of on startup) or as @DependsOn certain other beans (making sure that specific other beans are created before the current bean, beyond what the latter’s direct dependencies imply).
Conditionally Include @Configuration Classes or @Bean Methods

It is often useful to conditionally enable or disable a complete @Configuration class or even individual @Bean methods, based on some arbitrary system state. One common example of this is to use the @Profile annotation to activate beans only when a specific profile has been enabled in the Spring Environment (see Bean Definition Profiles for details).

The @Profile annotation is actually implemented by using a much more flexible annotation called @Conditional. The @Conditional annotation indicates specific org.springframework.context.annotation.Condition implementations that should be consulted before a @Bean is registered.

Implementations of the Condition interface provide a matches(…​) method that returns true or false. For example, the following listing shows the actual Condition implementation used for @Profile:

Java
@Override
public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
    // Read the @Profile annotation attributes
    MultiValueMap<String, Object> attrs = metadata.getAllAnnotationAttributes(Profile.class.getName());
    if (attrs != null) {
        for (Object value : attrs.get("value")) {
            if (context.getEnvironment().acceptsProfiles(((String[]) value))) {
                return true;
            }
        }
        return false;
    }
    return true;
}
Kotlin
override fun matches(context: ConditionContext, metadata: AnnotatedTypeMetadata): Boolean {
    // Read the @Profile annotation attributes
    val attrs = metadata.getAllAnnotationAttributes(Profile::class.java.name)
    if (attrs != null) {
        for (value in attrs["value"]!!) {
            if (context.environment.acceptsProfiles(Profiles .of(*value as Array<String>))) {
                return true
            }
        }
        return false
    }
    return true
}

See the @Conditional javadoc for more detail.

Combining Java and XML Configuration

Spring’s @Configuration class support does not aim to be a 100% complete replacement for Spring XML. Some facilities, such as Spring XML namespaces, remain an ideal way to configure the container. In cases where XML is convenient or necessary, you have a choice: either instantiate the container in an “XML-centric” way by using, for example, ClassPathXmlApplicationContext, or instantiate it in a “Java-centric” way by using AnnotationConfigApplicationContext and the @ImportResource annotation to import XML as needed.

XML-centric Use of @Configuration Classes

It may be preferable to bootstrap the Spring container from XML and include @Configuration classes in an ad-hoc fashion. For example, in a large existing codebase that uses Spring XML, it is easier to create @Configuration classes on an as-needed basis and include them from the existing XML files. Later in this section, we cover the options for using @Configuration classes in this kind of “XML-centric” situation.

Declaring @Configuration classes as plain Spring <bean/> elements

Remember that @Configuration classes are ultimately bean definitions in the container. In this series examples, we create a @Configuration class named AppConfig and include it within system-test-config.xml as a <bean/> definition. Because <context:annotation-config/> is switched on, the container recognizes the @Configuration annotation and processes the @Bean methods declared in AppConfig properly.

The following example shows an ordinary configuration class in Java:

Java
@Configuration
public class AppConfig {

    @Autowired
    private DataSource dataSource;

    @Bean
    public AccountRepository accountRepository() {
        return new JdbcAccountRepository(dataSource);
    }

    @Bean
    public TransferService transferService() {
        return new TransferService(accountRepository());
    }
}
Kotlin
@Configuration
class AppConfig {

    @Autowired
    private lateinit var dataSource: DataSource

    @Bean
    fun accountRepository(): AccountRepository {
        return JdbcAccountRepository(dataSource)
    }

    @Bean
    fun transferService() = TransferService(accountRepository())
}

The following example shows part of a sample system-test-config.xml file:

<beans>
    <!-- enable processing of annotations such as @Autowired and @Configuration -->
    <context:annotation-config/>
    <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>

    <bean class="com.acme.AppConfig"/>

    <bean class="org.springframework.jdbc.datasource.DriverManagerDataSource">
        <property name="url" value="${jdbc.url}"/>
        <property name="username" value="${jdbc.username}"/>
        <property name="password" value="${jdbc.password}"/>
    </bean>
</beans>

The following example shows a possible jdbc.properties file:

jdbc.url=jdbc:hsqldb:hsql://localhost/xdb
jdbc.username=sa
jdbc.password=
Java
public static void main(String[] args) {
    ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath:/com/acme/system-test-config.xml");
    TransferService transferService = ctx.getBean(TransferService.class);
    // ...
}
Kotlin
fun main() {
    val ctx = ClassPathXmlApplicationContext("classpath:/com/acme/system-test-config.xml")
    val transferService = ctx.getBean<TransferService>()
    // ...
}
In system-test-config.xml file, the AppConfig <bean/> does not declare an id element. While it would be acceptable to do so, it is unnecessary, given that no other bean ever refers to it, and it is unlikely to be explicitly fetched from the container by name. Similarly, the DataSource bean is only ever autowired by type, so an explicit bean id is not strictly required.
Using <context:component-scan/> to pick up @Configuration classes

Because @Configuration is meta-annotated with @Component, @Configuration-annotated classes are automatically candidates for component scanning. Using the same scenario as describe in the previous example, we can redefine system-test-config.xml to take advantage of component-scanning. Note that, in this case, we need not explicitly declare <context:annotation-config/>, because <context:component-scan/> enables the same functionality.

The following example shows the modified system-test-config.xml file:

<beans>
    <!-- picks up and registers AppConfig as a bean definition -->
    <context:component-scan base-package="com.acme"/>
    <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>

    <bean class="org.springframework.jdbc.datasource.DriverManagerDataSource">
        <property name="url" value="${jdbc.url}"/>
        <property name="username" value="${jdbc.username}"/>
        <property name="password" value="${jdbc.password}"/>
    </bean>
</beans>
@Configuration Class-centric Use of XML with @ImportResource

In applications where @Configuration classes are the primary mechanism for configuring the container, it is still likely necessary to use at least some XML. In these scenarios, you can use @ImportResource and define only as much XML as you need. Doing so achieves a “Java-centric” approach to configuring the container and keeps XML to a bare minimum. The following example (which includes a configuration class, an XML file that defines a bean, a properties file, and the main class) shows how to use the @ImportResource annotation to achieve “Java-centric” configuration that uses XML as needed:

Java
@Configuration
@ImportResource("classpath:/com/acme/properties-config.xml")
public class AppConfig {

    @Value("${jdbc.url}")
    private String url;

    @Value("${jdbc.username}")
    private String username;

    @Value("${jdbc.password}")
    private String password;

    @Bean
    public DataSource dataSource() {
        return new DriverManagerDataSource(url, username, password);
    }
}
Kotlin
@Configuration
@ImportResource("classpath:/com/acme/properties-config.xml")
class AppConfig {

    @Value("\${jdbc.url}")
    private lateinit var url: String

    @Value("\${jdbc.username}")
    private lateinit var username: String

    @Value("\${jdbc.password}")
    private lateinit var password: String

    @Bean
    fun dataSource(): DataSource {
        return DriverManagerDataSource(url, username, password)
    }
}
properties-config.xml
<beans>
    <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>
</beans>
jdbc.properties
jdbc.url=jdbc:hsqldb:hsql://localhost/xdb
jdbc.username=sa
jdbc.password=
Java
public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
    TransferService transferService = ctx.getBean(TransferService.class);
    // ...
}
Kotlin
import org.springframework.beans.factory.getBean

fun main() {
    val ctx = AnnotationConfigApplicationContext(AppConfig::class.java)
    val transferService = ctx.getBean<TransferService>()
    // ...
}

Environment Abstraction

The Environment interface is an abstraction integrated in the container that models two key aspects of the application environment: profiles and properties.

A profile is a named, logical group of bean definitions to be registered with the container only if the given profile is active. Beans may be assigned to a profile whether defined in XML or with annotations. The role of the Environment object with relation to profiles is in determining which profiles (if any) are currently active, and which profiles (if any) should be active by default.

Properties play an important role in almost all applications and may originate from a variety of sources: properties files, JVM system properties, system environment variables, JNDI, servlet context parameters, ad-hoc Properties objects, Map objects, and so on. The role of the Environment object with relation to properties is to provide the user with a convenient service interface for configuring property sources and resolving properties from them.

Bean Definition Profiles

Bean definition profiles provide a mechanism in the core container that allows for registration of different beans in different environments. The word, “environment,” can mean different things to different users, and this feature can help with many use cases, including:

  • Working against an in-memory datasource in development versus looking up that same datasource from JNDI when in QA or production.

  • Registering monitoring infrastructure only when deploying an application into a performance environment.

  • Registering customized implementations of beans for customer A versus customer B deployments.

Consider the first use case in a practical application that requires a DataSource. In a test environment, the configuration might resemble the following:

Java
@Bean
public DataSource dataSource() {
    return new EmbeddedDatabaseBuilder()
        .setType(EmbeddedDatabaseType.HSQL)
        .addScript("my-schema.sql")
        .addScript("my-test-data.sql")
        .build();
}
Kotlin
@Bean
fun dataSource(): DataSource {
    return EmbeddedDatabaseBuilder()
            .setType(EmbeddedDatabaseType.HSQL)
            .addScript("my-schema.sql")
            .addScript("my-test-data.sql")
            .build()
}

Now consider how this application can be deployed into a QA or production environment, assuming that the datasource for the application is registered with the production application server’s JNDI directory. Our dataSource bean now looks like the following listing:

Java
@Bean(destroyMethod="")
public DataSource dataSource() throws Exception {
    Context ctx = new InitialContext();
    return (DataSource) ctx.lookup("java:comp/env/jdbc/datasource");
}
Kotlin
@Bean(destroyMethod = "")
fun dataSource(): DataSource {
    val ctx = InitialContext()
    return ctx.lookup("java:comp/env/jdbc/datasource") as DataSource
}

The problem is how to switch between using these two variations based on the current environment. Over time, Spring users have devised a number of ways to get this done, usually relying on a combination of system environment variables and XML <import/> statements containing ${placeholder} tokens that resolve to the correct configuration file path depending on the value of an environment variable. Bean definition profiles is a core container feature that provides a solution to this problem.

If we generalize the use case shown in the preceding example of environment-specific bean definitions, we end up with the need to register certain bean definitions in certain contexts but not in others. You could say that you want to register a certain profile of bean definitions in situation A and a different profile in situation B. We start by updating our configuration to reflect this need.

Using @Profile

The @Profile annotation lets you indicate that a component is eligible for registration when one or more specified profiles are active. Using our preceding example, we can rewrite the dataSource configuration as follows:

Java
@Configuration
@Profile("development")
public class StandaloneDataConfig {

    @Bean
    public DataSource dataSource() {
        return new EmbeddedDatabaseBuilder()
            .setType(EmbeddedDatabaseType.HSQL)
            .addScript("classpath:com/bank/config/sql/schema.sql")
            .addScript("classpath:com/bank/config/sql/test-data.sql")
            .build();
    }
}
Kotlin
@Configuration
@Profile("development")
class StandaloneDataConfig {

    @Bean
    fun dataSource(): DataSource {
        return EmbeddedDatabaseBuilder()
                .setType(EmbeddedDatabaseType.HSQL)
                .addScript("classpath:com/bank/config/sql/schema.sql")
                .addScript("classpath:com/bank/config/sql/test-data.sql")
                .build()
    }
}
Java
@Configuration
@Profile("production")
public class JndiDataConfig {

    @Bean(destroyMethod="")
    public DataSource dataSource() throws Exception {
        Context ctx = new InitialContext();
        return (DataSource) ctx.lookup("java:comp/env/jdbc/datasource");
    }
}
Kotlin
@Configuration
@Profile("production")
class JndiDataConfig {

    @Bean(destroyMethod = "")
    fun dataSource(): DataSource {
        val ctx = InitialContext()
        return ctx.lookup("java:comp/env/jdbc/datasource") as DataSource
    }
}
As mentioned earlier, with @Bean methods, you typically choose to use programmatic JNDI lookups, by using either Spring’s JndiTemplate/JndiLocatorDelegate helpers or the straight JNDI InitialContext usage shown earlier but not the JndiObjectFactoryBean variant, which would force you to declare the return type as the FactoryBean type.

The profile string may contain a simple profile name (for example, production) or a profile expression. A profile expression allows for more complicated profile logic to be expressed (for example, production & us-east). The following operators are supported in profile expressions:

  • !: A logical “not” of the profile

  • &: A logical “and” of the profiles

  • |: A logical “or” of the profiles

You cannot mix the & and | operators without using parentheses. For example, production & us-east | eu-central is not a valid expression. It must be expressed as production & (us-east | eu-central).

You can use @Profile as a meta-annotation for the purpose of creating a custom composed annotation. The following example defines a custom @Production annotation that you can use as a drop-in replacement for @Profile("production"):

Java
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Profile("production")
public @interface Production {
}
Kotlin
@Target(AnnotationTarget.TYPE)
@Retention(AnnotationRetention.RUNTIME)
@Profile("production")
annotation class Production
If a @Configuration class is marked with @Profile, all of the @Bean methods and @Import annotations associated with that class are bypassed unless one or more of the specified profiles are active. If a @Component or @Configuration class is marked with @Profile({"p1", "p2"}), that class is not registered or processed unless profiles 'p1' or 'p2' have been activated. If a given profile is prefixed with the NOT operator (!), the annotated element is registered only if the profile is not active. For example, given @Profile({"p1", "!p2"}), registration will occur if profile 'p1' is active or if profile 'p2' is not active.

@Profile can also be declared at the method level to include only one particular bean of a configuration class (for example, for alternative variants of a particular bean), as the following example shows:

Java
@Configuration
public class AppConfig {

    @Bean("dataSource")
    @Profile("development") (1)
    public DataSource standaloneDataSource() {
        return new EmbeddedDatabaseBuilder()
            .setType(EmbeddedDatabaseType.HSQL)
            .addScript("classpath:com/bank/config/sql/schema.sql")
            .addScript("classpath:com/bank/config/sql/test-data.sql")
            .build();
    }

    @Bean("dataSource")
    @Profile("production") (2)
    public DataSource jndiDataSource() throws Exception {
        Context ctx = new InitialContext();
        return (DataSource) ctx.lookup("java:comp/env/jdbc/datasource");
    }
}
1 The standaloneDataSource method is available only in the development profile.
2 The jndiDataSource method is available only in the production profile.
Kotlin
@Configuration
class AppConfig {

    @Bean("dataSource")
    @Profile("development") (1)
    fun standaloneDataSource(): DataSource {
        return EmbeddedDatabaseBuilder()
                .setType(EmbeddedDatabaseType.HSQL)
                .addScript("classpath:com/bank/config/sql/schema.sql")
                .addScript("classpath:com/bank/config/sql/test-data.sql")
                .build()
    }

    @Bean("dataSource")
    @Profile("production") (2)
    fun jndiDataSource() =
        InitialContext().lookup("java:comp/env/jdbc/datasource") as DataSource
}
1 The standaloneDataSource method is available only in the development profile.
2 The jndiDataSource method is available only in the production profile.

With @Profile on @Bean methods, a special scenario may apply: In the case of overloaded @Bean methods of the same Java method name (analogous to constructor overloading), a @Profile condition needs to be consistently declared on all overloaded methods. If the conditions are inconsistent, only the condition on the first declaration among the overloaded methods matters. Therefore, @Profile can not be used to select an overloaded method with a particular argument signature over another. Resolution between all factory methods for the same bean follows Spring’s constructor resolution algorithm at creation time.

If you want to define alternative beans with different profile conditions, use distinct Java method names that point to the same bean name by using the @Bean name attribute, as shown in the preceding example. If the argument signatures are all the same (for example, all of the variants have no-arg factory methods), this is the only way to represent such an arrangement in a valid Java class in the first place (since there can only be one method of a particular name and argument signature).

XML Bean Definition Profiles

The XML counterpart is the profile attribute of the <beans> element. Our preceding sample configuration can be rewritten in two XML files, as follows:

<beans profile="development"
    xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:jdbc="http://www.springframework.org/schema/jdbc"
    xsi:schemaLocation="...">

    <jdbc:embedded-database id="dataSource">
        <jdbc:script location="classpath:com/bank/config/sql/schema.sql"/>
        <jdbc:script location="classpath:com/bank/config/sql/test-data.sql"/>
    </jdbc:embedded-database>
</beans>
<beans profile="production"
    xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:jee="http://www.springframework.org/schema/jee"
    xsi:schemaLocation="...">

    <jee:jndi-lookup id="dataSource" jndi-name="java:comp/env/jdbc/datasource"/>
</beans>

It is also possible to avoid that split and nest <beans/> elements within the same file, as the following example shows:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:jdbc="http://www.springframework.org/schema/jdbc"
    xmlns:jee="http://www.springframework.org/schema/jee"
    xsi:schemaLocation="...">

    <!-- other bean definitions -->

    <beans profile="development">
        <jdbc:embedded-database id="dataSource">
            <jdbc:script location="classpath:com/bank/config/sql/schema.sql"/>
            <jdbc:script location="classpath:com/bank/config/sql/test-data.sql"/>
        </jdbc:embedded-database>
    </beans>

    <beans profile="production">
        <jee:jndi-lookup id="dataSource" jndi-name="java:comp/env/jdbc/datasource"/>
    </beans>
</beans>

The spring-bean.xsd has been constrained to allow such elements only as the last ones in the file. This should help provide flexibility without incurring clutter in the XML files.

The XML counterpart does not support the profile expressions described earlier. It is possible, however, to negate a profile by using the ! operator. It is also possible to apply a logical “and” by nesting the profiles, as the following example shows:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:jdbc="http://www.springframework.org/schema/jdbc"
    xmlns:jee="http://www.springframework.org/schema/jee"
    xsi:schemaLocation="...">

    <!-- other bean definitions -->

    <beans profile="production">
        <beans profile="us-east">
            <jee:jndi-lookup id="dataSource" jndi-name="java:comp/env/jdbc/datasource"/>
        </beans>
    </beans>
</beans>

In the preceding example, the dataSource bean is exposed if both the production and us-east profiles are active.

Activating a Profile

Now that we have updated our configuration, we still need to instruct Spring which profile is active. If we started our sample application right now, we would see a NoSuchBeanDefinitionException thrown, because the container could not find the Spring bean named dataSource.

Activating a profile can be done in several ways, but the most straightforward is to do it programmatically against the Environment API which is available through an ApplicationContext. The following example shows how to do so:

Java
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
ctx.getEnvironment().setActiveProfiles("development");
ctx.register(SomeConfig.class, StandaloneDataConfig.class, JndiDataConfig.class);
ctx.refresh();
Kotlin
val ctx = AnnotationConfigApplicationContext().apply {
    environment.setActiveProfiles("development")
    register(SomeConfig::class.java, StandaloneDataConfig::class.java, JndiDataConfig::class.java)
    refresh()
}

In addition, you can also declaratively activate profiles through the spring.profiles.active property, which may be specified through system environment variables, JVM system properties, servlet context parameters in web.xml, or even as an entry in JNDI (see PropertySource Abstraction). In integration tests, active profiles can be declared by using the @ActiveProfiles annotation in the spring-test module (see context configuration with environment profiles).

Note that profiles are not an “either-or” proposition. You can activate multiple profiles at once. Programmatically, you can provide multiple profile names to the setActiveProfiles() method, which accepts String…​ varargs. The following example activates multiple profiles:

Java
ctx.getEnvironment().setActiveProfiles("profile1", "profile2");
Kotlin
ctx.getEnvironment().setActiveProfiles("profile1", "profile2")

Declaratively, spring.profiles.active may accept a comma-separated list of profile names, as the following example shows:

    -Dspring.profiles.active="profile1,profile2"
Default Profile

The default profile represents the profile that is enabled by default. Consider the following example:

Java
@Configuration
@Profile("default")
public class DefaultDataConfig {

    @Bean
    public DataSource dataSource() {
        return new EmbeddedDatabaseBuilder()
            .setType(EmbeddedDatabaseType.HSQL)
            .addScript("classpath:com/bank/config/sql/schema.sql")
            .build();
    }
}
Kotlin
@Configuration
@Profile("default")
class DefaultDataConfig {

    @Bean
    fun dataSource(): DataSource {
        return EmbeddedDatabaseBuilder()
                .setType(EmbeddedDatabaseType.HSQL)
                .addScript("classpath:com/bank/config/sql/schema.sql")
                .build()
    }
}

If no profile is active, the dataSource is created. You can see this as a way to provide a default definition for one or more beans. If any profile is enabled, the default profile does not apply.

You can change the name of the default profile by using setDefaultProfiles() on the Environment or ,declaratively, by using the spring.profiles.default property.

PropertySource Abstraction

Spring’s Environment abstraction provides search operations over a configurable hierarchy of property sources. Consider the following listing:

Java
ApplicationContext ctx = new GenericApplicationContext();
Environment env = ctx.getEnvironment();
boolean containsMyProperty = env.containsProperty("my-property");
System.out.println("Does my environment contain the 'my-property' property? " + containsMyProperty);
Kotlin
val ctx = GenericApplicationContext()
val env = ctx.environment
val containsMyProperty = env.containsProperty("my-property")
println("Does my environment contain the 'my-property' property? $containsMyProperty")

In the preceding snippet, we see a high-level way of asking Spring whether the my-property property is defined for the current environment. To answer this question, the Environment object performs a search over a set of PropertySource objects. A PropertySource is a simple abstraction over any source of key-value pairs, and Spring’s StandardEnvironment is configured with two PropertySource objects — one representing the set of JVM system properties (System.getProperties()) and one representing the set of system environment variables (System.getenv()).

These default property sources are present for StandardEnvironment, for use in standalone applications. StandardServletEnvironment is populated with additional default property sources including servlet config and servlet context parameters. It can optionally enable a JndiPropertySource. See the javadoc for details.

Concretely, when you use the StandardEnvironment, the call to env.containsProperty("my-property") returns true if a my-property system property or my-property environment variable is present at runtime.

The search performed is hierarchical. By default, system properties have precedence over environment variables. So, if the my-property property happens to be set in both places during a call to env.getProperty("my-property"), the system property value “wins” and is returned. Note that property values are not merged but rather completely overridden by a preceding entry.

For a common StandardServletEnvironment, the full hierarchy is as follows, with the highest-precedence entries at the top:

  1. ServletConfig parameters (if applicable — for example, in case of a DispatcherServlet context)

  2. ServletContext parameters (web.xml context-param entries)

  3. JNDI environment variables (java:comp/env/ entries)

  4. JVM system properties (-D command-line arguments)

  5. JVM system environment (operating system environment variables)

Most importantly, the entire mechanism is configurable. Perhaps you have a custom source of properties that you want to integrate into this search. To do so, implement and instantiate your own PropertySource and add it to the set of PropertySources for the current Environment. The following example shows how to do so:

Java
ConfigurableApplicationContext ctx = new GenericApplicationContext();
MutablePropertySources sources = ctx.getEnvironment().getPropertySources();
sources.addFirst(new MyPropertySource());
Kotlin
val ctx = GenericApplicationContext()
val sources = ctx.environment.propertySources
sources.addFirst(MyPropertySource())

In the preceding code, MyPropertySource has been added with highest precedence in the search. If it contains a my-property property, the property is detected and returned, in favor of any my-property property in any other PropertySource. The MutablePropertySources API exposes a number of methods that allow for precise manipulation of the set of property sources.

Using @PropertySource

The @PropertySource annotation provides a convenient and declarative mechanism for adding a PropertySource to Spring’s Environment.

Given a file called app.properties that contains the key-value pair testbean.name=myTestBean, the following @Configuration class uses @PropertySource in such a way that a call to testBean.getName() returns myTestBean:

Java
@Configuration
@PropertySource("classpath:/com/myco/app.properties")
public class AppConfig {

    @Autowired
    Environment env;

    @Bean
    public TestBean testBean() {
        TestBean testBean = new TestBean();
        testBean.setName(env.getProperty("testbean.name"));
        return testBean;
    }
}
Kotlin
@Configuration
@PropertySource("classpath:/com/myco/app.properties")
class AppConfig {

    @Autowired
    private lateinit var env: Environment

    @Bean
    fun testBean() = TestBean().apply {
        name = env.getProperty("testbean.name")!!
    }
}

Any ${…​} placeholders present in a @PropertySource resource location are resolved against the set of property sources already registered against the environment, as the following example shows:

Java
@Configuration
@PropertySource("classpath:/com/${my.placeholder:default/path}/app.properties")
public class AppConfig {

    @Autowired
    Environment env;

    @Bean
    public TestBean testBean() {
        TestBean testBean = new TestBean();
        testBean.setName(env.getProperty("testbean.name"));
        return testBean;
    }
}
Kotlin
@Configuration
@PropertySource("classpath:/com/\${my.placeholder:default/path}/app.properties")
class AppConfig {

    @Autowired
    private lateinit var env: Environment

    @Bean
    fun testBean() = TestBean().apply {
        name = env.getProperty("testbean.name")!!
    }
}

Assuming that my.placeholder is present in one of the property sources already registered (for example, system properties or environment variables), the placeholder is resolved to the corresponding value. If not, then default/path is used as a default. If no default is specified and a property cannot be resolved, an IllegalArgumentException is thrown.

The @PropertySource annotation is repeatable, according to Java 8 conventions. However, all such @PropertySource annotations need to be declared at the same level, either directly on the configuration class or as meta-annotations within the same custom annotation. Mixing direct annotations and meta-annotations is not recommended, since direct annotations effectively override meta-annotations.

Placeholder Resolution in Statements

Historically, the value of placeholders in elements could be resolved only against JVM system properties or environment variables. This is no longer the case. Because the Environment abstraction is integrated throughout the container, it is easy to route resolution of placeholders through it. This means that you may configure the resolution process in any way you like. You can change the precedence of searching through system properties and environment variables or remove them entirely. You can also add your own property sources to the mix, as appropriate.

Concretely, the following statement works regardless of where the customer property is defined, as long as it is available in the Environment:

<beans>
    <import resource="com/bank/service/${customer}-config.xml"/>
</beans>

Registering a LoadTimeWeaver

The LoadTimeWeaver is used by Spring to dynamically transform classes as they are loaded into the Java virtual machine (JVM).

To enable load-time weaving, you can add the @EnableLoadTimeWeaving to one of your @Configuration classes, as the following example shows:

Java
@Configuration
@EnableLoadTimeWeaving
public class AppConfig {
}
Kotlin
@Configuration
@EnableLoadTimeWeaving
class AppConfig

Alternatively, for XML configuration, you can use the context:load-time-weaver element:

<beans>
    <context:load-time-weaver/>
</beans>

Once configured for the ApplicationContext, any bean within that ApplicationContext may implement LoadTimeWeaverAware, thereby receiving a reference to the load-time weaver instance. This is particularly useful in combination with Spring’s JPA support where load-time weaving may be necessary for JPA class transformation. Consult the LocalContainerEntityManagerFactoryBean javadoc for more detail. For more on AspectJ load-time weaving, see Load-time Weaving with AspectJ in the Spring Framework.

Additional Capabilities of the ApplicationContext

As discussed in the chapter introduction, the org.springframework.beans.factory package provides basic functionality for managing and manipulating beans, including in a programmatic way. The org.springframework.context package adds the ApplicationContext interface, which extends the BeanFactory interface, in addition to extending other interfaces to provide additional functionality in a more application framework-oriented style. Many people use the ApplicationContext in a completely declarative fashion, not even creating it programmatically, but instead relying on support classes such as ContextLoader to automatically instantiate an ApplicationContext as part of the normal startup process of a Java EE web application.

To enhance BeanFactory functionality in a more framework-oriented style, the context package also provides the following functionality:

  • Access to messages in i18n-style, through the MessageSource interface.

  • Access to resources, such as URLs and files, through the ResourceLoader interface.

  • Event publication, namely to beans that implement the ApplicationListener interface, through the use of the ApplicationEventPublisher interface.

  • Loading of multiple (hierarchical) contexts, letting each be focused on one particular layer, such as the web layer of an application, through the HierarchicalBeanFactory interface.

Internationalization using MessageSource

The ApplicationContext interface extends an interface called MessageSource and, therefore, provides internationalization (“i18n”) functionality. Spring also provides the HierarchicalMessageSource interface, which can resolve messages hierarchically. Together, these interfaces provide the foundation upon which Spring effects message resolution. The methods defined on these interfaces include:

  • String getMessage(String code, Object[] args, String default, Locale loc): The basic method used to retrieve a message from the MessageSource. When no message is found for the specified locale, the default message is used. Any arguments passed in become replacement values, using the MessageFormat functionality provided by the standard library.

  • String getMessage(String code, Object[] args, Locale loc): Essentially the same as the previous method but with one difference: No default message can be specified. If the message cannot be found, a NoSuchMessageException is thrown.

  • String getMessage(MessageSourceResolvable resolvable, Locale locale): All properties used in the preceding methods are also wrapped in a class named MessageSourceResolvable, which you can use with this method.

When an ApplicationContext is loaded, it automatically searches for a MessageSource bean defined in the context. The bean must have the name messageSource. If such a bean is found, all calls to the preceding methods are delegated to the message source. If no message source is found, the ApplicationContext attempts to find a parent containing a bean with the same name. If it does, it uses that bean as the MessageSource. If the ApplicationContext cannot find any source for messages, an empty DelegatingMessageSource is instantiated in order to be able to accept calls to the methods defined above.

Spring provides two MessageSource implementations, ResourceBundleMessageSource and StaticMessageSource. Both implement HierarchicalMessageSource in order to do nested messaging. The StaticMessageSource is rarely used but provides programmatic ways to add messages to the source. The following example shows ResourceBundleMessageSource:

<beans>
    <bean id="messageSource"
            class="org.springframework.context.support.ResourceBundleMessageSource">
        <property name="basenames">
            <list>
                <value>format</value>
                <value>exceptions</value>
                <value>windows</value>
            </list>
        </property>
    </bean>
</beans>

The example assumes that you have three resource bundles called format, exceptions and windows defined in your classpath. Any request to resolve a message is handled in the JDK-standard way of resolving messages through ResourceBundle objects. For the purposes of the example, assume the contents of two of the above resource bundle files are as follows:

    # in format.properties
    message=Alligators rock!
    # in exceptions.properties
    argument.required=The {0} argument is required.

The next example shows a program to execute the MessageSource functionality. Remember that all ApplicationContext implementations are also MessageSource implementations and so can be cast to the MessageSource interface.

Java
public static void main(String[] args) {
    MessageSource resources = new ClassPathXmlApplicationContext("beans.xml");
    String message = resources.getMessage("message", null, "Default", Locale.ENGLISH);
    System.out.println(message);
}
Kotlin
fun main() {
    val resources = ClassPathXmlApplicationContext("beans.xml")
    val message = resources.getMessage("message", null, "Default", Locale.ENGLISH)
    println(message)
}

The resulting output from the above program is as follows:

Alligators rock!

To summarize, the MessageSource is defined in a file called beans.xml, which exists at the root of your classpath. The messageSource bean definition refers to a number of resource bundles through its basenames property. The three files that are passed in the list to the basenames property exist as files at the root of your classpath and are called format.properties, exceptions.properties, and windows.properties, respectively.

The next example shows arguments passed to the message lookup. These arguments are converted into String objects and inserted into placeholders in the lookup message.

<beans>

    <!-- this MessageSource is being used in a web application -->
    <bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource">
        <property name="basename" value="exceptions"/>
    </bean>

    <!-- lets inject the above MessageSource into this POJO -->
    <bean id="example" class="com.something.Example">
        <property name="messages" ref="messageSource"/>
    </bean>

</beans>
Java
public class Example {

    private MessageSource messages;

    public void setMessages(MessageSource messages) {
        this.messages = messages;
    }

    public void execute() {
        String message = this.messages.getMessage("argument.required",
            new Object [] {"userDao"}, "Required", Locale.ENGLISH);
        System.out.println(message);
    }
}
Kotlin
    class Example {

    lateinit var messages: MessageSource

    fun execute() {
        val message = messages.getMessage("argument.required",
                arrayOf("userDao"), "Required", Locale.ENGLISH)
        println(message)
    }
}

The resulting output from the invocation of the execute() method is as follows:

The userDao argument is required.

With regard to internationalization (“i18n”), Spring’s various MessageSource implementations follow the same locale resolution and fallback rules as the standard JDK ResourceBundle. In short, and continuing with the example messageSource defined previously, if you want to resolve messages against the British (en-GB) locale, you would create files called format_en_GB.properties, exceptions_en_GB.properties, and windows_en_GB.properties, respectively.

Typically, locale resolution is managed by the surrounding environment of the application. In the following example, the locale against which (British) messages are resolved is specified manually:

# in exceptions_en_GB.properties
argument.required=Ebagum lad, the {0} argument is required, I say, required.
Java
public static void main(final String[] args) {
    MessageSource resources = new ClassPathXmlApplicationContext("beans.xml");
    String message = resources.getMessage("argument.required",
        new Object [] {"userDao"}, "Required", Locale.UK);
    System.out.println(message);
}
Kotlin
fun main() {
    val resources = ClassPathXmlApplicationContext("beans.xml")
    val message = resources.getMessage("argument.required",
            arrayOf("userDao"), "Required", Locale.UK)
    println(message)
}

The resulting output from the running of the above program is as follows:

Ebagum lad, the 'userDao' argument is required, I say, required.

You can also use the MessageSourceAware interface to acquire a reference to any MessageSource that has been defined. Any bean that is defined in an ApplicationContext that implements the MessageSourceAware interface is injected with the application context’s MessageSource when the bean is created and configured.

As an alternative to ResourceBundleMessageSource, Spring provides a ReloadableResourceBundleMessageSource class. This variant supports the same bundle file format but is more flexible than the standard JDK based ResourceBundleMessageSource implementation. In particular, it allows for reading files from any Spring resource location (not only from the classpath) and supports hot reloading of bundle property files (while efficiently caching them in between). See the ReloadableResourceBundleMessageSource javadoc for details.

Standard and Custom Events

Event handling in the ApplicationContext is provided through the ApplicationEvent class and the ApplicationListener interface. If a bean that implements the ApplicationListener interface is deployed into the context, every time an ApplicationEvent gets published to the ApplicationContext, that bean is notified. Essentially, this is the standard Observer design pattern.

As of Spring 4.2, the event infrastructure has been significantly improved and offers an annotation-based model as well as the ability to publish any arbitrary event (that is, an object that does not necessarily extend from ApplicationEvent). When such an object is published, we wrap it in an event for you.

The following table describes the standard events that Spring provides:

Table 9. Built-in Events
Event Explanation

ContextRefreshedEvent

Published when the ApplicationContext is initialized or refreshed (for example, by using the refresh() method on the ConfigurableApplicationContext interface). Here, “initialized” means that all beans are loaded, post-processor beans are detected and activated, singletons are pre-instantiated, and the ApplicationContext object is ready for use. As long as the context has not been closed, a refresh can be triggered multiple times, provided that the chosen ApplicationContext actually supports such “hot” refreshes. For example, XmlWebApplicationContext supports hot refreshes, but GenericApplicationContext does not.

ContextStartedEvent

Published when the ApplicationContext is started by using the start() method on the ConfigurableApplicationContext interface. Here, “started” means that all Lifecycle beans receive an explicit start signal. Typically, this signal is used to restart beans after an explicit stop, but it may also be used to start components that have not been configured for autostart (for example, components that have not already started on initialization).

ContextStoppedEvent

Published when the ApplicationContext is stopped by using the stop() method on the ConfigurableApplicationContext interface. Here, “stopped” means that all Lifecycle beans receive an explicit stop signal. A stopped context may be restarted through a start() call.

ContextClosedEvent

Published when the ApplicationContext is being closed by using the close() method on the ConfigurableApplicationContext interface or via a JVM shutdown hook. Here, "closed" means that all singleton beans will be destroyed. Once the context is closed, it reaches its end of life and cannot be refreshed or restarted.

RequestHandledEvent

A web-specific event telling all beans that an HTTP request has been serviced. This event is published after the request is complete. This event is only applicable to web applications that use Spring’s DispatcherServlet.

ServletRequestHandledEvent

A subclass of RequestHandledEvent that adds Servlet-specific context information.

You can also create and publish your own custom events. The following example shows a simple class that extends Spring’s ApplicationEvent base class:

Java
public class BlackListEvent extends ApplicationEvent {

    private final String address;
    private final String content;

    public BlackListEvent(Object source, String address, String content) {
        super(source);
        this.address = address;
        this.content = content;
    }

    // accessor and other methods...
}
Kotlin
class BlackListEvent(source: Any,
                    val address: String,
                    val content: String) : ApplicationEvent(source)

To publish a custom ApplicationEvent, call the publishEvent() method on an ApplicationEventPublisher. Typically, this is done by creating a class that implements ApplicationEventPublisherAware and registering it as a Spring bean. The following example shows such a class:

Java
public class EmailService implements ApplicationEventPublisherAware {

    private List<String> blackList;
    private ApplicationEventPublisher publisher;

    public void setBlackList(List<String> blackList) {
        this.blackList = blackList;
    }

    public void setApplicationEventPublisher(ApplicationEventPublisher publisher) {
        this.publisher = publisher;
    }

    public void sendEmail(String address, String content) {
        if (blackList.contains(address)) {
            publisher.publishEvent(new BlackListEvent(this, address, content));
            return;
        }
        // send email...
    }
}
Kotlin
class EmailService : ApplicationEventPublisherAware {

    private lateinit var blackList: List<String>
    private lateinit var publisher: ApplicationEventPublisher

    fun setBlackList(blackList: List<String>) {
        this.blackList = blackList
    }

    override fun setApplicationEventPublisher(publisher: ApplicationEventPublisher) {
        this.publisher = publisher
    }

    fun sendEmail(address: String, content: String) {
        if (blackList!!.contains(address)) {
            publisher!!.publishEvent(BlackListEvent(this, address, content))
            return
        }
        // send email...
    }
}

At configuration time, the Spring container detects that EmailService implements ApplicationEventPublisherAware and automatically calls setApplicationEventPublisher(). In reality, the parameter passed in is the Spring container itself. You are interacting with the application context through its ApplicationEventPublisher interface.

To receive the custom ApplicationEvent, you can create a class that implements ApplicationListener and register it as a Spring bean. The following example shows such a class:

Java
public class BlackListNotifier implements ApplicationListener<BlackListEvent> {

    private String notificationAddress;

    public void setNotificationAddress(String notificationAddress) {
        this.notificationAddress = notificationAddress;
    }

    public void onApplicationEvent(BlackListEvent event) {
        // notify appropriate parties via notificationAddress...
    }
}
Kotlin
class BlackListNotifier : ApplicationListener<BlackListEvent> {

    lateinit var notificationAddres: String

    override fun onApplicationEvent(event: BlackListEvent) {
        // notify appropriate parties via notificationAddress...
    }
}

Notice that ApplicationListener is generically parameterized with the type of your custom event (BlackListEvent in the preceding example). This means that the onApplicationEvent() method can remain type-safe, avoiding any need for downcasting. You can register as many event listeners as you wish, but note that, by default, event listeners receive events synchronously. This means that the publishEvent() method blocks until all listeners have finished processing the event. One advantage of this synchronous and single-threaded approach is that, when a listener receives an event, it operates inside the transaction context of the publisher if a transaction context is available. If another strategy for event publication becomes necessary, see the javadoc for Spring’s ApplicationEventMulticaster interface and SimpleApplicationEventMulticaster implementation for configuration options.

The following example shows the bean definitions used to register and configure each of the classes above:

<bean id="emailService" class="example.EmailService">
    <property name="blackList">
        <list>
            <value>[email protected]</value>
            <value>[email protected]</value>
            <value>[email protected]</value>
        </list>
    </property>
</bean>

<bean id="blackListNotifier" class="example.BlackListNotifier">
    <property name="notificationAddress" value="[email protected]"/>
</bean>

Putting it all together, when the sendEmail() method of the emailService bean is called, if there are any email messages that should be blacklisted, a custom event of type BlackListEvent is published. The blackListNotifier bean is registered as an ApplicationListener and receives the BlackListEvent, at which point it can notify appropriate parties.

Spring’s eventing mechanism is designed for simple communication between Spring beans within the same application context. However, for more sophisticated enterprise integration needs, the separately maintained Spring Integration project provides complete support for building lightweight, pattern-oriented, event-driven architectures that build upon the well-known Spring programming model.
Annotation-based Event Listeners

As of Spring 4.2, you can register an event listener on any public method of a managed bean by using the @EventListener annotation. The BlackListNotifier can be rewritten as follows:

Java
public class BlackListNotifier {

    private String notificationAddress;

    public void setNotificationAddress(String notificationAddress) {
        this.notificationAddress = notificationAddress;
    }

    @EventListener
    public void processBlackListEvent(BlackListEvent event) {
        // notify appropriate parties via notificationAddress...
    }
}
Kotlin
class BlackListNotifier {

    lateinit var notificationAddress: String

    @EventListener
    fun processBlackListEvent(event: BlackListEvent) {
        // notify appropriate parties via notificationAddress...
    }
}

The method signature once again declares the event type to which it listens, but, this time, with a flexible name and without implementing a specific listener interface. The event type can also be narrowed through generics as long as the actual event type resolves your generic parameter in its implementation hierarchy.

If your method should listen to several events or if you want to define it with no parameter at all, the event types can also be specified on the annotation itself. The following example shows how to do so:

Java
@EventListener({ContextStartedEvent.class, ContextRefreshedEvent.class})
public void handleContextStart() {
    // ...
}
Kotlin
@EventListener(ContextStartedEvent::class, ContextRefreshedEvent::class)
fun handleContextStart() {
    // ...
}

It is also possible to add additional runtime filtering by using the condition attribute of the annotation that defines a SpEL expression , which should match to actually invoke the method for a particular event.

The following example shows how our notifier can be rewritten to be invoked only if the content attribute of the event is equal to my-event:

Java
@EventListener(condition = "#blEvent.content == 'my-event'")
public void processBlackListEvent(BlackListEvent blEvent) {
    // notify appropriate parties via notificationAddress...
}
Kotlin
@EventListener(condition = "#blEvent.content == 'my-event'")
fun processBlackListEvent(blEvent: BlackListEvent) {
    // notify appropriate parties via notificationAddress...
}

Each SpEL expression evaluates against a dedicated context. The following table lists the items made available to the context so that you can use them for conditional event processing:

Table 10. Event SpEL available metadata
Name Location Description Example

Event

root object

The actual ApplicationEvent.

#root.event or event

Arguments array

root object

The arguments (as an object array) used to invoke the method.

#root.args or args; args[0] to access the first argument, etc.

Argument name

evaluation context

The name of any of the method arguments. If, for some reason, the names are not available (for example, because there is no debug information in the compiled byte code), individual arguments are also available using the #a<#arg> syntax where <#arg> stands for the argument index (starting from 0).

#blEvent or #a0 (you can also use #p0 or #p<#arg> parameter notation as an alias)

Note that #root.event gives you access to the underlying event, even if your method signature actually refers to an arbitrary object that was published.

If you need to publish an event as the result of processing another event, you can change the method signature to return the event that should be published, as the following example shows:

Java
@EventListener
public ListUpdateEvent handleBlackListEvent(BlackListEvent event) {
    // notify appropriate parties via notificationAddress and
    // then publish a ListUpdateEvent...
}
Kotlin
@EventListener
fun handleBlackListEvent(event: BlackListEvent): ListUpdateEvent {
    // notify appropriate parties via notificationAddress and
    // then publish a ListUpdateEvent...
}
This feature is not supported for asynchronous listeners.

This new method publishes a new ListUpdateEvent for every BlackListEvent handled by the method above. If you need to publish several events, you can return a Collection of events instead.

Asynchronous Listeners

If you want a particular listener to process events asynchronously, you can reuse the regular @Async support. The following example shows how to do so:

Java
@EventListener
@Async
public void processBlackListEvent(BlackListEvent event) {
    // BlackListEvent is processed in a separate thread
}
Kotlin
@EventListener
@Async
fun processBlackListEvent(event: BlackListEvent) {
    // BlackListEvent is processed in a separate thread
}

Be aware of the following limitations when using asynchronous events:

  • If an asynchronous event listener throws an Exception, it is not propagated to the caller. See AsyncUncaughtExceptionHandler for more details.

  • Asynchronous event listener methods cannot publish a subsequent event by returning a value. If you need to publish another event as the result of the processing, inject an ApplicationEventPublisher to publish the event manually.

Ordering Listeners

If you need one listener to be invoked before another one, you can add the @Order annotation to the method declaration, as the following example shows:

Java
@EventListener
@Order(42)
public void processBlackListEvent(BlackListEvent event) {
    // notify appropriate parties via notificationAddress...
}
Kotlin
@EventListener
@Order(42)
fun processBlackListEvent(event: BlackListEvent) {
    // notify appropriate parties via notificationAddress...
}
Generic Events

You can also use generics to further define the structure of your event. Consider using an EntityCreatedEvent<T> where T is the type of the actual entity that got created. For example, you can create the following listener definition to receive only EntityCreatedEvent for a Person:

Java
@EventListener
public void onPersonCreated(EntityCreatedEvent<Person> event) {
    // ...
}
Kotlin
@EventListener
fun onPersonCreated(event: EntityCreatedEvent<Person>) {
    // ...
}

Due to type erasure, this works only if the event that is fired resolves the generic parameters on which the event listener filters (that is, something like class PersonCreatedEvent extends EntityCreatedEvent<Person> { …​ }).

In certain circumstances, this may become quite tedious if all events follow the same structure (as should be the case for the event in the preceding example). In such a case, you can implement ResolvableTypeProvider to guide the framework beyond what the runtime environment provides. The following event shows how to do so:

Java
public class EntityCreatedEvent<T> extends ApplicationEvent implements ResolvableTypeProvider {

    public EntityCreatedEvent(T entity) {
        super(entity);
    }

    @Override
    public ResolvableType getResolvableType() {
        return ResolvableType.forClassWithGenerics(getClass(), ResolvableType.forInstance(getSource()));
    }
}
Kotlin
class EntityCreatedEvent<T>(entity: T) : ApplicationEvent(entity), ResolvableTypeProvider {

    override fun getResolvableType(): ResolvableType? {
        return ResolvableType.forClassWithGenerics(javaClass, ResolvableType.forInstance(getSource()))
    }
}
This works not only for ApplicationEvent but any arbitrary object that you send as an event.

Convenient Access to Low-level Resources

For optimal usage and understanding of application contexts, you should familiarize yourself with Spring’s Resource abstraction, as described in Resources.

An application context is a ResourceLoader, which can be used to load Resource objects. A Resource is essentially a more feature rich version of the JDK java.net.URL class. In fact, the implementations of the Resource wrap an instance of java.net.URL, where appropriate. A Resource can obtain low-level resources from almost any location in a transparent fashion, including from the classpath, a filesystem location, anywhere describable with a standard URL, and some other variations. If the resource location string is a simple path without any special prefixes, where those resources come from is specific and appropriate to the actual application context type.

You can configure a bean deployed into the application context to implement the special callback interface, ResourceLoaderAware, to be automatically called back at initialization time with the application context itself passed in as the ResourceLoader. You can also expose properties of type Resource, to be used to access static resources. They are injected into it like any other properties. You can specify those Resource properties as simple String paths and rely on automatic conversion from those text strings to actual Resource objects when the bean is deployed.

The location path or paths supplied to an ApplicationContext constructor are actually resource strings and, in simple form, are treated appropriately according to the specific context implementation. For example ClassPathXmlApplicationContext treats a simple location path as a classpath location. You can also use location paths (resource strings) with special prefixes to force loading of definitions from the classpath or a URL, regardless of the actual context type.

Convenient ApplicationContext Instantiation for Web Applications

You can create ApplicationContext instances declaratively by using, for example, a ContextLoader. Of course, you can also create ApplicationContext instances programmatically by using one of the ApplicationContext implementations.

You can register an ApplicationContext by using the ContextLoaderListener, as the following example shows:

<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>/WEB-INF/daoContext.xml /WEB-INF/applicationContext.xml</param-value>
</context-param>

<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

The listener inspects the contextConfigLocation parameter. If the parameter does not exist, the listener uses /WEB-INF/applicationContext.xml as a default. When the parameter does exist, the listener separates the String by using predefined delimiters (comma, semicolon, and whitespace) and uses the values as locations where application contexts are searched. Ant-style path patterns are supported as well. Examples are /WEB-INF/*Context.xml (for all files with names that end with Context.xml and that reside in the WEB-INF directory) and /WEB-INF/**/*Context.xml (for all such files in any subdirectory of WEB-INF).

Deploying a Spring ApplicationContext as a Java EE RAR File

It is possible to deploy a Spring ApplicationContext as a RAR file, encapsulating the context and all of its required bean classes and library JARs in a Java EE RAR deployment unit. This is the equivalent of bootstrapping a stand-alone ApplicationContext (only hosted in Java EE environment) being able to access the Java EE servers facilities. RAR deployment is a more natural alternative to a scenario of deploying a headless WAR file — in effect, a WAR file without any HTTP entry points that is used only for bootstrapping a Spring ApplicationContext in a Java EE environment.

RAR deployment is ideal for application contexts that do not need HTTP entry points but rather consist only of message endpoints and scheduled jobs. Beans in such a context can use application server resources such as the JTA transaction manager and JNDI-bound JDBC DataSource instances and JMS ConnectionFactory instances and can also register with the platform’s JMX server — all through Spring’s standard transaction management and JNDI and JMX support facilities. Application components can also interact with the application server’s JCA WorkManager through Spring’s TaskExecutor abstraction.

See the javadoc of the SpringContextResourceAdapter class for the configuration details involved in RAR deployment.

For a simple deployment of a Spring ApplicationContext as a Java EE RAR file:

  1. Package all application classes into a RAR file (which is a standard JAR file with a different file extension). .Add all required library JARs into the root of the RAR archive. .Add a META-INF/ra.xml deployment descriptor (as shown in the javadoc for SpringContextResourceAdapter) and the corresponding Spring XML bean definition file(s) (typically META-INF/applicationContext.xml).

  2. Drop the resulting RAR file into your application server’s deployment directory.

Such RAR deployment units are usually self-contained. They do not expose components to the outside world, not even to other modules of the same application. Interaction with a RAR-based ApplicationContext usually occurs through JMS destinations that it shares with other modules. A RAR-based ApplicationContext may also, for example, schedule some jobs or react to new files in the file system (or the like). If it needs to allow synchronous access from the outside, it could (for example) export RMI endpoints, which may be used by other application modules on the same machine.

The BeanFactory

The BeanFactory API provides the underlying basis for Spring’s IoC functionality. Its specific contracts are mostly used in integration with other parts of Spring and related third-party frameworks, and its DefaultListableBeanFactory implementation is a key delegate within the higher-level GenericApplicationContext container.

BeanFactory and related interfaces (such as BeanFactoryAware, InitializingBean, DisposableBean) are important integration points for other framework components. By not requiring any annotations or even reflection, they allow for very efficient interaction between the container and its components. Application-level beans may use the same callback interfaces but typically prefer declarative dependency injection instead, either through annotations or through programmatic configuration.

Note that the core BeanFactory API level and its DefaultListableBeanFactory implementation do not make assumptions about the configuration format or any component annotations to be used. All of these flavors come in through extensions (such as XmlBeanDefinitionReader and AutowiredAnnotationBeanPostProcessor) and operate on shared BeanDefinition objects as a core metadata representation. This is the essence of what makes Spring’s container so flexible and extensible.

BeanFactory or ApplicationContext?

This section explains the differences between the BeanFactory and ApplicationContext container levels and the implications on bootstrapping.

You should use an ApplicationContext unless you have a good reason for not doing so, with GenericApplicationContext and its subclass AnnotationConfigApplicationContext as the common implementations for custom bootstrapping. These are the primary entry points to Spring’s core container for all common purposes: loading of configuration files, triggering a classpath scan, programmatically registering bean definitions and annotated classes, and (as of 5.0) registering functional bean definitions.

Because an ApplicationContext includes all the functionality of a BeanFactory, it is generally recommended over a plain BeanFactory, except for scenarios where full control over bean processing is needed. Within an ApplicationContext (such as the GenericApplicationContext implementation), several kinds of beans are detected by convention (that is, by bean name or by bean type — in particular, post-processors), while a plain DefaultListableBeanFactory is agnostic about any special beans.

For many extended container features, such as annotation processing and AOP proxying, the BeanPostProcessor extension point is essential. If you use only a plain DefaultListableBeanFactory, such post-processors do not get detected and activated by default. This situation could be confusing, because nothing is actually wrong with your bean configuration. Rather, in such a scenario, the container needs to be fully bootstrapped through additional setup.

The following table lists features provided by the BeanFactory and ApplicationContext interfaces and implementations.

Table 11. Feature Matrix
Feature BeanFactory ApplicationContext

Bean instantiation/wiring

Yes

Yes

Integrated lifecycle management

No

Yes

Automatic BeanPostProcessor registration

No

Yes

Automatic BeanFactoryPostProcessor registration

No

Yes

Convenient MessageSource access (for internalization)

No

Yes

Built-in ApplicationEvent publication mechanism

No

Yes

To explicitly register a bean post-processor with a DefaultListableBeanFactory, you need to programmatically call addBeanPostProcessor, as the following example shows:

Java
DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
// populate the factory with bean definitions

// now register any needed BeanPostProcessor instances
factory.addBeanPostProcessor(new AutowiredAnnotationBeanPostProcessor());
factory.addBeanPostProcessor(new MyBeanPostProcessor());

// now start using the factory
Kotlin
val factory = DefaultListableBeanFactory()
// populate the factory with bean definitions

// now register any needed BeanPostProcessor instances
factory.addBeanPostProcessor(AutowiredAnnotationBeanPostProcessor())
factory.addBeanPostProcessor(MyBeanPostProcessor())

// now start using the factory

To apply a BeanFactoryPostProcessor to a plain DefaultListableBeanFactory, you need to call its postProcessBeanFactory method, as the following example shows:

Java
DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
reader.loadBeanDefinitions(new FileSystemResource("beans.xml"));

// bring in some property values from a Properties file
PropertySourcesPlaceholderConfigurer cfg = new PropertySourcesPlaceholderConfigurer();
cfg.setLocation(new FileSystemResource("jdbc.properties"));

// now actually do the replacement
cfg.postProcessBeanFactory(factory);
Kotlin
val factory = DefaultListableBeanFactory()
val reader = XmlBeanDefinitionReader(factory)
reader.loadBeanDefinitions(FileSystemResource("beans.xml"))

// bring in some property values from a Properties file
val cfg = PropertySourcesPlaceholderConfigurer()
cfg.setLocation(FileSystemResource("jdbc.properties"))

// now actually do the replacement
cfg.postProcessBeanFactory(factory)

In both cases, the explicit registration steps are inconvenient, which is why the various ApplicationContext variants are preferred over a plain DefaultListableBeanFactory in Spring-backed applications, especially when relying on BeanFactoryPostProcessor and BeanPostProcessor instances for extended container functionality in a typical enterprise setup.

An AnnotationConfigApplicationContext has all common annotation post-processors registered and may bring in additional processors underneath the covers through configuration annotations, such as @EnableTransactionManagement. At the abstraction level of Spring’s annotation-based configuration model, the notion of bean post-processors becomes a mere internal container detail.

Resources

This chapter covers how Spring handles resources and how you can work with resources in Spring. It includes the following topics:

Introduction

Java’s standard java.net.URL class and standard handlers for various URL prefixes, unfortunately, are not quite adequate enough for all access to low-level resources. For example, there is no standardized URL implementation that may be used to access a resource that needs to be obtained from the classpath or relative to a ServletContext. While it is possible to register new handlers for specialized URL prefixes (similar to existing handlers for prefixes such as http:), this is generally quite complicated, and the URL interface still lacks some desirable functionality, such as a method to check for the existence of the resource being pointed to.

The Resource Interface

Spring’s Resource interface is meant to be a more capable interface for abstracting access to low-level resources. The following listing shows the Resource interface definition:

Java
public interface Resource extends InputStreamSource {

    boolean exists();

    boolean isOpen();

    URL getURL() throws IOException;

    File getFile() throws IOException;

    Resource createRelative(String relativePath) throws IOException;

    String getFilename();

    String getDescription();
}
Kotlin
interface Resource : InputStreamSource {

    fun exists(): Boolean

    val isOpen: Boolean

    val url: URL

    val file: File

    @Throws(IOException::class)
    fun createRelative(relativePath: String): Resource

    val filename: String

    val description: String
}

As the definition of the Resource interface shows, it extends the InputStreamSource interface. The following listing shows the definition of the InputStreamSource interface:

Java
public interface InputStreamSource {

    InputStream getInputStream() throws IOException;
}
Kotlin
interface InputStreamSource {

    val inputStream: InputStream
}

Some of the most important methods from the Resource interface are:

  • getInputStream(): Locates and opens the resource, returning an InputStream for reading from the resource. It is expected that each invocation returns a fresh InputStream. It is the responsibility of the caller to close the stream.

  • exists(): Returns a boolean indicating whether this resource actually exists in physical form.

  • isOpen(): Returns a boolean indicating whether this resource represents a handle with an open stream. If true, the InputStream cannot be read multiple times and must be read once only and then closed to avoid resource leaks. Returns false for all usual resource implementations, with the exception of InputStreamResource.

  • getDescription(): Returns a description for this resource, to be used for error output when working with the resource. This is often the fully qualified file name or the actual URL of the resource.

Other methods let you obtain an actual URL or File object representing the resource (if the underlying implementation is compatible and supports that functionality).

Spring itself uses the Resource abstraction extensively, as an argument type in many method signatures when a resource is needed. Other methods in some Spring APIs (such as the constructors to various ApplicationContext implementations) take a String which in unadorned or simple form is used to create a Resource appropriate to that context implementation or, via special prefixes on the String path, let the caller specify that a specific Resource implementation must be created and used.

While the Resource interface is used a lot with Spring and by Spring, it is actually very useful to use as a general utility class by itself in your own code, for access to resources, even when your code does not know or care about any other parts of Spring. While this couples your code to Spring, it really only couples it to this small set of utility classes, which serve as a more capable replacement for URL and can be considered equivalent to any other library you would use for this purpose.

The Resource abstraction does not replace functionality. It wraps it where possible. For example, a UrlResource wraps a URL and uses the wrapped URL to do its work.

Built-in Resource Implementations

Spring includes the following Resource implementations:

UrlResource

UrlResource wraps a java.net.URL and can be used to access any object that is normally accessible with a URL, such as files, an HTTP target, an FTP target, and others. All URLs have a standardized String representation, such that appropriate standardized prefixes are used to indicate one URL type from another. This includes file: for accessing filesystem paths, http: for accessing resources through the HTTP protocol, ftp: for accessing resources through FTP, and others.

A UrlResource is created by Java code by explicitly using the UrlResource constructor but is often created implicitly when you call an API method that takes a String argument meant to represent a path. For the latter case, a JavaBeans PropertyEditor ultimately decides which type of Resource to create. If the path string contains well-known (to it, that is) prefix (such as classpath:), it creates an appropriate specialized Resource for that prefix. However, if it does not recognize the prefix, it assume the string is a standard URL string and creates a UrlResource.

ClassPathResource

This class represents a resource that should be obtained from the classpath. It uses either the thread context class loader, a given class loader, or a given class for loading resources.

This Resource implementation supports resolution as java.io.File if the class path resource resides in the file system but not for classpath resources that reside in a jar and have not been expanded (by the servlet engine or whatever the environment is) to the filesystem. To address this, the various Resource implementations always support resolution as a java.net.URL.

A ClassPathResource is created by Java code by explicitly using the ClassPathResource constructor but is often created implicitly when you call an API method that takes a String argument meant to represent a path. For the latter case, a JavaBeans PropertyEditor recognizes the special prefix, classpath:, on the string path and creates a ClassPathResource in that case.

FileSystemResource

This is a Resource implementation for java.io.File and java.nio.file.Path handles. It supports resolution as a File and as a URL.

ServletContextResource

This is a Resource implementation for ServletContext resources that interprets relative paths within the relevant web application’s root directory.

It always supports stream access and URL access but allows java.io.File access only when the web application archive is expanded and the resource is physically on the filesystem. Whether or not it is expanded and on the filesystem or accessed directly from the JAR or somewhere else like a database (which is conceivable) is actually dependent on the Servlet container.

InputStreamResource

An InputStreamResource is a Resource implementation for a given InputStream. It should be used only if no specific Resource implementation is applicable. In particular, prefer ByteArrayResource or any of the file-based Resource implementations where possible.

In contrast to other Resource implementations, this is a descriptor for an already-opened resource. Therefore, it returns true from isOpen(). Do not use it if you need to keep the resource descriptor somewhere or if you need to read a stream multiple times.

ByteArrayResource

This is a Resource implementation for a given byte array. It creates a ByteArrayInputStream for the given byte array.

It is useful for loading content from any given byte array without having to resort to a single-use InputStreamResource.

The ResourceLoader

The ResourceLoader interface is meant to be implemented by objects that can return (that is, load) Resource instances. The following listing shows the ResourceLoader interface definition:

Java
public interface ResourceLoader {

    Resource getResource(String location);
}
Kotlin
interface ResourceLoader {

    fun getResource(location: String): Resource
}

All application contexts implement the ResourceLoader interface. Therefore, all application contexts may be used to obtain Resource instances.

When you call getResource() on a specific application context, and the location path specified doesn’t have a specific prefix, you get back a Resource type that is appropriate to that particular application context. For example, assume the following snippet of code was executed against a ClassPathXmlApplicationContext instance:

Java
Resource template = ctx.getResource("some/resource/path/myTemplate.txt");
Kotlin
val template = ctx.getResource("some/resource/path/myTemplate.txt")

Against a ClassPathXmlApplicationContext, that code returns a ClassPathResource. If the same method were executed against a FileSystemXmlApplicationContext instance, it would return a FileSystemResource. For a WebApplicationContext, it would return a ServletContextResource. It would similarly return appropriate objects for each context.

As a result, you can load resources in a fashion appropriate to the particular application context.

On the other hand, you may also force ClassPathResource to be used, regardless of the application context type, by specifying the special classpath: prefix, as the following example shows:

Java
Resource template = ctx.getResource("classpath:some/resource/path/myTemplate.txt");
Kotlin
val template = ctx.getResource("classpath:some/resource/path/myTemplate.txt")

Similarly, you can force a UrlResource to be used by specifying any of the standard java.net.URL prefixes. The following pair of examples use the file and http prefixes:

Java
Resource template = ctx.getResource("file:///some/resource/path/myTemplate.txt");
Kotlin
val template = ctx.getResource("file:///some/resource/path/myTemplate.txt")
Java
Resource template = ctx.getResource("https://myhost.com/resource/path/myTemplate.txt");
Kotlin
val template = ctx.getResource("https://myhost.com/resource/path/myTemplate.txt")

The following table summarizes the strategy for converting String objects to Resource objects:

Table 12. Resource strings
Prefix Example Explanation

classpath:

classpath:com/myapp/config.xml

Loaded from the classpath.

file:

file:///data/config.xml

Loaded as a URL from the filesystem. See also FileSystemResource Caveats.

http:

https://myserver/logo.png

Loaded as a URL.

(none)

/data/config.xml

Depends on the underlying ApplicationContext.

The ResourceLoaderAware interface

The ResourceLoaderAware interface is a special callback interface which identifies components that expect to be provided with a ResourceLoader reference. The following listing shows the definition of the ResourceLoaderAware interface:

Java
public interface ResourceLoaderAware {

    void setResourceLoader(ResourceLoader resourceLoader);
}
Kotlin
interface ResourceLoaderAware {

    fun setResourceLoader(resourceLoader: ResourceLoader)
}

When a class implements ResourceLoaderAware and is deployed into an application context (as a Spring-managed bean), it is recognized as ResourceLoaderAware by the application context. The application context then invokes setResourceLoader(ResourceLoader), supplying itself as the argument (remember, all application contexts in Spring implement the ResourceLoader interface).

Since an ApplicationContext is a ResourceLoader, the bean could also implement the ApplicationContextAware interface and use the supplied application context directly to load resources. However, in general, it is better to use the specialized ResourceLoader interface if that is all you need. The code would be coupled only to the resource loading interface (which can be considered a utility interface) and not to the whole Spring ApplicationContext interface.

In application components, you may also rely upon autowiring of the ResourceLoader as an alternative to implementing the ResourceLoaderAware interface. The “traditional” constructor and byType autowiring modes (as described in 自动装配协作者(对bean的相关依赖进行自动注入)(Autowiring Collaborators)) are capable of providing a ResourceLoader for either a constructor argument or a setter method parameter, respectively. For more flexibility (including the ability to autowire fields and multiple parameter methods), consider using the annotation-based autowiring features. In that case, the ResourceLoader is autowired into a field, constructor argument, or method parameter that expects the ResourceLoader type as long as the field, constructor, or method in question carries the @Autowired annotation. For more information, see Using @Autowired.

Resources as Dependencies

If the bean itself is going to determine and supply the resource path through some sort of dynamic process, it probably makes sense for the bean to use the ResourceLoader interface to load resources. For example, consider the loading of a template of some sort, where the specific resource that is needed depends on the role of the user. If the resources are static, it makes sense to eliminate the use of the ResourceLoader interface completely, have the bean expose the Resource properties it needs, and expect them to be injected into it.

What makes it trivial to then inject these properties is that all application contexts register and use a special JavaBeans PropertyEditor, which can convert String paths to Resource objects. So, if myBean has a template property of type Resource, it can be configured with a simple string for that resource, as the following example shows:

<bean id="myBean" class="...">
    <property name="template" value="some/resource/path/myTemplate.txt"/>
</bean>

Note that the resource path has no prefix. Consequently, because the application context itself is going to be used as the ResourceLoader, the resource itself is loaded through a ClassPathResource, a FileSystemResource, or a ServletContextResource, depending on the exact type of the context.

If you need to force a specific Resource type to be used, you can use a prefix. The following two examples show how to force a ClassPathResource and a UrlResource (the latter being used to access a filesystem file):

<property name="template" value="classpath:some/resource/path/myTemplate.txt">
<property name="template" value="file:///some/resource/path/myTemplate.txt"/>

Application Contexts and Resource Paths

This section covers how to create application contexts with resources, including shortcuts that work with XML, how to use wildcards, and other details.

Constructing Application Contexts

An application context constructor (for a specific application context type) generally takes a string or array of strings as the location paths of the resources, such as XML files that make up the definition of the context.

When such a location path does not have a prefix, the specific Resource type built from that path and used to load the bean definitions depends on and is appropriate to the specific application context. For example, consider the following example, which creates a ClassPathXmlApplicationContext:

Java
ApplicationContext ctx = new ClassPathXmlApplicationContext("conf/appContext.xml");
Kotlin
val ctx = ClassPathXmlApplicationContext("conf/appContext.xml")

The bean definitions are loaded from the classpath, because a ClassPathResource is used. However, consider the following example, which creates a FileSystemXmlApplicationContext:

Java
ApplicationContext ctx =
    new FileSystemXmlApplicationContext("conf/appContext.xml");
Kotlin
val ctx = FileSystemXmlApplicationContext("conf/appContext.xml")

Now the bean definition is loaded from a filesystem location (in this case, relative to the current working directory).

Note that the use of the special classpath prefix or a standard URL prefix on the location path overrides the default type of Resource created to load the definition. Consider the following example:

Java
ApplicationContext ctx =
    new FileSystemXmlApplicationContext("classpath:conf/appContext.xml");
Kotlin
val ctx = FileSystemXmlApplicationContext("classpath:conf/appContext.xml")

Using FileSystemXmlApplicationContext loads the bean definitions from the classpath. However, it is still a FileSystemXmlApplicationContext. If it is subsequently used as a ResourceLoader, any unprefixed paths are still treated as filesystem paths.

Constructing ClassPathXmlApplicationContext Instances — Shortcuts

The ClassPathXmlApplicationContext exposes a number of constructors to enable convenient instantiation. The basic idea is that you can supply merely a string array that contains only the filenames of the XML files themselves (without the leading path information) and also supplies a Class. The ClassPathXmlApplicationContext then derives the path information from the supplied class.

Consider the following directory layout:

com/
  foo/
    services.xml
    daos.xml
    MessengerService.class

The following example shows how a ClassPathXmlApplicationContext instance composed of the beans defined in files named services.xml and daos.xml (which are on the classpath) can be instantiated:

Java
ApplicationContext ctx = new ClassPathXmlApplicationContext(
    new String[] {"services.xml", "daos.xml"}, MessengerService.class);
Kotlin
val ctx = ClassPathXmlApplicationContext(arrayOf("services.xml", "daos.xml"), MessengerService::class.java)

See the ClassPathXmlApplicationContext javadoc for details on the various constructors.

Wildcards in Application Context Constructor Resource Paths

The resource paths in application context constructor values may be simple paths (as shown earlier), each of which has a one-to-one mapping to a target Resource or, alternately, may contain the special "classpath*:" prefix or internal Ant-style regular expressions (matched by using Spring’s PathMatcher utility). Both of the latter are effectively wildcards.

One use for this mechanism is when you need to do component-style application assembly. All components can 'publish' context definition fragments to a well-known location path, and, when the final application context is created using the same path prefixed with classpath*:, all component fragments are automatically picked up.

Note that this wildcarding is specific to the use of resource paths in application context constructors (or when you use the PathMatcher utility class hierarchy directly) and is resolved at construction time. It has nothing to do with the Resource type itself. You cannot use the classpath*: prefix to construct an actual Resource, as a resource points to just one resource at a time.

Ant-style Patterns

Path locations can contain Ant-style patterns, as the following example shows:

/WEB-INF/-context.xml
com/mycompany//applicationContext.xml
file:C:/some/path/-context.xml
classpath:com/mycompany//applicationContext.xml

When the path location contains an Ant-style pattern, the resolver follows a more complex procedure to try to resolve the wildcard. It produces a Resource for the path up to the last non-wildcard segment and obtains a URL from it. If this URL is not a jar: URL or container-specific variant (such as zip: in WebLogic, wsjar in WebSphere, and so on), a java.io.File is obtained from it and used to resolve the wildcard by traversing the filesystem. In the case of a jar URL, the resolver either gets a java.net.JarURLConnection from it or manually parses the jar URL and then traverses the contents of the jar file to resolve the wildcards.

Implications on Portability

If the specified path is already a file URL (either implicitly because the base ResourceLoader is a filesystem one or explicitly), wildcarding is guaranteed to work in a completely portable fashion.

If the specified path is a classpath location, the resolver must obtain the last non-wildcard path segment URL by making a Classloader.getResource() call. Since this is just a node of the path (not the file at the end), it is actually undefined (in the ClassLoader javadoc) exactly what sort of a URL is returned in this case. In practice, it is always a java.io.File representing the directory (where the classpath resource resolves to a filesystem location) or a jar URL of some sort (where the classpath resource resolves to a jar location). Still, there is a portability concern on this operation.

If a jar URL is obtained for the last non-wildcard segment, the resolver must be able to get a java.net.JarURLConnection from it or manually parse the jar URL, to be able to walk the contents of the jar and resolve the wildcard. This does work in most environments but fails in others, and we strongly recommend that the wildcard resolution of resources coming from jars be thoroughly tested in your specific environment before you rely on it.

The classpath*: Prefix

When constructing an XML-based application context, a location string may use the special classpath*: prefix, as the following example shows:

Java
ApplicationContext ctx =
    new ClassPathXmlApplicationContext("classpath*:conf/appContext.xml");
Kotlin
val ctx = ClassPathXmlApplicationContext("classpath*:conf/appContext.xml")

This special prefix specifies that all classpath resources that match the given name must be obtained (internally, this essentially happens through a call to ClassLoader.getResources(…​)) and then merged to form the final application context definition.

The wildcard classpath relies on the getResources() method of the underlying classloader. As most application servers nowadays supply their own classloader implementation, the behavior might differ, especially when dealing with jar files. A simple test to check if classpath* works is to use the classloader to load a file from within a jar on the classpath: getClass().getClassLoader().getResources("<someFileInsideTheJar>"). Try this test with files that have the same name but are placed inside two different locations. In case an inappropriate result is returned, check the application server documentation for settings that might affect the classloader behavior.

You can also combine the classpath*: prefix with a PathMatcher pattern in the rest of the location path (for example, classpath*:META-INF/*-beans.xml). In this case, the resolution strategy is fairly simple: A ClassLoader.getResources() call is used on the last non-wildcard path segment to get all the matching resources in the class loader hierarchy and then, off each resource, the same PathMatcher resolution strategy described earlier is used for the wildcard subpath.

Other Notes Relating to Wildcards

Note that classpath*:, when combined with Ant-style patterns, only works reliably with at least one root directory before the pattern starts, unless the actual target files reside in the file system. This means that a pattern such as classpath*:*.xml might not retrieve files from the root of jar files but rather only from the root of expanded directories.

Spring’s ability to retrieve classpath entries originates from the JDK’s ClassLoader.getResources() method, which only returns file system locations for an empty string (indicating potential roots to search). Spring evaluates URLClassLoader runtime configuration and the java.class.path manifest in jar files as well, but this is not guaranteed to lead to portable behavior.

The scanning of classpath packages requires the presence of corresponding directory entries in the classpath. When you build JARs with Ant, do not activate the files-only switch of the JAR task. Also, classpath directories may not get exposed based on security policies in some environments — for example, stand-alone applications on JDK 1.7.0_45 and higher (which requires 'Trusted-Library' to be set up in your manifests. See https://stackoverflow.com/questions/19394570/java-jre-7u45-breaks-classloader-getresources).

On JDK 9’s module path (Jigsaw), Spring’s classpath scanning generally works as expected. Putting resources into a dedicated directory is highly recommendable here as well, avoiding the aforementioned portability problems with searching the jar file root level.

Ant-style patterns with classpath: resources are not guaranteed to find matching resources if the root package to search is available in multiple class path locations. Consider the following example of a resource location:

com/mycompany/package1/service-context.xml

Now consider an Ant-style path that someone might use to try to find that file:

classpath:com/mycompany/**/service-context.xml

Such a resource may be in only one location, but when a path such as the preceding example is used to try to resolve it, the resolver works off the (first) URL returned by getResource("com/mycompany");. If this base package node exists in multiple classloader locations, the actual end resource may not be there. Therefore, in such a case you should prefer using classpath*: with the same Ant-style pattern, which searches all class path locations that contain the root package.

FileSystemResource Caveats

A FileSystemResource that is not attached to a FileSystemApplicationContext (that is, when a FileSystemApplicationContext is not the actual ResourceLoader) treats absolute and relative paths as you would expect. Relative paths are relative to the current working directory, while absolute paths are relative to the root of the filesystem.

For backwards compatibility (historical) reasons however, this changes when the FileSystemApplicationContext is the ResourceLoader. The FileSystemApplicationContext forces all attached FileSystemResource instances to treat all location paths as relative, whether they start with a leading slash or not. In practice, this means the following examples are equivalent:

Java
ApplicationContext ctx =
    new FileSystemXmlApplicationContext("conf/context.xml");
Kotlin
val ctx = FileSystemXmlApplicationContext("conf/context.xml")
Java
ApplicationContext ctx =
    new FileSystemXmlApplicationContext("/conf/context.xml");
Kotlin
val ctx = FileSystemXmlApplicationContext("/conf/context.xml")

The following examples are also equivalent (even though it would make sense for them to be different, as one case is relative and the other absolute):

Java
FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("some/resource/path/myTemplate.txt");
Kotlin
val ctx: FileSystemXmlApplicationContext = ...
ctx.getResource("some/resource/path/myTemplate.txt")
Java
FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("/some/resource/path/myTemplate.txt");
Kotlin
val ctx: FileSystemXmlApplicationContext = ...
ctx.getResource("/some/resource/path/myTemplate.txt")

In practice, if you need true absolute filesystem paths, you should avoid using absolute paths with FileSystemResource or FileSystemXmlApplicationContext and force the use of a UrlResource by using the file: URL prefix. The following examples show how to do so:

Java
// actual context type doesn't matter, the Resource will always be UrlResource
ctx.getResource("file:///some/resource/path/myTemplate.txt");
Kotlin
// actual context type doesn't matter, the Resource will always be UrlResource
ctx.getResource("file:///some/resource/path/myTemplate.txt")
Java
// force this FileSystemXmlApplicationContext to load its definition via a UrlResource
ApplicationContext ctx =
    new FileSystemXmlApplicationContext("file:///conf/context.xml");
Kotlin
// force this FileSystemXmlApplicationContext to load its definition via a UrlResource
val ctx = FileSystemXmlApplicationContext("file:///conf/context.xml")

Validation, Data Binding, and Type Conversion

There are pros and cons for considering validation as business logic, and Spring offers a design for validation (and data binding) that does not exclude either one of them. Specifically, validation should not be tied to the web tier and should be easy to localize, and it should be possible to plug in any available validator. Considering these concerns, Spring has come up with a Validator interface that is both basic and eminently usable in every layer of an application.

Data binding is useful for letting user input be dynamically bound to the domain model of an application (or whatever objects you use to process user input). Spring provides the aptly named DataBinder to do exactly that. The Validator and the DataBinder make up the validation package, which is primarily used in but not limited to the MVC framework.

The BeanWrapper is a fundamental concept in the Spring Framework and is used in a lot of places. However, you probably do not need to use the BeanWrapper directly. Because this is reference documentation, however, we felt that some explanation might be in order. We explain the BeanWrapper in this chapter, since, if you are going to use it at all, you are most likely do so when trying to bind data to objects.

Spring’s DataBinder and the lower-level BeanWrapper both use PropertyEditorSupport implementations to parse and format property values. The PropertyEditor and PropertyEditorSupport types are part of the JavaBeans specification and are also explained in this chapter. Spring 3 introduced a core.convert package that provides a general type conversion facility, as well as a higher-level “format” package for formatting UI field values. You can use these packages as simpler alternatives to PropertyEditorSupport implementations. They are also discussed in this chapter.

JSR-303/JSR-349 Bean Validation

As of version 4.0, Spring Framework supports Bean Validation 1.0 (JSR-303) and Bean Validation 1.1 (JSR-349) for setup support and adapting them to Spring’s Validator interface.

An application can choose to enable Bean Validation once globally, as described in Spring Validation, and use it exclusively for all validation needs.

An application can also register additional Spring Validator instances for each DataBinder instance, as described in Configuring a DataBinder. This may be useful for plugging in validation logic without the use of annotations.

Validation by Using Spring’s Validator Interface

Spring features a Validator interface that you can use to validate objects. The Validator interface works by using an Errors object so that, while validating, validators can report validation failures to the Errors object.

Consider the following example of a small data object:

Java
public class Person {

    private String name;
    private int age;

    // the usual getters and setters...
}
Kotlin
class Person(val name: String, val age: Int)

The next example provides validation behavior for the Person class by implementing the following two methods of the org.springframework.validation.Validator interface:

  • supports(Class): Can this Validator validate instances of the supplied Class?

  • validate(Object, org.springframework.validation.Errors): Validates the given object and, in case of validation errors, registers those with the given Errors object.

Implementing a Validator is fairly straightforward, especially when you know of the ValidationUtils helper class that the Spring Framework also provides. The following example implements Validator for Person instances:

Java
public class PersonValidator implements Validator {

    /**
     * This Validator validates only Person instances
     */
    public boolean supports(Class clazz) {
        return Person.class.equals(clazz);
    }

    public void validate(Object obj, Errors e) {
        ValidationUtils.rejectIfEmpty(e, "name", "name.empty");
        Person p = (Person) obj;
        if (p.getAge() < 0) {
            e.rejectValue("age", "negativevalue");
        } else if (p.getAge() > 110) {
            e.rejectValue("age", "too.darn.old");
        }
    }
}
Kotlin
class PersonValidator : Validator {

    /*
     * This Validator validates only Person instances
     */
    override fun supports(clazz: Class<>): Boolean {
        return Person::class.java == clazz
    }

    override fun validate(obj: Any, e: Errors) {
        ValidationUtils.rejectIfEmpty(e, "name", "name.empty")
        val p = obj as Person
        if (p.age < 0) {
            e.rejectValue("age", "negativevalue")
        } else if (p.age > 110) {
            e.rejectValue("age", "too.darn.old")
        }
    }
}

The static rejectIfEmpty(..) method on the ValidationUtils class is used to reject the name property if it is null or the empty string. Have a look at the ValidationUtils javadoc to see what functionality it provides besides the example shown previously.

While it is certainly possible to implement a single Validator class to validate each of the nested objects in a rich object, it may be better to encapsulate the validation logic for each nested class of object in its own Validator implementation. A simple example of a “rich” object would be a Customer that is composed of two String properties (a first and a second name) and a complex Address object. Address objects may be used independently of Customer objects, so a distinct AddressValidator has been implemented. If you want your CustomerValidator to reuse the logic contained within the AddressValidator class without resorting to copy-and-paste, you can dependency-inject or instantiate an AddressValidator within your CustomerValidator, as the following example shows:

Java
public class CustomerValidator implements Validator {

    private final Validator addressValidator;

    public CustomerValidator(Validator addressValidator) {
        if (addressValidator == null) {
            throw new IllegalArgumentException("The supplied [Validator] is " +
                "required and must not be null.");
        }
        if (!addressValidator.supports(Address.class)) {
            throw new IllegalArgumentException("The supplied [Validator] must " +
                "support the validation of [Address] instances.");
        }
        this.addressValidator = addressValidator;
    }

    /**
     * This Validator validates Customer instances, and any subclasses of Customer too
     */
    public boolean supports(Class clazz) {
        return Customer.class.isAssignableFrom(clazz);
    }

    public void validate(Object target, Errors errors) {
        ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "field.required");
        ValidationUtils.rejectIfEmptyOrWhitespace(errors, "surname", "field.required");
        Customer customer = (Customer) target;
        try {
            errors.pushNestedPath("address");
            ValidationUtils.invokeValidator(this.addressValidator, customer.getAddress(), errors);
        } finally {
            errors.popNestedPath();
        }
    }
}
Kotlin
class CustomerValidator(private val addressValidator: Validator) : Validator {

    init {
        if (addressValidator == null) {
            throw IllegalArgumentException("The supplied [Validator] is required and must not be null.")
        }
        if (!addressValidator.supports(Address::class.java)) {
            throw IllegalArgumentException("The supplied [Validator] must support the validation of [Address] instances.")
        }
    }

    /*
    * This Validator validates Customer instances, and any subclasses of Customer too
    */
    override fun supports(clazz: Class<>): Boolean {
        return Customer::class.java.isAssignableFrom(clazz)
    }

    override fun validate(target: Any, errors: Errors) {
        ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "field.required")
        ValidationUtils.rejectIfEmptyOrWhitespace(errors, "surname", "field.required")
        val customer = target as Customer
        try {
            errors.pushNestedPath("address")
            ValidationUtils.invokeValidator(this.addressValidator, customer.address, errors)
        } finally {
            errors.popNestedPath()
        }
    }
}

Validation errors are reported to the Errors object passed to the validator. In the case of Spring Web MVC, you can use the <spring:bind/> tag to inspect the error messages, but you can also inspect the Errors object yourself. More information about the methods it offers can be found in the javadoc.

Resolving Codes to Error Messages

We covered databinding and validation. This section covers outputting messages that correspond to validation errors. In the example shown in the preceding section, we rejected the name and age fields. If we want to output the error messages by using a MessageSource, we can do so using the error code we provide when rejecting the field ('name' and 'age' in this case). When you call (either directly, or indirectly, by using, for example, the ValidationUtils class) rejectValue or one of the other reject methods from the Errors interface, the underlying implementation not only registers the code you passed in but also registers a number of additional error codes. The MessageCodesResolver determines which error codes the Errors interface registers. By default, the DefaultMessageCodesResolver is used, which (for example) not only registers a message with the code you gave but also registers messages that include the field name you passed to the reject method. So, if you reject a field by using rejectValue("age", "too.darn.old"), apart from the too.darn.old code, Spring also registers too.darn.old.age and too.darn.old.age.int (the first includes the field name and the second includes the type of the field). This is done as a convenience to aid developers when targeting error messages.

More information on the MessageCodesResolver and the default strategy can be found in the javadoc of MessageCodesResolver and DefaultMessageCodesResolver, respectively.

Bean Manipulation and the BeanWrapper

The org.springframework.beans package adheres to the JavaBeans standard. A JavaBean is a class with a default no-argument constructor and that follows a naming convention where (for example) a property named bingoMadness would have a setter method setBingoMadness(..) and a getter method getBingoMadness(). For more information about JavaBeans and the specification, see javabeans.

One quite important class in the beans package is the BeanWrapper interface and its corresponding implementation (BeanWrapperImpl). As quoted from the javadoc, the BeanWrapper offers functionality to set and get property values (individually or in bulk), get property descriptors, and query properties to determine if they are readable or writable. Also, the BeanWrapper offers support for nested properties, enabling the setting of properties on sub-properties to an unlimited depth. The BeanWrapper also supports the ability to add standard JavaBeans PropertyChangeListeners and VetoableChangeListeners, without the need for supporting code in the target class. Last but not least, the BeanWrapper provides support for setting indexed properties. The BeanWrapper usually is not used by application code directly but is used by the DataBinder and the BeanFactory.

The way the BeanWrapper works is partly indicated by its name: it wraps a bean to perform actions on that bean, such as setting and retrieving properties.

Setting and Getting Basic and Nested Properties

Setting and getting properties is done by using the setPropertyValue, setPropertyValues, getPropertyValue, and getPropertyValues methods which come with a couple of overloaded variants. Springs javadoc describes them in more detail. The JavaBeans specification has conventions for indicating properties of an object. The following table shows some examples of these conventions:

Table 13. Examples of properties
Expression Explanation

name

Indicates the property name that corresponds to the getName() or isName() and setName(..) methods.

account.name

Indicates the nested property name of the property account that corresponds to (for example) the getAccount().setName() or getAccount().getName() methods.

account[2]

Indicates the third element of the indexed property account. Indexed properties can be of type array, list, or other naturally ordered collection.

account[COMPANYNAME]

Indicates the value of the map entry indexed by the COMPANYNAME key of the account Map property.

(This next section is not vitally important to you if you do not plan to work with the BeanWrapper directly. If you use only the DataBinder and the BeanFactory and their default implementations, you should skip ahead to the section on PropertyEditors.)

The following two example classes use the BeanWrapper to get and set properties:

Java
public class Company {

    private String name;
    private Employee managingDirector;

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Employee getManagingDirector() {
        return this.managingDirector;
    }

    public void setManagingDirector(Employee managingDirector) {
        this.managingDirector = managingDirector;
    }
}
Kotlin
class Company {
    var name: String? = null
    var managingDirector: Employee? = null
}
Java
public class Employee {

    private String name;

    private float salary;

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public float getSalary() {
        return salary;
    }

    public void setSalary(float salary) {
        this.salary = salary;
    }
}
Kotlin
class Employee {
    var name: String? = null
    var salary: Float? = null
}

The following code snippets show some examples of how to retrieve and manipulate some of the properties of instantiated Companies and Employees:

Java
BeanWrapper company = new BeanWrapperImpl(new Company());
// setting the company name..
company.setPropertyValue("name", "Some Company Inc.");
// ... can also be done like this:
PropertyValue value = new PropertyValue("name", "Some Company Inc.");
company.setPropertyValue(value);

// ok, let's create the director and tie it to the company:
BeanWrapper jim = new BeanWrapperImpl(new Employee());
jim.setPropertyValue("name", "Jim Stravinsky");
company.setPropertyValue("managingDirector", jim.getWrappedInstance());

// retrieving the salary of the managingDirector through the company
Float salary = (Float) company.getPropertyValue("managingDirector.salary");
Kotlin
val company = BeanWrapperImpl(Company())
// setting the company name..
company.setPropertyValue("name", "Some Company Inc.")
// ... can also be done like this:
val value = PropertyValue("name", "Some Company Inc.")
company.setPropertyValue(value)

// ok, let's create the director and tie it to the company:
val jim = BeanWrapperImpl(Employee())
jim.setPropertyValue("name", "Jim Stravinsky")
company.setPropertyValue("managingDirector", jim.wrappedInstance)

// retrieving the salary of the managingDirector through the company
val salary = company.getPropertyValue("managingDirector.salary") as Float?

Built-in PropertyEditor Implementations

Spring uses the concept of a PropertyEditor to effect the conversion between an Object and a String. It can be handy to represent properties in a different way than the object itself. For example, a Date can be represented in a human readable way (as the String: '2007-14-09'), while we can still convert the human readable form back to the original date (or, even better, convert any date entered in a human readable form back to Date objects). This behavior can be achieved by registering custom editors of type java.beans.PropertyEditor. Registering custom editors on a BeanWrapper or, alternatively, in a specific IoC container (as mentioned in the previous chapter), gives it the knowledge of how to convert properties to the desired type. For more about PropertyEditor, see the javadoc of the java.beans package from Oracle.

A couple of examples where property editing is used in Spring:

  • Setting properties on beans is done by using PropertyEditor implementations. When you use String as the value of a property of some bean that you declare in an XML file, Spring (if the setter of the corresponding property has a Class parameter) uses ClassEditor to try to resolve the parameter to a Class object.

  • Parsing HTTP request parameters in Spring’s MVC framework is done by using all kinds of PropertyEditor implementations that you can manually bind in all subclasses of the CommandController.

Spring has a number of built-in PropertyEditor implementations to make life easy. They are all located in the org.springframework.beans.propertyeditors package. Most, (but not all, as indicated in the following table) are, by default, registered by BeanWrapperImpl. Where the property editor is configurable in some fashion, you can still register your own variant to override the default one. The following table describes the various PropertyEditor implementations that Spring provides:

Table 14. Built-in PropertyEditor Implementations
Class Explanation

ByteArrayPropertyEditor

Editor for byte arrays. Converts strings to their corresponding byte representations. Registered by default by BeanWrapperImpl.

ClassEditor

Parses Strings that represent classes to actual classes and vice-versa. When a class is not found, an IllegalArgumentException is thrown. By default, registered by BeanWrapperImpl.

CustomBooleanEditor

Customizable property editor for Boolean properties. By default, registered by BeanWrapperImpl but can be overridden by registering a custom instance of it as a custom editor.

CustomCollectionEditor

Property editor for collections, converting any source Collection to a given target Collection type.

CustomDateEditor

Customizable property editor for java.util.Date, supporting a custom DateFormat. NOT registered by default. Must be user-registered with the appropriate format as needed.

CustomNumberEditor

Customizable property editor for any Number subclass, such as Integer, Long, Float, or Double. By default, registered by BeanWrapperImpl but can be overridden by registering a custom instance of it as a custom editor.

FileEditor

Resolves strings to java.io.File objects. By default, registered by BeanWrapperImpl.

InputStreamEditor

One-way property editor that can take a string and produce (through an intermediate ResourceEditor and Resource) an InputStream so that InputStream properties may be directly set as strings. Note that the default usage does not close the InputStream for you. By default, registered by BeanWrapperImpl.

LocaleEditor

Can resolve strings to Locale objects and vice-versa (the string format is [country][variant], same as the toString() method of Locale). By default, registered by BeanWrapperImpl.

PatternEditor

Can resolve strings to java.util.regex.Pattern objects and vice-versa.

PropertiesEditor

Can convert strings (formatted with the format defined in the javadoc of the java.util.Properties class) to Properties objects. By default, registered by BeanWrapperImpl.

StringTrimmerEditor

Property editor that trims strings. Optionally allows transforming an empty string into a null value. NOT registered by default — must be user-registered.

URLEditor

Can resolve a string representation of a URL to an actual URL object. By default, registered by BeanWrapperImpl.

Spring uses the java.beans.PropertyEditorManager to set the search path for property editors that might be needed. The search path also includes sun.bean.editors, which includes PropertyEditor implementations for types such as Font, Color, and most of the primitive types. Note also that the standard JavaBeans infrastructure automatically discovers PropertyEditor classes (without you having to register them explicitly) if they are in the same package as the class they handle and have the same name as that class, with Editor appended. For example, one could have the following class and package structure, which would be sufficient for the SomethingEditor class to be recognized and used as the PropertyEditor for Something-typed properties.

com
  chank
    pop
      Something
      SomethingEditor // the PropertyEditor for the Something class

Note that you can also use the standard BeanInfo JavaBeans mechanism here as well (described to some extent here). The following example use the BeanInfo mechanism to explicitly register one or more PropertyEditor instances with the properties of an associated class:

com
  chank
    pop
      Something
      SomethingBeanInfo // the BeanInfo for the Something class

The following Java source code for the referenced SomethingBeanInfo class associates a CustomNumberEditor with the age property of the Something class:

Java
public class SomethingBeanInfo extends SimpleBeanInfo {

    public PropertyDescriptor[] getPropertyDescriptors() {
        try {
            final PropertyEditor numberPE = new CustomNumberEditor(Integer.class, true);
            PropertyDescriptor ageDescriptor = new PropertyDescriptor("age", Something.class) {
                public PropertyEditor createPropertyEditor(Object bean) {
                    return numberPE;
                };
            };
            return new PropertyDescriptor[] { ageDescriptor };
        }
        catch (IntrospectionException ex) {
            throw new Error(ex.toString());
        }
    }
}
Kotlin
class SomethingBeanInfo : SimpleBeanInfo() {

    override fun getPropertyDescriptors(): Array<PropertyDescriptor> {
        try {
            val numberPE = CustomNumberEditor(Int::class.java, true)
            val ageDescriptor = object : PropertyDescriptor("age", Something::class.java) {
                override fun createPropertyEditor(bean: Any): PropertyEditor {
                    return numberPE
                }
            }
            return arrayOf(ageDescriptor)
        } catch (ex: IntrospectionException) {
            throw Error(ex.toString())
        }

    }
}
Registering Additional Custom PropertyEditor Implementations

When setting bean properties as string values, a Spring IoC container ultimately uses standard JavaBeans PropertyEditor implementations to convert these strings to the complex type of the property. Spring pre-registers a number of custom PropertyEditor implementations (for example, to convert a class name expressed as a string into a Class object). Additionally, Java’s standard JavaBeans PropertyEditor lookup mechanism lets a PropertyEditor for a class be named appropriately and placed in the same package as the class for which it provides support, so that it can be found automatically.

If there is a need to register other custom PropertyEditors, several mechanisms are available. The most manual approach, which is not normally convenient or recommended, is to use the registerCustomEditor() method of the ConfigurableBeanFactory interface, assuming you have a BeanFactory reference. Another (slightly more convenient) mechanism is to use a special bean factory post-processor called CustomEditorConfigurer. Although you can use bean factory post-processors with BeanFactory implementations, the CustomEditorConfigurer has a nested property setup, so we strongly recommend that you use it with the ApplicationContext, where you can deploy it in similar fashion to any other bean and where it can be automatically detected and applied.

Note that all bean factories and application contexts automatically use a number of built-in property editors, through their use a BeanWrapper to handle property conversions. The standard property editors that the BeanWrapper registers are listed in the previous section. Additionally, ApplicationContexts also override or add additional editors to handle resource lookups in a manner appropriate to the specific application context type.

Standard JavaBeans PropertyEditor instances are used to convert property values expressed as strings to the actual complex type of the property. You can use CustomEditorConfigurer, a bean factory post-processor, to conveniently add support for additional PropertyEditor instances to an ApplicationContext.

Consider the following example, which defines a user class called ExoticType and another class called DependsOnExoticType, which needs ExoticType set as a property:

Java
package example;

public class ExoticType {

    private String name;

    public ExoticType(String name) {
        this.name = name;
    }
}

public class DependsOnExoticType {

    private ExoticType type;

    public void setType(ExoticType type) {
        this.type = type;
    }
}
Kotlin
package example

class ExoticType(val name: String)

class DependsOnExoticType {

    var type: ExoticType? = null
}

When things are properly set up, we want to be able to assign the type property as a string, which a PropertyEditor converts into an actual ExoticType instance. The following bean definition shows how to set up this relationship:

<bean id="sample" class="example.DependsOnExoticType">
    <property name="type" value="aNameForExoticType"/>
</bean>

The PropertyEditor implementation could look similar to the following:

Java
// converts string representation to ExoticType object
package example;

public class ExoticTypeEditor extends PropertyEditorSupport {

    public void setAsText(String text) {
        setValue(new ExoticType(text.toUpperCase()));
    }
}
Kotlin
// converts string representation to ExoticType object
package example

import java.beans.PropertyEditorSupport

class ExoticTypeEditor : PropertyEditorSupport() {

    override fun setAsText(text: String) {
        value = ExoticType(text.toUpperCase())
    }
}

Finally, the following example shows how to use CustomEditorConfigurer to register the new PropertyEditor with the ApplicationContext, which will then be able to use it as needed:

<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
    <property name="customEditors">
        <map>
            <entry key="example.ExoticType" value="example.ExoticTypeEditor"/>
        </map>
    </property>
</bean>
Using PropertyEditorRegistrar

Another mechanism for registering property editors with the Spring container is to create and use a PropertyEditorRegistrar. This interface is particularly useful when you need to use the same set of property editors in several different situations. You can write a corresponding registrar and reuse it in each case. PropertyEditorRegistrar instances work in conjunction with an interface called PropertyEditorRegistry, an interface that is implemented by the Spring BeanWrapper (and DataBinder). PropertyEditorRegistrar instances are particularly convenient when used in conjunction with CustomEditorConfigurer (described here), which exposes a property called setPropertyEditorRegistrars(..). PropertyEditorRegistrar instances added to a CustomEditorConfigurer in this fashion can easily be shared with DataBinder and Spring MVC controllers. Furthermore, it avoids the need for synchronization on custom editors: A PropertyEditorRegistrar is expected to create fresh PropertyEditor instances for each bean creation attempt.

The following example shows how to create your own PropertyEditorRegistrar implementation:

Java
package com.foo.editors.spring;

public final class CustomPropertyEditorRegistrar implements PropertyEditorRegistrar {

    public void registerCustomEditors(PropertyEditorRegistry registry) {

        // it is expected that new PropertyEditor instances are created
        registry.registerCustomEditor(ExoticType.class, new ExoticTypeEditor());

        // you could register as many custom property editors as are required here...
    }
}
Kotlin
package com.foo.editors.spring

import org.springframework.beans.PropertyEditorRegistrar
import org.springframework.beans.PropertyEditorRegistry

class CustomPropertyEditorRegistrar : PropertyEditorRegistrar {

    override fun registerCustomEditors(registry: PropertyEditorRegistry) {

        // it is expected that new PropertyEditor instances are created
        registry.registerCustomEditor(ExoticType::class.java, ExoticTypeEditor())

        // you could register as many custom property editors as are required here...
    }
}

See also the org.springframework.beans.support.ResourceEditorRegistrar for an example PropertyEditorRegistrar implementation. Notice how in its implementation of the registerCustomEditors(..) method ,it creates new instances of each property editor.

The next example shows how to configure a CustomEditorConfigurer and inject an instance of our CustomPropertyEditorRegistrar into it:

<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
    <property name="propertyEditorRegistrars">
        <list>
            <ref bean="customPropertyEditorRegistrar"/>
        </list>
    </property>
</bean>

<bean id="customPropertyEditorRegistrar"
    class="com.foo.editors.spring.CustomPropertyEditorRegistrar"/>

Finally (and in a bit of a departure from the focus of this chapter for those of you using Spring’s MVC web framework), using PropertyEditorRegistrars in conjunction with data-binding Controllers (such as SimpleFormController) can be very convenient. The following example uses a PropertyEditorRegistrar in the implementation of an initBinder(..) method:

Java
public final class RegisterUserController extends SimpleFormController {

    private final PropertyEditorRegistrar customPropertyEditorRegistrar;

    public RegisterUserController(PropertyEditorRegistrar propertyEditorRegistrar) {
        this.customPropertyEditorRegistrar = propertyEditorRegistrar;
    }

    protected void initBinder(HttpServletRequest request,
            ServletRequestDataBinder binder) throws Exception {
        this.customPropertyEditorRegistrar.registerCustomEditors(binder);
    }

    // other methods to do with registering a User
}
Kotlin
class RegisterUserController(
    private val customPropertyEditorRegistrar: PropertyEditorRegistrar) : SimpleFormController() {

    protected fun initBinder(request: HttpServletRequest,
                            binder: ServletRequestDataBinder) {
        this.customPropertyEditorRegistrar.registerCustomEditors(binder)
    }

    // other methods to do with registering a User
}

This style of PropertyEditor registration can lead to concise code (the implementation of initBinder(..) is only one line long) and lets common PropertyEditor registration code be encapsulated in a class and then shared amongst as many Controllers as needed.

Spring Type Conversion

Spring 3 introduced a core.convert package that provides a general type conversion system. The system defines an SPI to implement type conversion logic and an API to perform type conversions at runtime. Within a Spring container, you can use this system as an alternative to PropertyEditor implementations to convert externalized bean property value strings to the required property types. You can also use the public API anywhere in your application where type conversion is needed.

Converter SPI

The SPI to implement type conversion logic is simple and strongly typed, as the following interface definition shows:

Java
package org.springframework.core.convert.converter;

public interface Converter<S, T> {

    T convert(S source);
}
Kotlin
package org.springframework.core.convert.converter

interface Converter<S, T> {

    fun convert(source: S): T
}

To create your own converter, implement the Converter interface and parameterize S as the type you are converting from and T as the type you are converting to. You can also transparently apply such a converter if a collection or array of S needs to be converted to an array or collection of T, provided that a delegating array or collection converter has been registered as well (which DefaultConversionService does by default).

For each call to convert(S), the source argument is guaranteed to not be null. Your Converter may throw any unchecked exception if conversion fails. Specifically, it should throw an IllegalArgumentException to report an invalid source value. Take care to ensure that your Converter implementation is thread-safe.

Several converter implementations are provided in the core.convert.support package as a convenience. These include converters from strings to numbers and other common types. The following listing shows the StringToInteger class, which is a typical Converter implementation:

Java
package org.springframework.core.convert.support;

final class StringToInteger implements Converter<String, Integer> {

    public Integer convert(String source) {
        return Integer.valueOf(source);
    }
}
Kotlin
package org.springframework.core.convert.support

import org.springframework.core.convert.converter.Converter

internal class StringToInteger : Converter<String, Int> {

    override fun convert(source: String): Int? {
        return Integer.valueOf(source)
    }
}

Using ConverterFactory

When you need to centralize the conversion logic for an entire class hierarchy (for example, when converting from String to Enum objects), you can implement ConverterFactory, as the following example shows:

Java
package org.springframework.core.convert.converter;

public interface ConverterFactory<S, R> {

    <T extends R> Converter<S, T> getConverter(Class<T> targetType);
}
Kotlin
package org.springframework.core.convert.converter

interface ConverterFactory<S, R> {

    fun <T : R> getConverter(targetType: Class<T>): Converter<S, T>
}

Parameterize S to be the type you are converting from and R to be the base type defining the range of classes you can convert to. Then implement getConverter(Class<T>), where T is a subclass of R.

Consider the StringToEnumConverterFactory as an example:

Java
package org.springframework.core.convert.support;

final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {

    public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
        return new StringToEnumConverter(targetType);
    }

    private final class StringToEnumConverter<T extends Enum> implements Converter<String, T> {

        private Class<T> enumType;

        public StringToEnumConverter(Class<T> enumType) {
            this.enumType = enumType;
        }

        public T convert(String source) {
            return (T) Enum.valueOf(this.enumType, source.trim());
        }
    }
}

Using GenericConverter

When you require a sophisticated Converter implementation, consider using the GenericConverter interface. With a more flexible but less strongly typed signature than Converter, a GenericConverter supports converting between multiple source and target types. In addition, a GenericConverter makes available source and target field context that you can use when you implement your conversion logic. Such context lets a type conversion be driven by a field annotation or by generic information declared on a field signature. The following listing shows the interface definition of GenericConverter:

Java
package org.springframework.core.convert.converter;

public interface GenericConverter {

    public Set<ConvertiblePair> getConvertibleTypes();

    Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
}
Kotlin
package org.springframework.core.convert.converter

interface GenericConverter {

    fun getConvertibleTypes(): Set<ConvertiblePair>?

    fun convert(@Nullable source: Any?, sourceType: TypeDescriptor, targetType: TypeDescriptor): Any?
}

To implement a GenericConverter, have getConvertibleTypes() return the supported source→target type pairs. Then implement convert(Object, TypeDescriptor, TypeDescriptor) to contain your conversion logic. The source TypeDescriptor provides access to the source field that holds the value being converted. The target TypeDescriptor provides access to the target field where the converted value is to be set.

A good example of a GenericConverter is a converter that converts between a Java array and a collection. Such an ArrayToCollectionConverter introspects the field that declares the target collection type to resolve the collection’s element type. This lets each element in the source array be converted to the collection element type before the collection is set on the target field.

Because GenericConverter is a more complex SPI interface, you should use it only when you need it. Favor Converter or ConverterFactory for basic type conversion needs.
Using ConditionalGenericConverter

Sometimes, you want a Converter to run only if a specific condition holds true. For example, you might want to run a Converter only if a specific annotation is present on the target field, or you might want to run a Converter only if a specific method (such as a static valueOf method) is defined on the target class. ConditionalGenericConverter is the union of the GenericConverter and ConditionalConverter interfaces that lets you define such custom matching criteria:

Java
public interface ConditionalConverter {

    boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType);
}

public interface ConditionalGenericConverter extends GenericConverter, ConditionalConverter {
}
Kotlin
interface ConditionalConverter {

    fun matches(sourceType: TypeDescriptor, targetType: TypeDescriptor): Boolean
}

interface ConditionalGenericConverter : GenericConverter, ConditionalConverter

A good example of a ConditionalGenericConverter is an EntityConverter that converts between a persistent entity identifier and an entity reference. Such an EntityConverter might match only if the target entity type declares a static finder method (for example, findAccount(Long)). You might perform such a finder method check in the implementation of matches(TypeDescriptor, TypeDescriptor).

The ConversionService API

ConversionService defines a unified API for executing type conversion logic at runtime. Converters are often executed behind the following facade interface:

Java
package org.springframework.core.convert;

public interface ConversionService {

    boolean canConvert(Class<?> sourceType, Class<?> targetType);

    <T> T convert(Object source, Class<T> targetType);

    boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType);

    Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);

}
Kotlin
package org.springframework.core.convert

interface ConversionService {

    fun canConvert(sourceType: Class<*>, targetType: Class<*>): Boolean

    fun <T> convert(source: Any, targetType: Class<T>): T

    fun canConvert(sourceType: TypeDescriptor, targetType: TypeDescriptor): Boolean

    fun convert(source: Any, sourceType: TypeDescriptor, targetType: TypeDescriptor): Any

}

Most ConversionService implementations also implement ConverterRegistry, which provides an SPI for registering converters. Internally, a ConversionService implementation delegates to its registered converters to carry out type conversion logic.

A robust ConversionService implementation is provided in the core.convert.support package. GenericConversionService is the general-purpose implementation suitable for use in most environments. ConversionServiceFactory provides a convenient factory for creating common ConversionService configurations.

Configuring a ConversionService

A ConversionService is a stateless object designed to be instantiated at application startup and then shared between multiple threads. In a Spring application, you typically configure a ConversionService instance for each Spring container (or ApplicationContext). Spring picks up that ConversionService and uses it whenever a type conversion needs to be performed by the framework. You can also inject this ConversionService into any of your beans and invoke it directly.

If no ConversionService is registered with Spring, the original PropertyEditor-based system is used.

To register a default ConversionService with Spring, add the following bean definition with an id of conversionService:

<bean id="conversionService"
    class="org.springframework.context.support.ConversionServiceFactoryBean"/>

A default ConversionService can convert between strings, numbers, enums, collections, maps, and other common types. To supplement or override the default converters with your own custom converters, set the converters property. Property values can implement any of the Converter, ConverterFactory, or GenericConverter interfaces.

<bean id="conversionService"
        class="org.springframework.context.support.ConversionServiceFactoryBean">
    <property name="converters">
        <set>
            <bean class="example.MyCustomConverter"/>
        </set>
    </property>
</bean>

It is also common to use a ConversionService within a Spring MVC application. See Conversion and Formatting in the Spring MVC chapter.

In certain situations, you may wish to apply formatting during conversion. See The FormatterRegistry SPI for details on using FormattingConversionServiceFactoryBean.

Using a ConversionService Programmatically

To work with a ConversionService instance programmatically, you can inject a reference to it like you would for any other bean. The following example shows how to do so:

Java
@Service
public class MyService {

    public MyService(ConversionService conversionService) {
        this.conversionService = conversionService;
    }

    public void doIt() {
        this.conversionService.convert(...)
    }
}
Kotlin
@Service
class MyService(private val conversionService: ConversionService) {

    fun doIt() {
        conversionService.convert(...)
    }
}

For most use cases, you can use the convert method that specifies the targetType, but it does not work with more complex types, such as a collection of a parameterized element. For example, if you want to convert a List of Integer to a List of String programmatically, you need to provide a formal definition of the source and target types.

Fortunately, TypeDescriptor provides various options to make doing so straightforward, as the following example shows:

Java
DefaultConversionService cs = new DefaultConversionService();

List<Integer> input = ...
cs.convert(input,
    TypeDescriptor.forObject(input), // List<Integer> type descriptor
    TypeDescriptor.collection(List.class, TypeDescriptor.valueOf(String.class)));
Kotlin
val cs = DefaultConversionService()

val input: List<Integer> = ...
cs.convert(input,
        TypeDescriptor.forObject(input), // List<Integer> type descriptor
        TypeDescriptor.collection(List::class.java, TypeDescriptor.valueOf(String::class.java)))

Note that DefaultConversionService automatically registers converters that are appropriate for most environments. This includes collection converters, scalar converters, and basic Object-to-String converters. You can register the same converters with any ConverterRegistry by using the static addDefaultConverters method on the DefaultConversionService class.

Converters for value types are reused for arrays and collections, so there is no need to create a specific converter to convert from a Collection of S to a Collection of T, assuming that standard collection handling is appropriate.

Spring Field Formatting

As discussed in the previous section, core.convert is a general-purpose type conversion system. It provides a unified ConversionService API as well as a strongly typed Converter SPI for implementing conversion logic from one type to another. A Spring container uses this system to bind bean property values. In addition, both the Spring Expression Language (SpEL) and DataBinder use this system to bind field values. For example, when SpEL needs to coerce a Short to a Long to complete an expression.setValue(Object bean, Object value) attempt, the core.convert system performs the coercion.

Now consider the type conversion requirements of a typical client environment, such as a web or desktop application. In such environments, you typically convert from String to support the client postback process, as well as back to String to support the view rendering process. In addition, you often need to localize String values. The more general core.convert Converter SPI does not address such formatting requirements directly. To directly address them, Spring 3 introduced a convenient Formatter SPI that provides a simple and robust alternative to PropertyEditor implementations for client environments.

In general, you can use the Converter SPI when you need to implement general-purpose type conversion logic — for example, for converting between a java.util.Date and a Long. You can use the Formatter SPI when you work in a client environment (such as a web application) and need to parse and print localized field values. The ConversionService provides a unified type conversion API for both SPIs.

The Formatter SPI

The Formatter SPI to implement field formatting logic is simple and strongly typed. The following listing shows the Formatter interface definition:

Java
package org.springframework.format;

public interface Formatter<T> extends Printer<T>, Parser<T> {
}

Formatter extends from the Printer and Parser building-block interfaces. The following listing shows the definitions of those two interfaces:

Java
public interface Printer<T> {

    String print(T fieldValue, Locale locale);
}
Kotlin
interface Printer<T> {

    fun print(fieldValue: T, locale: Locale): String
}
Java
import java.text.ParseException;

public interface Parser<T> {

    T parse(String clientValue, Locale locale) throws ParseException;
}
Kotlin
interface Parser<T> {

    @Throws(ParseException::class)
    fun parse(clientValue: String, locale: Locale): T
}

To create your own Formatter, implement the Formatter interface shown earlier. Parameterize T to be the type of object you wish to format — for example, java.util.Date. Implement the print() operation to print an instance of T for display in the client locale. Implement the parse() operation to parse an instance of T from the formatted representation returned from the client locale. Your Formatter should throw a ParseException or an IllegalArgumentException if a parse attempt fails. Take care to ensure that your Formatter implementation is thread-safe.

The format subpackages provide several Formatter implementations as a convenience. The number package provides NumberStyleFormatter, CurrencyStyleFormatter, and PercentStyleFormatter to format Number objects that use a java.text.NumberFormat. The datetime package provides a DateFormatter to format java.util.Date objects with a java.text.DateFormat. The datetime.joda package provides comprehensive datetime formatting support based on the Joda-Time library.

The following DateFormatter is an example Formatter implementation:

Java
package org.springframework.format.datetime;

public final class DateFormatter implements Formatter<Date> {

    private String pattern;

    public DateFormatter(String pattern) {
        this.pattern = pattern;
    }

    public String print(Date date, Locale locale) {
        if (date == null) {
            return "";
        }
        return getDateFormat(locale).format(date);
    }

    public Date parse(String formatted, Locale locale) throws ParseException {
        if (formatted.length() == 0) {
            return null;
        }
        return getDateFormat(locale).parse(formatted);
    }

    protected DateFormat getDateFormat(Locale locale) {
        DateFormat dateFormat = new SimpleDateFormat(this.pattern, locale);
        dateFormat.setLenient(false);
        return dateFormat;
    }
}
Kotlin
class DateFormatter(private val pattern: String) : Formatter<Date> {

    override fun print(date: Date, locale: Locale)
            = getDateFormat(locale).format(date)

    @Throws(ParseException::class)
    override fun parse(formatted: String, locale: Locale)
            = getDateFormat(locale).parse(formatted)

    protected fun getDateFormat(locale: Locale): DateFormat {
        val dateFormat = SimpleDateFormat(this.pattern, locale)
        dateFormat.isLenient = false
        return dateFormat
    }
}

The Spring team welcomes community-driven Formatter contributions. See GitHub Issues to contribute.

Annotation-driven Formatting

Field formatting can be configured by field type or annotation. To bind an annotation to a Formatter, implement AnnotationFormatterFactory. The following listing shows the definition of the AnnotationFormatterFactory interface:

Java
package org.springframework.format;

public interface AnnotationFormatterFactory<A extends Annotation> {

    Set<Class<?>> getFieldTypes();

    Printer<?> getPrinter(A annotation, Class<?> fieldType);

    Parser<?> getParser(A annotation, Class<?> fieldType);
}
Kotlin
package org.springframework.format

interface AnnotationFormatterFactory<A : Annotation> {

    val fieldTypes: Set<Class<*>>

    fun getPrinter(annotation: A, fieldType: Class<*>): Printer<*>

    fun getParser(annotation: A, fieldType: Class<*>): Parser<*>
}

To create an implementation: . Parameterize A to be the field annotationType with which you wish to associate formatting logic — for example org.springframework.format.annotation.DateTimeFormat. . Have getFieldTypes() return the types of fields on which the annotation can be used. . Have getPrinter() return a Printer to print the value of an annotated field. . Have getParser() return a Parser to parse a clientValue for an annotated field.

The following example AnnotationFormatterFactory implementation binds the @NumberFormat annotation to a formatter to let a number style or pattern be specified:

Java
public final class NumberFormatAnnotationFormatterFactory
        implements AnnotationFormatterFactory<NumberFormat> {

    public Set<Class<?>> getFieldTypes() {
        return new HashSet<Class<?>>(asList(new Class<?>[] {
            Short.class, Integer.class, Long.class, Float.class,
            Double.class, BigDecimal.class, BigInteger.class }));
    }

    public Printer<Number> getPrinter(NumberFormat annotation, Class<?> fieldType) {
        return configureFormatterFrom(annotation, fieldType);
    }

    public Parser<Number> getParser(NumberFormat annotation, Class<?> fieldType) {
        return configureFormatterFrom(annotation, fieldType);
    }

    private Formatter<Number> configureFormatterFrom(NumberFormat annotation, Class<?> fieldType) {
        if (!annotation.pattern().isEmpty()) {
            return new NumberStyleFormatter(annotation.pattern());
        } else {
            Style style = annotation.style();
            if (style == Style.PERCENT) {
                return new PercentStyleFormatter();
            } else if (style == Style.CURRENCY) {
                return new CurrencyStyleFormatter();
            } else {
                return new NumberStyleFormatter();
            }
        }
    }
}
Kotlin
class NumberFormatAnnotationFormatterFactory : AnnotationFormatterFactory<NumberFormat> {

    override fun getFieldTypes(): Set<Class<*>> {
        return setOf(Short::class.java, Int::class.java, Long::class.java, Float::class.java, Double::class.java, BigDecimal::class.java, BigInteger::class.java)
    }

    override fun getPrinter(annotation: NumberFormat, fieldType: Class<*>): Printer<Number> {
        return configureFormatterFrom(annotation, fieldType)
    }

    override fun getParser(annotation: NumberFormat, fieldType: Class<*>): Parser<Number> {
        return configureFormatterFrom(annotation, fieldType)
    }

    private fun configureFormatterFrom(annotation: NumberFormat, fieldType: Class<*>): Formatter<Number> {
        return if (annotation.pattern.isNotEmpty()) {
            NumberStyleFormatter(annotation.pattern)
        } else {
            val style = annotation.style
            when {
                style === NumberFormat.Style.PERCENT -> PercentStyleFormatter()
                style === NumberFormat.Style.CURRENCY -> CurrencyStyleFormatter()
                else -> NumberStyleFormatter()
            }
        }
    }
}

To trigger formatting, you can annotate fields with @NumberFormat, as the following example shows:

Java
public class MyModel {

    @NumberFormat(style=Style.CURRENCY)
    private BigDecimal decimal;
}
Kotlin
class MyModel(
    @field:NumberFormat(style = Style.CURRENCY) private val decimal: BigDecimal
)
Format Annotation API

A portable format annotation API exists in the org.springframework.format.annotation package. You can use @NumberFormat to format Number fields such as Double and Long, and @DateTimeFormat to format java.util.Date, java.util.Calendar, Long (for millisecond timestamps) as well as JSR-310 java.time and Joda-Time value types.

The following example uses @DateTimeFormat to format a java.util.Date as an ISO Date (yyyy-MM-dd):

Java
public class MyModel {

    @DateTimeFormat(iso=ISO.DATE)
    private Date date;
}
Kotlin
class MyModel(
    @DateTimeFormat(iso= ISO.DATE) private val date: Date
)

The FormatterRegistry SPI

The FormatterRegistry is an SPI for registering formatters and converters. FormattingConversionService is an implementation of FormatterRegistry suitable for most environments. You can programmatically or declaratively configure this variant as a Spring bean, e.g. by using FormattingConversionServiceFactoryBean. Because this implementation also implements ConversionService, you can directly configure it for use with Spring’s DataBinder and the Spring Expression Language (SpEL).

The following listing shows the FormatterRegistry SPI:

Java
package org.springframework.format;

public interface FormatterRegistry extends ConverterRegistry {

    void addFormatterForFieldType(Class<?> fieldType, Printer<?> printer, Parser<?> parser);

    void addFormatterForFieldType(Class<?> fieldType, Formatter<?> formatter);

    void addFormatterForFieldType(Formatter<?> formatter);

    void addFormatterForAnnotation(AnnotationFormatterFactory<?> factory);
}
Kotlin
package org.springframework.format

interface FormatterRegistry : ConverterRegistry {

    fun addFormatterForFieldType(fieldType: Class<*>, printer: Printer<*>, parser: Parser<*>)

    fun addFormatterForFieldType(fieldType: Class<*>, formatter: Formatter<*>)

    fun addFormatterForFieldType(formatter: Formatter<*>)

    fun addFormatterForAnnotation(factory: AnnotationFormatterFactory<*>)
}

As shown in the preceding listing, you can register formatters by field type or by annotation.

The FormatterRegistry SPI lets you configure formatting rules centrally, instead of duplicating such configuration across your controllers. For example, you might want to enforce that all date fields are formatted a certain way or that fields with a specific annotation are formatted in a certain way. With a shared FormatterRegistry, you define these rules once, and they are applied whenever formatting is needed.

The FormatterRegistrar SPI

FormatterRegistrar is an SPI for registering formatters and converters through the FormatterRegistry. The following listing shows its interface definition:

Java
package org.springframework.format;

public interface FormatterRegistrar {

    void registerFormatters(FormatterRegistry registry);
}
Kotlin
package org.springframework.format

interface FormatterRegistrar {

    fun registerFormatters(registry: FormatterRegistry)
}

A FormatterRegistrar is useful when registering multiple related converters and formatters for a given formatting category, such as date formatting. It can also be useful where declarative registration is insufficient — for example, when a formatter needs to be indexed under a specific field type different from its own <T> or when registering a Printer/Parser pair. The next section provides more information on converter and formatter registration.

Configuring Formatting in Spring MVC

See Conversion and Formatting in the Spring MVC chapter.

Configuring a Global Date and Time Format

By default, date and time fields that are not annotated with @DateTimeFormat are converted from strings by using the DateFormat.SHORT style. If you prefer, you can change this by defining your own global format.

To do so, you need to ensure that Spring does not register default formatters. Instead, you should register all formatters manually. Use the org.springframework.format.datetime.joda.JodaTimeFormatterRegistrar or org.springframework.format.datetime.DateFormatterRegistrar class, depending on whether you use the Joda-Time library.

For example, the following Java configuration registers a global yyyyMMdd format (this example does not depend on the Joda-Time library):

Java
@Configuration
public class AppConfig {

    @Bean
    public FormattingConversionService conversionService() {

        // Use the DefaultFormattingConversionService but do not register defaults
        DefaultFormattingConversionService conversionService = new DefaultFormattingConversionService(false);

        // Ensure @NumberFormat is still supported
        conversionService.addFormatterForFieldAnnotation(new NumberFormatAnnotationFormatterFactory());

        // Register date conversion with a specific global format
        DateFormatterRegistrar registrar = new DateFormatterRegistrar();
        registrar.setFormatter(new DateFormatter("yyyyMMdd"));
        registrar.registerFormatters(conversionService);

        return conversionService;
    }
}
Kotlin
@Configuration
class AppConfig {

    @Bean
    fun conversionService(): FormattingConversionService {
        // Use the DefaultFormattingConversionService but do not register defaults
        return DefaultFormattingConversionService(false).apply {
            // Ensure @NumberFormat is still supported
            addFormatterForFieldAnnotation(NumberFormatAnnotationFormatterFactory())
            // Register date conversion with a specific global format
            val registrar = DateFormatterRegistrar()
            registrar.setFormatter(DateFormatter("yyyyMMdd"))
            registrar.registerFormatters(this)
        }
    }
}

If you prefer XML-based configuration, you can use a FormattingConversionServiceFactoryBean. The following example shows how to do so (this time using Joda Time):

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd>

    <bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
        <property name="registerDefaultFormatters" value="false" />
        <property name="formatters">
            <set>
                <bean class="org.springframework.format.number.NumberFormatAnnotationFormatterFactory" />
            </set>
        </property>
        <property name="formatterRegistrars">
            <set>
                <bean class="org.springframework.format.datetime.joda.JodaTimeFormatterRegistrar">
                    <property name="dateFormatter">
                        <bean class="org.springframework.format.datetime.joda.DateTimeFormatterFactoryBean">
                            <property name="pattern" value="yyyyMMdd"/>
                        </bean>
                    </property>
                </bean>
            </set>
        </property>
    </bean>
</beans>
Joda-Time provides separate distinct types to represent date, time, and date-time values. The dateFormatter, timeFormatter, and dateTimeFormatter properties of the JodaTimeFormatterRegistrar should be used to configure the different formats for each type. The DateTimeFormatterFactoryBean provides a convenient way to create formatters.
If you use Spring MVC, remember to explicitly configure the conversion service that is used. For Java-based @Configuration, this means extending the WebMvcConfigurationSupport class and overriding the mvcConversionService() method. For XML, you should use the conversion-service attribute of the mvc:annotation-driven element. See Conversion and Formatting for details.

Spring Validation

Spring 3 introduced several enhancements to its validation support. First, the JSR-303 Bean Validation API is fully supported. Second, when used programmatically, Spring’s DataBinder can validate objects as well as bind to them. Third, Spring MVC has support for declaratively validating @Controller inputs.

Overview of the JSR-303 Bean Validation API

JSR-303 standardizes validation constraint declaration and metadata for the Java platform. By using this API, you annotate domain model properties with declarative validation constraints and the runtime enforces them. You can use a number of built-in constraints. You can also define your own custom constraints.

Consider the following example, which shows a simple PersonForm model with two properties:

Java
public class PersonForm {
    private String name;
    private int age;
}
Kotlin
class PersonForm(
        private val name: String,
        private val age: Int
)

JSR-303 lets you define declarative validation constraints against such properties, as the following example shows:

Java
public class PersonForm {

    @NotNull
    @Size(max=64)
    private String name;

    @Min(0)
    private int age;
}
Kotlin
class PersonForm(
    @get:NotNull @get:Size(max=64)
    private val name: String,
    @get:Min(0)
    private val age: Int
)

When a JSR-303 Validator validates an instance of this class, these constraints are enforced.

For general information on JSR-303 and JSR-349, see the Bean Validation website. For information on the specific capabilities of the default reference implementation, see the Hibernate Validator documentation. To learn how to set up a bean validation provider as a Spring bean, keep reading.

Configuring a Bean Validation Provider

Spring provides full support for the Bean Validation API. This includes convenient support for bootstrapping a JSR-303 or JSR-349 Bean Validation provider as a Spring bean. This lets you inject a javax.validation.ValidatorFactory or javax.validation.Validator wherever validation is needed in your application.

You can use the LocalValidatorFactoryBean to configure a default Validator as a Spring bean, as the following example shows:

<bean id="validator"
    class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean"/>

The basic configuration in the preceding example triggers bean validation to initialize by using its default bootstrap mechanism. A JSR-303 or JSR-349 provider, such as the Hibernate Validator, is expected to be present in the classpath and is automatically detected.

Injecting a Validator

LocalValidatorFactoryBean implements both javax.validation.ValidatorFactory and javax.validation.Validator, as well as Spring’s org.springframework.validation.Validator. You can inject a reference to either of these interfaces into beans that need to invoke validation logic.

You can inject a reference to javax.validation.Validator if you prefer to work with the Bean Validation API directly, as the following example shows:

Java
import javax.validation.Validator;

@Service
public class MyService {

    @Autowired
    private Validator validator;
}
Kotlin
import javax.validation.Validator;

@Service
class MyService(@Autowired private val validator: Validator)

You can inject a reference to org.springframework.validation.Validator if your bean requires the Spring Validation API, as the following example shows:

Java
import org.springframework.validation.Validator;

@Service
public class MyService {

    @Autowired
    private Validator validator;
}
Kotlin
import org.springframework.validation.Validator

@Service
class MyService(@Autowired private val validator: Validator)
Configuring Custom Constraints

Each bean validation constraint consists of two parts: * A @Constraint annotation that declares the constraint and its configurable properties. * An implementation of the javax.validation.ConstraintValidator interface that implements the constraint’s behavior.

To associate a declaration with an implementation, each @Constraint annotation references a corresponding ConstraintValidator implementation class. At runtime, a ConstraintValidatorFactory instantiates the referenced implementation when the constraint annotation is encountered in your domain model.

By default, the LocalValidatorFactoryBean configures a SpringConstraintValidatorFactory that uses Spring to create ConstraintValidator instances. This lets your custom ConstraintValidators benefit from dependency injection like any other Spring bean.

The following example shows a custom @Constraint declaration followed by an associated ConstraintValidator implementation that uses Spring for dependency injection:

Java
@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy=MyConstraintValidator.class)
public @interface MyConstraint {
}
Kotlin
@Target(AnnotationTarget.FUNCTION, AnnotationTarget.FIELD)
@Retention(AnnotationRetention.RUNTIME)
@Constraint(validatedBy = MyConstraintValidator::class)
annotation class MyConstraint
Java
import javax.validation.ConstraintValidator;

public class MyConstraintValidator implements ConstraintValidator {

    @Autowired;
    private Foo aDependency;

    // ...
}
Kotlin
import javax.validation.ConstraintValidator

class MyConstraintValidator(private val aDependency: Foo) : ConstraintValidator {

    // ...
}

As the preceding example shows, a ConstraintValidator implementation can have its dependencies @Autowired as any other Spring bean.

Spring-driven Method Validation

You can integrate the method validation feature supported by Bean Validation 1.1 (and, as a custom extension, also by Hibernate Validator 4.3) into a Spring context through a MethodValidationPostProcessor bean definition, as follows:

<bean class="org.springframework.validation.beanvalidation.MethodValidationPostProcessor"/>

To be eligible for Spring-driven method validation, all target classes need to be annotated with Spring’s @Validated annotation. (Optionally, you can also declare the validation groups to use.) See the MethodValidationPostProcessor javadoc for setup details with the Hibernate Validator and Bean Validation 1.1 providers.

Additional Configuration Options

The default LocalValidatorFactoryBean configuration suffices for most cases. There are a number of configuration options for various Bean Validation constructs, from message interpolation to traversal resolution. See the LocalValidatorFactoryBean javadoc for more information on these options.

Configuring a DataBinder

Since Spring 3, you can configure a DataBinder instance with a Validator. Once configured, you can invoke the Validator by calling binder.validate(). Any validation Errors are automatically added to the binder’s BindingResult.

The following example shows how to use a DataBinder programmatically to invoke validation logic after binding to a target object:

Java
Foo target = new Foo();
DataBinder binder = new DataBinder(target);
binder.setValidator(new FooValidator());

// bind to the target object
binder.bind(propertyValues);

// validate the target object
binder.validate();

// get BindingResult that includes any validation errors
BindingResult results = binder.getBindingResult();
Kotlin
val target = Foo()
val binder = DataBinder(target)
binder.validator = FooValidator()

// bind to the target object
binder.bind(propertyValues)

// validate the target object
binder.validate()

// get BindingResult that includes any validation errors
val results = binder.bindingResult

You can also configure a DataBinder with multiple Validator instances through dataBinder.addValidators and dataBinder.replaceValidators. This is useful when combining globally configured bean validation with a Spring Validator configured locally on a DataBinder instance. See [validation-mvc-configuring].

Spring MVC 3 Validation

See Validation in the Spring MVC chapter.

Spring Expression Language (SpEL)

The Spring Expression Language (“SpEL” for short) is a powerful expression language that supports querying and manipulating an object graph at runtime. The language syntax is similar to Unified EL but offers additional features, most notably method invocation and basic string templating functionality.

While there are several other Java expression languages available — OGNL, MVEL, and JBoss EL, to name a few — the Spring Expression Language was created to provide the Spring community with a single well supported expression language that can be used across all the products in the Spring portfolio. Its language features are driven by the requirements of the projects in the Spring portfolio, including tooling requirements for code completion support within the Eclipse-based Spring Tool Suite. That said, SpEL is based on a technology-agnostic API that lets other expression language implementations be integrated, should the need arise.

While SpEL serves as the foundation for expression evaluation within the Spring portfolio, it is not directly tied to Spring and can be used independently. To be self contained, many of the examples in this chapter use SpEL as if it were an independent expression language. This requires creating a few bootstrapping infrastructure classes, such as the parser. Most Spring users need not deal with this infrastructure and can, instead, author only expression strings for evaluation. An example of this typical use is the integration of SpEL into creating XML or annotation-based bean definitions, as shown in Expression support for defining bean definitions.

This chapter covers the features of the expression language, its API, and its language syntax. In several places, Inventor and Society classes are used as the target objects for expression evaluation. These class declarations and the data used to populate them are listed at the end of the chapter.

The expression language supports the following functionality:

  • Literal expressions

  • Boolean and relational operators

  • Regular expressions

  • Class expressions

  • Accessing properties, arrays, lists, and maps

  • Method invocation

  • Relational operators

  • Assignment

  • Calling constructors

  • Bean references

  • Array construction

  • Inline lists

  • Inline maps

  • Ternary operator

  • Variables

  • User-defined functions

  • Collection projection

  • Collection selection

  • Templated expressions

Evaluation

This section introduces the simple use of SpEL interfaces and its expression language. The complete language reference can be found in Language Reference.

The following code introduces the SpEL API to evaluate the literal string expression, Hello World.

Java
ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("'Hello World'"); (1)
String message = (String) exp.getValue();
1 The value of the message variable is 'Hello World'.
Kotlin
val parser = SpelExpressionParser()
val exp = parser.parseExpression("'Hello World'") (1)
val message = exp.value as String
1 The value of the message variable is 'Hello World'.

The SpEL classes and interfaces you are most likely to use are located in the org.springframework.expression package and its sub-packages, such as spel.support.

The ExpressionParser interface is responsible for parsing an expression string. In the preceding example, the expression string is a string literal denoted by the surrounding single quotation marks. The Expression interface is responsible for evaluating the previously defined expression string. Two exceptions that can be thrown, ParseException and EvaluationException, when calling parser.parseExpression and exp.getValue, respectively.

SpEL supports a wide range of features, such as calling methods, accessing properties, and calling constructors.

In the following example of method invocation, we call the concat method on the string literal:

Java
ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("'Hello World'.concat('!')"); (1)
String message = (String) exp.getValue();
1 The value of message is now 'Hello World!'.
Kotlin
val parser = SpelExpressionParser()
val exp = parser.parseExpression("'Hello World'.concat('!')") (1)
val message = exp.value as String
1 The value of message is now 'Hello World!'.

The following example of calling a JavaBean property calls the String property Bytes:

Java
ExpressionParser parser = new SpelExpressionParser();

// invokes 'getBytes()'
Expression exp = parser.parseExpression("'Hello World'.bytes"); (1)
byte[] bytes = (byte[]) exp.getValue();
1 This line converts the literal to a byte array.
Kotlin
val parser = SpelExpressionParser()

// invokes 'getBytes()'
val exp = parser.parseExpression("'Hello World'.bytes") (1)
val bytes = exp.value as ByteArray
1 This line converts the literal to a byte array.

SpEL also supports nested properties by using the standard dot notation (such as prop1.prop2.prop3) and also the corresponding setting of property values. Public fields may also be accessed.

The following example shows how to use dot notation to get the length of a literal:

Java
ExpressionParser parser = new SpelExpressionParser();

// invokes 'getBytes().length'
Expression exp = parser.parseExpression("'Hello World'.bytes.length"); (1)
int length = (Integer) exp.getValue();
1 'Hello World'.bytes.length gives the length of the literal.
Kotlin
val parser = SpelExpressionParser()

// invokes 'getBytes().length'
val exp = parser.parseExpression("'Hello World'.bytes.length") (1)
val length = exp.value as Int
1 'Hello World'.bytes.length gives the length of the literal.

The String’s constructor can be called instead of using a string literal, as the following example shows:

Java
ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("new String('hello world').toUpperCase()"); (1)
String message = exp.getValue(String.class);
1 Construct a new String from the literal and make it be upper case.
Kotlin
val parser = SpelExpressionParser()
val exp = parser.parseExpression("new String('hello world').toUpperCase()")  (1)
val message = exp.getValue(String::class.java)
1 Construct a new String from the literal and make it be upper case.

Note the use of the generic method: public <T> T getValue(Class<T> desiredResultType). Using this method removes the need to cast the value of the expression to the desired result type. An EvaluationException is thrown if the value cannot be cast to the type T or converted by using the registered type converter.

The more common usage of SpEL is to provide an expression string that is evaluated against a specific object instance (called the root object). The following example shows how to retrieve the name property from an instance of the Inventor class or create a boolean condition:

Java
// Create and set a calendar
GregorianCalendar c = new GregorianCalendar();
c.set(1856, 7, 9);

// The constructor arguments are name, birthday, and nationality.
Inventor tesla = new Inventor("Nikola Tesla", c.getTime(), "Serbian");

ExpressionParser parser = new SpelExpressionParser();

Expression exp = parser.parseExpression("name"); // Parse name as an expression
String name = (String) exp.getValue(tesla);
// name == "Nikola Tesla"

exp = parser.parseExpression("name == 'Nikola Tesla'");
boolean result = exp.getValue(tesla, Boolean.class);
// result == true
Kotlin
// Create and set a calendar
val c = GregorianCalendar()
c.set(1856, 7, 9)

// The constructor arguments are name, birthday, and nationality.
val tesla = Inventor("Nikola Tesla", c.time, "Serbian")

val parser = SpelExpressionParser()

var exp = parser.parseExpression("name") // Parse name as an expression
val name = exp.getValue(tesla) as String
// name == "Nikola Tesla"

exp = parser.parseExpression("name == 'Nikola Tesla'")
val result = exp.getValue(tesla, Boolean::class.java)
// result == true

Understanding EvaluationContext

The EvaluationContext interface is used when evaluating an expression to resolve properties, methods, or fields and to help perform type conversion. Spring provides two implementations.

  • SimpleEvaluationContext: Exposes a subset of essential SpEL language features and configuration options, for categories of expressions that do not require the full extent of the SpEL language syntax and should be meaningfully restricted. Examples include but are not limited to data binding expressions and property-based filters.

  • StandardEvaluationContext: Exposes the full set of SpEL language features and configuration options. You can use it to specify a default root object and to configure every available evaluation-related strategy.

SimpleEvaluationContext is designed to support only a subset of the SpEL language syntax. It excludes Java type references, constructors, and bean references. It also requires you to explicitly choose the level of support for properties and methods in expressions. By default, the create() static factory method enables only read access to properties. You can also obtain a builder to configure the exact level of support needed, targeting one or some combination of the following:

  • Custom PropertyAccessor only (no reflection)

  • Data binding properties for read-only access

  • Data binding properties for read and write

Type Conversion

By default, SpEL uses the conversion service available in Spring core (org.springframework.core.convert.ConversionService). This conversion service comes with many built-in converters for common conversions but is also fully extensible so that you can add custom conversions between types. Additionally, it is generics-aware. This means that, when you work with generic types in expressions, SpEL attempts conversions to maintain type correctness for any objects it encounters.

What does this mean in practice? Suppose assignment, using setValue(), is being used to set a List property. The type of the property is actually List<Boolean>. SpEL recognizes that the elements of the list need to be converted to Boolean before being placed in it. The following example shows how to do so:

Java
class Simple {
    public List<Boolean> booleanList = new ArrayList<Boolean>();
}

Simple simple = new Simple();
simple.booleanList.add(true);

EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();

// "false" is passed in here as a String. SpEL and the conversion service
// will recognize that it needs to be a Boolean and convert it accordingly.
parser.parseExpression("booleanList[0]").setValue(context, simple, "false");

// b is false
Boolean b = simple.booleanList.get(0);
Kotlin
class Simple {
    var booleanList: MutableList<Boolean> = ArrayList()
}

val simple = Simple()
simple.booleanList.add(true)

val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()

// "false" is passed in here as a String. SpEL and the conversion service
// will recognize that it needs to be a Boolean and convert it accordingly.
parser.parseExpression("booleanList[0]").setValue(context, simple, "false")

// b is false
val b = simple.booleanList[0]

Parser Configuration

It is possible to configure the SpEL expression parser by using a parser configuration object (org.springframework.expression.spel.SpelParserConfiguration). The configuration object controls the behavior of some of the expression components. For example, if you index into an array or collection and the element at the specified index is null, you can automatically create the element. This is useful when using expressions made up of a chain of property references. If you index into an array or list and specifying an index that is beyond the end of the current size of the array or list, you can automatically grow the array or list to accommodate that index. The following example demonstrates how to automatically grow the list:

Java
class Demo {
    public List<String> list;
}

// Turn on:
// - auto null reference initialization
// - auto collection growing
SpelParserConfiguration config = new SpelParserConfiguration(true,true);

ExpressionParser parser = new SpelExpressionParser(config);

Expression expression = parser.parseExpression("list[3]");

Demo demo = new Demo();

Object o = expression.getValue(demo);

// demo.list will now be a real collection of 4 entries
// Each entry is a new empty String
Kotlin
class Demo {
    var list: List<String>? = null
}

// Turn on:
// - auto null reference initialization
// - auto collection growing
val config = SpelParserConfiguration(true, true)

val parser = SpelExpressionParser(config)

val expression = parser.parseExpression("list[3]")

val demo = Demo()

val o = expression.getValue(demo)

// demo.list will now be a real collection of 4 entries
// Each entry is a new empty String

SpEL Compilation

Spring Framework 4.1 includes a basic expression compiler. Expressions are usually interpreted, which provides a lot of dynamic flexibility during evaluation but does not provide optimum performance. For occasional expression usage, this is fine, but, when used by other components such as Spring Integration, performance can be very important, and there is no real need for the dynamism.

The SpEL compiler is intended to address this need. During evaluation, the compiler generates a Java class that embodies the expression behavior at runtime and uses that class to achieve much faster expression evaluation. Due to the lack of typing around expressions, the compiler uses information gathered during the interpreted evaluations of an expression when performing compilation. For example, it does not know the type of a property reference purely from the expression, but during the first interpreted evaluation, it finds out what it is. Of course, basing compilation on such derived information can cause trouble later if the types of the various expression elements change over time. For this reason, compilation is best suited to expressions whose type information is not going to change on repeated evaluations.

Consider the following basic expression:

someArray[0].someProperty.someOtherProperty < 0.1

Because the preceding expression involves array access, some property de-referencing, and numeric operations, the performance gain can be very noticeable. In an example micro benchmark run of 50000 iterations, it took 75ms to evaluate by using the interpreter and only 3ms using the compiled version of the expression.

Compiler Configuration

The compiler is not turned on by default, but you can turn it on in either of two different ways. You can turn it on by using the parser configuration process (discussed earlier) or by using a system property when SpEL usage is embedded inside another component. This section discusses both of these options.

The compiler can operate in one of three modes, which are captured in the org.springframework.expression.spel.SpelCompilerMode enum. The modes are as follows:

  • OFF (default): The compiler is switched off.

  • IMMEDIATE: In immediate mode, the expressions are compiled as soon as possible. This is typically after the first interpreted evaluation. If the compiled expression fails (typically due to a type changing, as described earlier), the caller of the expression evaluation receives an exception.

  • MIXED: In mixed mode, the expressions silently switch between interpreted and compiled mode over time. After some number of interpreted runs, they switch to compiled form and, if something goes wrong with the compiled form (such as a type changing, as described earlier), the expression automatically switches back to interpreted form again. Sometime later, it may generate another compiled form and switch to it. Basically, the exception that the user gets in IMMEDIATE mode is instead handled internally.

IMMEDIATE mode exists because MIXED mode could cause issues for expressions that have side effects. If a compiled expression blows up after partially succeeding, it may have already done something that has affected the state of the system. If this has happened, the caller may not want it to silently re-run in interpreted mode, since part of the expression may be running twice.

After selecting a mode, use the SpelParserConfiguration to configure the parser. The following example shows how to do so:

Java
SpelParserConfiguration config = new SpelParserConfiguration(SpelCompilerMode.IMMEDIATE,
    this.getClass().getClassLoader());

SpelExpressionParser parser = new SpelExpressionParser(config);

Expression expr = parser.parseExpression("payload");

MyMessage message = new MyMessage();

Object payload = expr.getValue(message);
Kotlin
val config = SpelParserConfiguration(SpelCompilerMode.IMMEDIATE,
        this.javaClass.classLoader)

val parser = SpelExpressionParser(config)

val expr = parser.parseExpression("payload")

val message = MyMessage()

val payload = expr.getValue(message)

When you specify the compiler mode, you can also specify a classloader (passing null is allowed). Compiled expressions are defined in a child classloader created under any that is supplied. It is important to ensure that, if a classloader is specified, it can see all the types involved in the expression evaluation process. If you do not specify a classloader, a default classloader is used (typically the context classloader for the thread that is running during expression evaluation).

The second way to configure the compiler is for use when SpEL is embedded inside some other component and it may not be possible to configure it through a configuration object. In these cases, it is possible to use a system property. You can set the spring.expression.compiler.mode property to one of the SpelCompilerMode enum values (off, immediate, or mixed).

Compiler Limitations

Since Spring Framework 4.1, the basic compilation framework is in place. However, the framework does not yet support compiling every kind of expression. The initial focus has been on the common expressions that are likely to be used in performance-critical contexts. The following kinds of expression cannot be compiled at the moment:

  • Expressions involving assignment

  • Expressions relying on the conversion service

  • Expressions using custom resolvers or accessors

  • Expressions using selection or projection

More types of expression will be compilable in the future.

Expressions in Bean Definitions

You can use SpEL expressions with XML-based or annotation-based configuration metadata for defining BeanDefinition instances. In both cases, the syntax to define the expression is of the form #{ <expression string> }.

XML Configuration

A property or constructor argument value can be set by using expressions, as the following example shows:

<bean id="numberGuess" class="org.spring.samples.NumberGuess">
    <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>

    <!-- other properties -->
</bean>

The systemProperties variable is predefined, so you can use it in your expressions, as the following example shows:

<bean id="taxCalculator" class="org.spring.samples.TaxCalculator">
    <property name="defaultLocale" value="#{ systemProperties['user.region'] }"/>

    <!-- other properties -->
</bean>

Note that you do not have to prefix the predefined variable with the # symbol in this context.

You can also refer to other bean properties by name, as the following example shows:

<bean id="numberGuess" class="org.spring.samples.NumberGuess">
    <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>

    <!-- other properties -->
</bean>

<bean id="shapeGuess" class="org.spring.samples.ShapeGuess">
    <property name="initialShapeSeed" value="#{ numberGuess.randomNumber }"/>

    <!-- other properties -->
</bean>

Annotation Configuration

To specify a default value, you can place the @Value annotation on fields, methods, and method or constructor parameters.

The following example sets the default value of a field variable:

Java
public class FieldValueTestBean {

    @Value("#{ systemProperties['user.region'] }")
    private String defaultLocale;

    public void setDefaultLocale(String defaultLocale) {
        this.defaultLocale = defaultLocale;
    }

    public String getDefaultLocale() {
        return this.defaultLocale;
    }
}
Kotlin
class FieldValueTestBean {

    @Value("#{ systemProperties['user.region'] }")
    var defaultLocale: String? = null
}

The following example shows the equivalent but on a property setter method:

Java
public class PropertyValueTestBean {

    private String defaultLocale;

    @Value("#{ systemProperties['user.region'] }")
    public void setDefaultLocale(String defaultLocale) {
        this.defaultLocale = defaultLocale;
    }

    public String getDefaultLocale() {
        return this.defaultLocale;
    }
}
Kotlin
class PropertyValueTestBean {

    @Value("#{ systemProperties['user.region'] }")
    var defaultLocale: String? = null
}

Autowired methods and constructors can also use the @Value annotation, as the following examples show:

Java
public class SimpleMovieLister {

    private MovieFinder movieFinder;
    private String defaultLocale;

    @Autowired
    public void configure(MovieFinder movieFinder,
            @Value("#{ systemProperties['user.region'] }") String defaultLocale) {
        this.movieFinder = movieFinder;
        this.defaultLocale = defaultLocale;
    }

    // ...
}
Kotlin
class SimpleMovieLister {

    private lateinit var movieFinder: MovieFinder
    private lateinit var defaultLocale: String

    @Autowired
    fun configure(movieFinder: MovieFinder,
                @Value("#{ systemProperties['user.region'] }") defaultLocale: String) {
        this.movieFinder = movieFinder
        this.defaultLocale = defaultLocale
    }

    // ...
}
Java
public class MovieRecommender {

    private String defaultLocale;

    private CustomerPreferenceDao customerPreferenceDao;

    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao,
            @Value("#{systemProperties['user.country']}") String defaultLocale) {
        this.customerPreferenceDao = customerPreferenceDao;
        this.defaultLocale = defaultLocale;
    }

    // ...
}
Kotlin
class MovieRecommender(private val customerPreferenceDao: CustomerPreferenceDao,
            @Value("#{systemProperties['user.country']}") private val defaultLocale: String) {
    // ...
}

Language Reference

This section describes how the Spring Expression Language works. It covers the following topics:

Literal Expressions

The types of literal expressions supported are strings, numeric values (int, real, hex), boolean, and null. Strings are delimited by single quotation marks. To put a single quotation mark itself in a string, use two single quotation mark characters.

The following listing shows simple usage of literals. Typically, they are not used in isolation like this but, rather, as part of a more complex expression — for example, using a literal on one side of a logical comparison operator.

Java
ExpressionParser parser = new SpelExpressionParser();

// evals to "Hello World"
String helloWorld = (String) parser.parseExpression("'Hello World'").getValue();

double avogadrosNumber = (Double) parser.parseExpression("6.0221415E+23").getValue();

// evals to 2147483647
int maxValue = (Integer) parser.parseExpression("0x7FFFFFFF").getValue();

boolean trueValue = (Boolean) parser.parseExpression("true").getValue();

Object nullValue = parser.parseExpression("null").getValue();
Kotlin
val parser = SpelExpressionParser()

// evals to "Hello World"
val helloWorld = parser.parseExpression("'Hello World'").value as String

val avogadrosNumber = parser.parseExpression("6.0221415E+23").value as Double

// evals to 2147483647
val maxValue = parser.parseExpression("0x7FFFFFFF").value as Int

val trueValue = parser.parseExpression("true").value as Boolean

val nullValue = parser.parseExpression("null").value

Numbers support the use of the negative sign, exponential notation, and decimal points. By default, real numbers are parsed by using Double.parseDouble().

Properties, Arrays, Lists, Maps, and Indexers

Navigating with property references is easy. To do so, use a period to indicate a nested property value. The instances of the Inventor class, pupin and tesla, were populated with data listed in the Classes used in the examples section. To navigate “down” and get Tesla’s year of birth and Pupin’s city of birth, we use the following expressions:

Java
// evals to 1856
int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context);

String city = (String) parser.parseExpression("placeOfBirth.City").getValue(context);
Kotlin
// evals to 1856
val year = parser.parseExpression("Birthdate.Year + 1900").getValue(context) as Int

val city = parser.parseExpression("placeOfBirth.City").getValue(context) as String

Case insensitivity is allowed for the first letter of property names. The contents of arrays and lists are obtained by using square bracket notation, as the following example shows:

Java
ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();

// Inventions Array

// evaluates to "Induction motor"
String invention = parser.parseExpression("inventions[3]").getValue(
        context, tesla, String.class);

// Members List

// evaluates to "Nikola Tesla"
String name = parser.parseExpression("Members[0].Name").getValue(
        context, ieee, String.class);

// List and Array navigation
// evaluates to "Wireless communication"
String invention = parser.parseExpression("Members[0].Inventions[6]").getValue(
        context, ieee, String.class);
Kotlin
val parser = SpelExpressionParser()
val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()

// Inventions Array

// evaluates to "Induction motor"
val invention = parser.parseExpression("inventions[3]").getValue(
        context, tesla, String::class.java)

// Members List

// evaluates to "Nikola Tesla"
val name = parser.parseExpression("Members[0].Name").getValue(
        context, ieee, String::class.java)

// List and Array navigation
// evaluates to "Wireless communication"
val invention = parser.parseExpression("Members[0].Inventions[6]").getValue(
        context, ieee, String::class.java)

The contents of maps are obtained by specifying the literal key value within the brackets. In the following example, because keys for the Officers map are strings, we can specify string literals:

Java
// Officer's Dictionary

Inventor pupin = parser.parseExpression("Officers['president']").getValue(
        societyContext, Inventor.class);

// evaluates to "Idvor"
String city = parser.parseExpression("Officers['president'].PlaceOfBirth.City").getValue(
        societyContext, String.class);

// setting values
parser.parseExpression("Officers['advisors'][0].PlaceOfBirth.Country").setValue(
        societyContext, "Croatia");
Kotlin
// Officer's Dictionary

val pupin = parser.parseExpression("Officers['president']").getValue(
        societyContext, Inventor::class.java)

// evaluates to "Idvor"
val city = parser.parseExpression("Officers['president'].PlaceOfBirth.City").getValue(
        societyContext, String::class.java)

// setting values
parser.parseExpression("Officers['advisors'][0].PlaceOfBirth.Country").setValue(
        societyContext, "Croatia")

Inline Lists

You can directly express lists in an expression by using {} notation.

Java
// evaluates to a Java list containing the four numbers
List numbers = (List) parser.parseExpression("{1,2,3,4}").getValue(context);

List listOfLists = (List) parser.parseExpression("{{'a','b'},{'x','y'}}").getValue(context);
Kotlin
// evaluates to a Java list containing the four numbers
val numbers = parser.parseExpression("{1,2,3,4}").getValue(context) as List<*>

val listOfLists = parser.parseExpression("{{'a','b'},{'x','y'}}").getValue(context) as List<*>

{} by itself means an empty list. For performance reasons, if the list is itself entirely composed of fixed literals, a constant list is created to represent the expression (rather than building a new list on each evaluation).

Inline Maps

You can also directly express maps in an expression by using {key:value} notation. The following example shows how to do so:

Java
// evaluates to a Java map containing the two entries
Map inventorInfo = (Map) parser.parseExpression("{name:'Nikola',dob:'10-July-1856'}").getValue(context);

Map mapOfMaps = (Map) parser.parseExpression("{name:{first:'Nikola',last:'Tesla'},dob:{day:10,month:'July',year:1856}}").getValue(context);
Kotlin
// evaluates to a Java map containing the two entries
val inventorInfo = parser.parseExpression("{name:'Nikola',dob:'10-July-1856'}").getValue(context) as Map<*, >

val mapOfMaps = parser.parseExpression("{name:{first:'Nikola',last:'Tesla'},dob:{day:10,month:'July',year:1856}}").getValue(context) as Map<, *>

{:} by itself means an empty map. For performance reasons, if the map is itself composed of fixed literals or other nested constant structures (lists or maps), a constant map is created to represent the expression (rather than building a new map on each evaluation). Quoting of the map keys is optional. The examples above do not use quoted keys.

Array Construction

You can build arrays by using the familiar Java syntax, optionally supplying an initializer to have the array populated at construction time. The following example shows how to do so:

Java
int[] numbers1 = (int[]) parser.parseExpression("new int[4]").getValue(context);

// Array with initializer
int[] numbers2 = (int[]) parser.parseExpression("new int[]{1,2,3}").getValue(context);

// Multi dimensional array
int[][] numbers3 = (int[][]) parser.parseExpression("new int[4][5]").getValue(context);
Kotlin
val numbers1 = parser.parseExpression("new int[4]").getValue(context) as IntArray

// Array with initializer
val numbers2 = parser.parseExpression("new int[]{1,2,3}").getValue(context) as IntArray

// Multi dimensional array
val numbers3 = parser.parseExpression("new int[4][5]").getValue(context) as Array<IntArray>

You cannot currently supply an initializer when you construct multi-dimensional array.

Methods

You can invoke methods by using typical Java programming syntax. You can also invoke methods on literals. Variable arguments are also supported. The following examples show how to invoke methods:

Java
// string literal, evaluates to "bc"
String bc = parser.parseExpression("'abc'.substring(1, 3)").getValue(String.class);

// evaluates to true
boolean isMember = parser.parseExpression("isMember('Mihajlo Pupin')").getValue(
        societyContext, Boolean.class);
Kotlin
// string literal, evaluates to "bc"
val bc = parser.parseExpression("'abc'.substring(1, 3)").getValue(String::class.java)

// evaluates to true
val isMember = parser.parseExpression("isMember('Mihajlo Pupin')").getValue(
        societyContext, Boolean::class.java)

Operators

The Spring Expression Language supports the following kinds of operators:

Relational Operators

The relational operators (equal, not equal, less than, less than or equal, greater than, and greater than or equal) are supported by using standard operator notation. The following listing shows a few examples of operators:

Java
// evaluates to true
boolean trueValue = parser.parseExpression("2 == 2").getValue(Boolean.class);

// evaluates to false
boolean falseValue = parser.parseExpression("2 < -5.0").getValue(Boolean.class);

// evaluates to true
boolean trueValue = parser.parseExpression("'black' < 'block'").getValue(Boolean.class);
Kotlin
// evaluates to true
val trueValue = parser.parseExpression("2 == 2").getValue(Boolean::class.java)

// evaluates to false
val falseValue = parser.parseExpression("2 < -5.0").getValue(Boolean::class.java)

// evaluates to true
val trueValue = parser.parseExpression("'black' < 'block'").getValue(Boolean::class.java)

Greater-than and less-than comparisons against null follow a simple rule: null is treated as nothing (that is NOT as zero). As a consequence, any other value is always greater than null (X > null is always true) and no other value is ever less than nothing (X < null is always false).

If you prefer numeric comparisons instead, avoid number-based null comparisons in favor of comparisons against zero (for example, X > 0 or X < 0).

In addition to the standard relational operators, SpEL supports the instanceof and regular expression-based matches operator. The following listing shows examples of both:

Java
// evaluates to false
boolean falseValue = parser.parseExpression(
        "'xyz' instanceof T(Integer)").getValue(Boolean.class);

// evaluates to true
boolean trueValue = parser.parseExpression(
        "'5.00' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class);

//evaluates to false
boolean falseValue = parser.parseExpression(
        "'5.0067' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class);
Kotlin
// evaluates to false
val falseValue = parser.parseExpression(
        "'xyz' instanceof T(Integer)").getValue(Boolean::class.java)

// evaluates to true
val trueValue = parser.parseExpression(
        "'5.00' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean::class.java)

//evaluates to false
val falseValue = parser.parseExpression(
        "'5.0067' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean::class.java)
Be careful with primitive types, as they are immediately boxed up to the wrapper type, so 1 instanceof T(int) evaluates to false while 1 instanceof T(Integer) evaluates to true, as expected.

Each symbolic operator can also be specified as a purely alphabetic equivalent. This avoids problems where the symbols used have special meaning for the document type in which the expression is embedded (such as in an XML document). The textual equivalents are:

  • lt (<)

  • gt (>)

  • le (<=)

  • ge (>=)

  • eq (==)

  • ne (!=)

  • div (/)

  • mod (%)

  • not (!).

All of the textual operators are case-insensitive.

Logical Operators

SpEL supports the following logical operators:

  • and

  • or

  • not

The following example shows how to use the logical operators

Java
// -- AND --

// evaluates to false
boolean falseValue = parser.parseExpression("true and false").getValue(Boolean.class);

// evaluates to true
String expression = "isMember('Nikola Tesla') and isMember('Mihajlo Pupin')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);

// -- OR --

// evaluates to true
boolean trueValue = parser.parseExpression("true or false").getValue(Boolean.class);

// evaluates to true
String expression = "isMember('Nikola Tesla') or isMember('Albert Einstein')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);

// -- NOT --

// evaluates to false
boolean falseValue = parser.parseExpression("!true").getValue(Boolean.class);

// -- AND and NOT --
String expression = "isMember('Nikola Tesla') and !isMember('Mihajlo Pupin')";
boolean falseValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);
Kotlin
// -- AND --

// evaluates to false
val falseValue = parser.parseExpression("true and false").getValue(Boolean::class.java)

// evaluates to true
val expression = "isMember('Nikola Tesla') and isMember('Mihajlo Pupin')"
val trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean::class.java)

// -- OR --

// evaluates to true
val trueValue = parser.parseExpression("true or false").getValue(Boolean::class.java)

// evaluates to true
val expression = "isMember('Nikola Tesla') or isMember('Albert Einstein')"
val trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean::class.java)

// -- NOT --

// evaluates to false
val falseValue = parser.parseExpression("!true").getValue(Boolean::class.java)

// -- AND and NOT --
val expression = "isMember('Nikola Tesla') and !isMember('Mihajlo Pupin')"
val falseValue = parser.parseExpression(expression).getValue(societyContext, Boolean::class.java)
Mathematical Operators

You can use the addition operator on both numbers and strings. You can use the subtraction, multiplication, and division operators only on numbers. You can also use the modulus (%) and exponential power (^) operators. Standard operator precedence is enforced. The following example shows the mathematical operators in use:

Java
// Addition
int two = parser.parseExpression("1 + 1").getValue(Integer.class);  // 2

String testString = parser.parseExpression(
        "'test' + ' ' + 'string'").getValue(String.class);  // 'test string'

// Subtraction
int four = parser.parseExpression("1 - -3").getValue(Integer.class);  // 4

double d = parser.parseExpression("1000.00 - 1e4").getValue(Double.class);  // -9000

// Multiplication
int six = parser.parseExpression("-2 * -3").getValue(Integer.class);  // 6

double twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double.class);  // 24.0

// Division
int minusTwo = parser.parseExpression("6 / -3").getValue(Integer.class);  // -2

double one = parser.parseExpression("8.0 / 4e0 / 2").getValue(Double.class);  // 1.0

// Modulus
int three = parser.parseExpression("7 % 4").getValue(Integer.class);  // 3

int one = parser.parseExpression("8 / 5 % 2").getValue(Integer.class);  // 1

// Operator precedence
int minusTwentyOne = parser.parseExpression("1+2-3*8").getValue(Integer.class);  // -21
Kotlin
// Addition
val two = parser.parseExpression("1 + 1").getValue(Int::class.java)  // 2

val testString = parser.parseExpression(
        "'test' + ' ' + 'string'").getValue(String::class.java)  // 'test string'

// Subtraction
val four = parser.parseExpression("1 - -3").getValue(Int::class.java)  // 4

val d = parser.parseExpression("1000.00 - 1e4").getValue(Double::class.java)  // -9000

// Multiplication
val six = parser.parseExpression("-2 * -3").getValue(Int::class.java)  // 6

val twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double::class.java)  // 24.0

// Division
val minusTwo = parser.parseExpression("6 / -3").getValue(Int::class.java)  // -2

val one = parser.parseExpression("8.0 / 4e0 / 2").getValue(Double::class.java)  // 1.0

// Modulus
val three = parser.parseExpression("7 % 4").getValue(Int::class.java)  // 3

val one = parser.parseExpression("8 / 5 % 2").getValue(Int::class.java)  // 1

// Operator precedence
val minusTwentyOne = parser.parseExpression("1+2-3*8").getValue(Int::class.java)  // -21
The Assignment Operator

To setting a property, use the assignment operator (=). This is typically done within a call to setValue but can also be done inside a call to getValue. The following listing shows both ways to use the assignment operator:

Java
Inventor inventor = new Inventor();
EvaluationContext context = SimpleEvaluationContext.forReadWriteDataBinding().build();

parser.parseExpression("Name").setValue(context, inventor, "Aleksandar Seovic");

// alternatively
String aleks = parser.parseExpression(
        "Name = 'Aleksandar Seovic'").getValue(context, inventor, String.class);
Kotlin
val inventor = Inventor()
val context = SimpleEvaluationContext.forReadWriteDataBinding().build()

parser.parseExpression("Name").setValue(context, inventor, "Aleksandar Seovic")

// alternatively
val aleks = parser.parseExpression(
        "Name = 'Aleksandar Seovic'").getValue(context, inventor, String::class.java)

Types

You can use the special T operator to specify an instance of java.lang.Class (the type). Static methods are invoked by using this operator as well. The StandardEvaluationContext uses a TypeLocator to find types, and the StandardTypeLocator (which can be replaced) is built with an understanding of the java.lang package. This means that T() references to types within java.lang do not need to be fully qualified, but all other type references must be. The following example shows how to use the T operator:

Java
Class dateClass = parser.parseExpression("T(java.util.Date)").getValue(Class.class);

Class stringClass = parser.parseExpression("T(String)").getValue(Class.class);

boolean trueValue = parser.parseExpression(
        "T(java.math.RoundingMode).CEILING < T(java.math.RoundingMode).FLOOR")
        .getValue(Boolean.class);
Kotlin
val dateClass = parser.parseExpression("T(java.util.Date)").getValue(Class::class.java)

val stringClass = parser.parseExpression("T(String)").getValue(Class::class.java)

val trueValue = parser.parseExpression(
        "T(java.math.RoundingMode).CEILING < T(java.math.RoundingMode).FLOOR")
        .getValue(Boolean::class.java)

Constructors

You can invoke constructors by using the new operator. You should use the fully qualified class name for all but the primitive types (int, float, and so on) and String. The following example shows how to use the new operator to invoke constructors:

Java
Inventor einstein = p.parseExpression(
        "new org.spring.samples.spel.inventor.Inventor('Albert Einstein', 'German')")
        .getValue(Inventor.class);

//create new inventor instance within add method of List
p.parseExpression(
        "Members.add(new org.spring.samples.spel.inventor.Inventor(
            'Albert Einstein', 'German'))").getValue(societyContext);
Kotlin
val einstein = p.parseExpression(
        "new org.spring.samples.spel.inventor.Inventor('Albert Einstein', 'German')")
        .getValue(Inventor::class.java)

//create new inventor instance within add method of List
p.parseExpression(
        "Members.add(new org.spring.samples.spel.inventor.Inventor('Albert Einstein', 'German'))")
        .getValue(societyContext)

Variables

You can reference variables in the expression by using the #variableName syntax. Variables are set by using the setVariable method on EvaluationContext implementations. The following example shows how to use variables:

Java
Inventor tesla = new Inventor("Nikola Tesla", "Serbian");

EvaluationContext context = SimpleEvaluationContext.forReadWriteDataBinding().build();
context.setVariable("newName", "Mike Tesla");

parser.parseExpression("Name = #newName").getValue(context, tesla);
System.out.println(tesla.getName())  // "Mike Tesla"
Kotlin
val tesla = Inventor("Nikola Tesla", "Serbian")

val context = SimpleEvaluationContext.forReadWriteDataBinding().build()
context.setVariable("newName", "Mike Tesla")

parser.parseExpression("Name = #newName").getValue(context, tesla)
println(tesla.name)  // "Mike Tesla"
The #this and #root Variables

The #this variable is always defined and refers to the current evaluation object (against which unqualified references are resolved). The #root variable is always defined and refers to the root context object. Although #this may vary as components of an expression are evaluated, #root always refers to the root. The following examples show how to use the #this and #root variables:

Java
// create an array of integers
List<Integer> primes = new ArrayList<Integer>();
primes.addAll(Arrays.asList(2,3,5,7,11,13,17));

// create parser and set variable 'primes' as the array of integers
ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataAccess();
context.setVariable("primes", primes);

// all prime numbers > 10 from the list (using selection ?{...})
// evaluates to [11, 13, 17]
List<Integer> primesGreaterThanTen = (List<Integer>) parser.parseExpression(
        "#primes.?[#this>10]").getValue(context);
Kotlin
// create an array of integers
val primes = ArrayList<Int>()
primes.addAll(listOf(2, 3, 5, 7, 11, 13, 17))

// create parser and set variable 'primes' as the array of integers
val parser = SpelExpressionParser()
val context = SimpleEvaluationContext.forReadOnlyDataAccess()
context.setVariable("primes", primes)

// all prime numbers > 10 from the list (using selection ?{...})
// evaluates to [11, 13, 17]
val primesGreaterThanTen = parser.parseExpression(
        "#primes.?[#this>10]").getValue(context) as List<Int>

Functions

You can extend SpEL by registering user-defined functions that can be called within the expression string. The function is registered through the EvaluationContext. The following example shows how to register a user-defined function:

Java
Method method = ...;

EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();
context.setVariable("myFunction", method);
Kotlin
val method: Method = ...

val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()
context.setVariable("myFunction", method)

For example, consider the following utility method that reverses a string:

Java
public abstract class StringUtils {

    public static String reverseString(String input) {
        StringBuilder backwards = new StringBuilder(input.length());
        for (int i = 0; i < input.length(); i++) {
            backwards.append(input.charAt(input.length() - 1 - i));
        }
        return backwards.toString();
    }
}
Kotlin
fun reverseString(input: String): String {
    val backwards = StringBuilder(input.length)
    for (i in 0 until input.length) {
        backwards.append(input[input.length - 1 - i])
    }
    return backwards.toString()
}

You can then register and use the preceding method, as the following example shows:

Java
ExpressionParser parser = new SpelExpressionParser();

EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();
context.setVariable("reverseString",
        StringUtils.class.getDeclaredMethod("reverseString", String.class));

String helloWorldReversed = parser.parseExpression(
        "#reverseString('hello')").getValue(context, String.class);
Kotlin
val parser = SpelExpressionParser()

val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()
context.setVariable("reverseString", ::reverseString::javaMethod)

val helloWorldReversed = parser.parseExpression(
        "#reverseString('hello')").getValue(context, String::class.java)

Bean References

If the evaluation context has been configured with a bean resolver, you can look up beans from an expression by using the @ symbol. The following example shows how to do so:

Java
ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setBeanResolver(new MyBeanResolver());

// This will end up calling resolve(context,"something") on MyBeanResolver during evaluation
Object bean = parser.parseExpression("@something").getValue(context);
Kotlin
val parser = SpelExpressionParser()
val context = StandardEvaluationContext()
context.setBeanResolver(MyBeanResolver())

// This will end up calling resolve(context,"something") on MyBeanResolver during evaluation
val bean = parser.parseExpression("@something").getValue(context)

To access a factory bean itself, you should instead prefix the bean name with an & symbol. The following example shows how to do so:

Java
ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setBeanResolver(new MyBeanResolver());

// This will end up calling resolve(context,"&foo") on MyBeanResolver during evaluation
Object bean = parser.parseExpression("&foo").getValue(context);
Kotlin
val parser = SpelExpressionParser()
val context = StandardEvaluationContext()
context.setBeanResolver(MyBeanResolver())

// This will end up calling resolve(context,"&foo") on MyBeanResolver during evaluation
val bean = parser.parseExpression("&foo").getValue(context)

Ternary Operator (If-Then-Else)

You can use the ternary operator for performing if-then-else conditional logic inside the expression. The following listing shows a minimal example:

Java
String falseString = parser.parseExpression(
        "false ? 'trueExp' : 'falseExp'").getValue(String.class);
Kotlin
val falseString = parser.parseExpression(
        "false ? 'trueExp' : 'falseExp'").getValue(String::class.java)

In this case, the boolean false results in returning the string value 'falseExp'. A more realistic example follows:

Java
parser.parseExpression("Name").setValue(societyContext, "IEEE");
societyContext.setVariable("queryName", "Nikola Tesla");

expression = "isMember(#queryName)? #queryName + ' is a member of the ' " +
        "+ Name + ' Society' : #queryName + ' is not a member of the ' + Name + ' Society'";

String queryResultString = parser.parseExpression(expression)
        .getValue(societyContext, String.class);
// queryResultString = "Nikola Tesla is a member of the IEEE Society"
Kotlin
parser.parseExpression("Name").setValue(societyContext, "IEEE")
societyContext.setVariable("queryName", "Nikola Tesla")

expression = "isMember(#queryName)? #queryName + ' is a member of the ' " + "+ Name + ' Society' : #queryName + ' is not a member of the ' + Name + ' Society'"

val queryResultString = parser.parseExpression(expression)
        .getValue(societyContext, String::class.java)
// queryResultString = "Nikola Tesla is a member of the IEEE Society"

See the next section on the Elvis operator for an even shorter syntax for the ternary operator.

The Elvis Operator

The Elvis operator is a shortening of the ternary operator syntax and is used in the Groovy language. With the ternary operator syntax, you usually have to repeat a variable twice, as the following example shows:

String name = "Elvis Presley";
String displayName = (name != null ? name : "Unknown");

Instead, you can use the Elvis operator (named for the resemblance to Elvis' hair style). The following example shows how to use the Elvis operator:

Java
ExpressionParser parser = new SpelExpressionParser();

String name = parser.parseExpression("name?:'Unknown'").getValue(String.class);
System.out.println(name);  // 'Unknown'
Kotlin
val parser = SpelExpressionParser()

val name = parser.parseExpression("name?:'Unknown'").getValue(String::class.java)
println(name)  // 'Unknown'

The following listing shows a more complex example:

Java
ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();

Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
String name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, tesla, String.class);
System.out.println(name);  // Nikola Tesla

tesla.setName(null);
name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, tesla, String.class);
System.out.println(name);  // Elvis Presley
Kotlin
val parser = SpelExpressionParser()
val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()

val tesla = Inventor("Nikola Tesla", "Serbian")
var name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, tesla, String::class.java)
println(name)  // Nikola Tesla

tesla.setName(null)
name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, tesla, String::class.java)
println(name)  // Elvis Presley

You can use the Elvis operator to apply default values in expressions. The following example shows how to use the Elvis operator in a @Value expression:

@Value("#{systemProperties['pop3.port'] ?: 25}")

This will inject a system property pop3.port if it is defined or 25 if not.

Safe Navigation Operator

The safe navigation operator is used to avoid a NullPointerException and comes from the Groovy language. Typically, when you have a reference to an object, you might need to verify that it is not null before accessing methods or properties of the object. To avoid this, the safe navigation operator returns null instead of throwing an exception. The following example shows how to use the safe navigation operator:

Java
ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();

Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
tesla.setPlaceOfBirth(new PlaceOfBirth("Smiljan"));

String city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, tesla, String.class);
System.out.println(city);  // Smiljan

tesla.setPlaceOfBirth(null);
city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, tesla, String.class);
System.out.println(city);  // null - does not throw NullPointerException!!!
Kotlin
val parser = SpelExpressionParser()
val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()

val tesla = Inventor("Nikola Tesla", "Serbian")
tesla.setPlaceOfBirth(PlaceOfBirth("Smiljan"))

var city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, tesla, String::class.java)
println(city)  // Smiljan

tesla.setPlaceOfBirth(null)
city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, tesla, String::class.java)
println(city)  // null - does not throw NullPointerException!!!

Collection Selection

Selection is a powerful expression language feature that lets you transform a source collection into another collection by selecting from its entries.

Selection uses a syntax of .?[selectionExpression]. It filters the collection and returns a new collection that contain a subset of the original elements. For example, selection lets us easily get a list of Serbian inventors, as the following example shows:

Java
List<Inventor> list = (List<Inventor>) parser.parseExpression(
        "Members.?[Nationality == 'Serbian']").getValue(societyContext);
Kotlin
val list = parser.parseExpression(
        "Members.?[Nationality == 'Serbian']").getValue(societyContext) as List<Inventor>

Selection is possible upon both lists and maps. For a list, the selection criteria is evaluated against each individual list element. Against a map, the selection criteria is evaluated against each map entry (objects of the Java type Map.Entry). Each map entry has its key and value accessible as properties for use in the selection.

The following expression returns a new map that consists of those elements of the original map where the entry value is less than 27:

Java
Map newMap = parser.parseExpression("map.?[value<27]").getValue();
Kotlin
val newMap = parser.parseExpression("map.?[value<27]").getValue()

In addition to returning all the selected elements, you can retrieve only the first or the last value. To obtain the first entry matching the selection, the syntax is .^[selectionExpression]. To obtain the last matching selection, the syntax is .$[selectionExpression].

Collection Projection

Projection lets a collection drive the evaluation of a sub-expression, and the result is a new collection. The syntax for projection is .![projectionExpression]. For example, suppose we have a list of inventors but want the list of cities where they were born. Effectively, we want to evaluate 'placeOfBirth.city' for every entry in the inventor list. The following example uses projection to do so:

Java
// returns ['Smiljan', 'Idvor' ]
List placesOfBirth = (List)parser.parseExpression("Members.![placeOfBirth.city]");
Kotlin
// returns ['Smiljan', 'Idvor' ]
val placesOfBirth = parser.parseExpression("Members.![placeOfBirth.city]") as List<*>

You can also use a map to drive projection and, in this case, the projection expression is evaluated against each entry in the map (represented as a Java Map.Entry). The result of a projection across a map is a list that consists of the evaluation of the projection expression against each map entry.

Expression templating

Expression templates allow mixing literal text with one or more evaluation blocks. Each evaluation block is delimited with prefix and suffix characters that you can define. A common choice is to use #{ } as the delimiters, as the following example shows:

Java
String randomPhrase = parser.parseExpression(
        "random number is #{T(java.lang.Math).random()}",
        new TemplateParserContext()).getValue(String.class);

// evaluates to "random number is 0.7038186818312008"
Kotlin
val randomPhrase = parser.parseExpression(
        "random number is #{T(java.lang.Math).random()}",
        TemplateParserContext()).getValue(String::class.java)

// evaluates to "random number is 0.7038186818312008"

The string is evaluated by concatenating the literal text 'random number is ' with the result of evaluating the expression inside the #{ } delimiter (in this case, the result of calling that random() method). The second argument to the parseExpression() method is of the type ParserContext. The ParserContext interface is used to influence how the expression is parsed in order to support the expression templating functionality. The definition of TemplateParserContext follows:

Java
public class TemplateParserContext implements ParserContext {

    public String getExpressionPrefix() {
        return "#{";
    }

    public String getExpressionSuffix() {
        return "}";
    }

    public boolean isTemplate() {
        return true;
    }
}
Kotlin
class TemplateParserContext : ParserContext {

    override fun getExpressionPrefix(): String {
        return "#{"
    }

    override fun getExpressionSuffix(): String {
        return "}"
    }

    override fun isTemplate(): Boolean {
        return true
    }
}

Classes Used in the Examples

This section lists the classes used in the examples throughout this chapter.

Inventor.Java
package org.spring.samples.spel.inventor;

import java.util.Date;
import java.util.GregorianCalendar;

public class Inventor {

    private String name;
    private String nationality;
    private String[] inventions;
    private Date birthdate;
    private PlaceOfBirth placeOfBirth;

    public Inventor(String name, String nationality) {
        GregorianCalendar c= new GregorianCalendar();
        this.name = name;
        this.nationality = nationality;
        this.birthdate = c.getTime();
    }

    public Inventor(String name, Date birthdate, String nationality) {
        this.name = name;
        this.nationality = nationality;
        this.birthdate = birthdate;
    }

    public Inventor() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getNationality() {
        return nationality;
    }

    public void setNationality(String nationality) {
        this.nationality = nationality;
    }

    public Date getBirthdate() {
        return birthdate;
    }

    public void setBirthdate(Date birthdate) {
        this.birthdate = birthdate;
    }

    public PlaceOfBirth getPlaceOfBirth() {
        return placeOfBirth;
    }

    public void setPlaceOfBirth(PlaceOfBirth placeOfBirth) {
        this.placeOfBirth = placeOfBirth;
    }

    public void setInventions(String[] inventions) {
        this.inventions = inventions;
    }

    public String[] getInventions() {
        return inventions;
    }
}
Inventor.kt
class Inventor(
    var name: String,
    var nationality: String,
    var inventions: Array<String>? = null,
    var birthdate: Date =  GregorianCalendar().time,
    var placeOfBirth: PlaceOfBirth? = null)
PlaceOfBirth.java
package org.spring.samples.spel.inventor;

public class PlaceOfBirth {

    private String city;
    private String country;

    public PlaceOfBirth(String city) {
        this.city=city;
    }

    public PlaceOfBirth(String city, String country) {
        this(city);
        this.country = country;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String s) {
        this.city = s;
    }

    public String getCountry() {
        return country;
    }

    public void setCountry(String country) {
        this.country = country;
    }
}
PlaceOfBirth.kt
class PlaceOfBirth(var city: String, var country: String? = null) {
Society.java
package org.spring.samples.spel.inventor;

import java.util.*;

public class Society {

    private String name;

    public static String Advisors = "advisors";
    public static String President = "president";

    private List<Inventor> members = new ArrayList<Inventor>();
    private Map officers = new HashMap();

    public List getMembers() {
        return members;
    }

    public Map getOfficers() {
        return officers;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public boolean isMember(String name) {
        for (Inventor inventor : members) {
            if (inventor.getName().equals(name)) {
                return true;
            }
        }
        return false;
    }
}
Society.kt
package org.spring.samples.spel.inventor

import java.util.*

class Society {

    val Advisors = "advisors"
    val President = "president"

    var name: String? = null

    val members = ArrayList<Inventor>()
    val officers = mapOf<Any, Any>()

    fun isMember(name: String): Boolean {
        for (inventor in members) {
            if (inventor.name == name) {
                return true
            }
        }
        return false
    }
}

Aspect Oriented Programming with Spring

Aspect-oriented Programming (AOP) complements Object-oriented Programming (OOP) by providing another way of thinking about program structure. The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect. Aspects enable the modularization of concerns (such as transaction management) that cut across multiple types and objects. (Such concerns are often termed “crosscutting” concerns in AOP literature.)

One of the key components of Spring is the AOP framework. While the Spring IoC container does not depend on AOP (meaning you do not need to use AOP if you don’t want to), AOP complements Spring IoC to provide a very capable middleware solution.

Spring AOP with AspectJ pointcuts

Spring provides simple and powerful ways of writing custom aspects by using either a schema-based approach or the @AspectJ annotation style. Both of these styles offer fully typed advice and use of the AspectJ pointcut language while still using Spring AOP for weaving.

This chapter discusses the schema- and @AspectJ-based AOP support. The lower-level AOP support is discussed in the following chapter.

AOP is used in the Spring Framework to:

  • Provide declarative enterprise services. The most important such service is declarative transaction management.

  • Let users implement custom aspects, complementing their use of OOP with AOP.

If you are interested only in generic declarative services or other pre-packaged declarative middleware services such as pooling, you do not need to work directly with Spring AOP, and can skip most of this chapter.

AOP Concepts

Let us begin by defining some central AOP concepts and terminology. These terms are not Spring-specific. Unfortunately, AOP terminology is not particularly intuitive. However, it would be even more confusing if Spring used its own terminology.

  • Aspect: A modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in enterprise Java applications. In Spring AOP, aspects are implemented by using regular classes (the schema-based approach) or regular classes annotated with the @Aspect annotation (the @AspectJ style).

  • Join point: A point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.

  • Advice: Action taken by an aspect at a particular join point. Different types of advice include “around”, “before” and “after” advice. (Advice types are discussed later.) Many AOP frameworks, including Spring, model an advice as an interceptor and maintain a chain of interceptors around the join point.

  • Pointcut: A predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name). The concept of join points as matched by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut expression language by default.

  • Introduction: Declaring additional methods or fields on behalf of a type. Spring AOP lets you introduce new interfaces (and a corresponding implementation) to any advised object. For example, you could use an introduction to make a bean implement an IsModified interface, to simplify caching. (An introduction is known as an inter-type declaration in the AspectJ community.)

  • Target object: An object being advised by one or more aspects. Also referred to as the “advised object”. Since Spring AOP is implemented by using runtime proxies, this object is always a proxied object.

  • AOP proxy: An object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). In the Spring Framework, an AOP proxy is a JDK dynamic proxy or a CGLIB proxy.

  • Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.

Spring AOP includes the following types of advice:

  • Before advice: Advice that runs before a join point but that does not have the ability to prevent execution flow proceeding to the join point (unless it throws an exception).

  • After returning advice: Advice to be run after a join point completes normally (for example, if a method returns without throwing an exception).

  • After throwing advice: Advice to be executed if a method exits by throwing an exception.

  • After (finally) advice: Advice to be executed regardless of the means by which a join point exits (normal or exceptional return).

  • Around advice: Advice that surrounds a join point such as a method invocation. This is the most powerful kind of advice. Around advice can perform custom behavior before and after the method invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the advised method execution by returning its own return value or throwing an exception.

Around advice is the most general kind of advice. Since Spring AOP, like AspectJ, provides a full range of advice types, we recommend that you use the least powerful advice type that can implement the required behavior. For example, if you need only to update a cache with the return value of a method, you are better off implementing an after returning advice than an around advice, although an around advice can accomplish the same thing. Using the most specific advice type provides a simpler programming model with less potential for errors. For example, you do not need to invoke the proceed() method on the JoinPoint used for around advice, and, hence, you cannot fail to invoke it.

All advice parameters are statically typed so that you work with advice parameters of the appropriate type (e.g. the type of the return value from a method execution) rather than Object arrays.

The concept of join points matched by pointcuts is the key to AOP, which distinguishes it from older technologies offering only interception. Pointcuts enable advice to be targeted independently of the object-oriented hierarchy. For example, you can apply an around advice providing declarative transaction management to a set of methods that span multiple objects (such as all business operations in the service layer).

Spring AOP Capabilities and Goals

Spring AOP is implemented in pure Java. There is no need for a special compilation process. Spring AOP does not need to control the class loader hierarchy and is thus suitable for use in a servlet container or application server.

Spring AOP currently supports only method execution join points (advising the execution of methods on Spring beans). Field interception is not implemented, although support for field interception could be added without breaking the core Spring AOP APIs. If you need to advise field access and update join points, consider a language such as AspectJ.

Spring AOP’s approach to AOP differs from that of most other AOP frameworks. The aim is not to provide the most complete AOP implementation (although Spring AOP is quite capable). Rather, the aim is to provide a close integration between AOP implementation and Spring IoC, to help solve common problems in enterprise applications.

Thus, for example, the Spring Framework’s AOP functionality is normally used in conjunction with the Spring IoC container. Aspects are configured by using normal bean definition syntax (although this allows powerful “auto-proxying” capabilities). This is a crucial difference from other AOP implementations. You cannot do some things easily or efficiently with Spring AOP, such as advise very fine-grained objects (typically, domain objects). AspectJ is the best choice in such cases. However, our experience is that Spring AOP provides an excellent solution to most problems in enterprise Java applications that are amenable to AOP.

Spring AOP never strives to compete with AspectJ to provide a comprehensive AOP solution. We believe that both proxy-based frameworks such as Spring AOP and full-blown frameworks such as AspectJ are valuable and that they are complementary, rather than in competition. Spring seamlessly integrates Spring AOP and IoC with AspectJ, to enable all uses of AOP within a consistent Spring-based application architecture. This integration does not affect the Spring AOP API or the AOP Alliance API. Spring AOP remains backward-compatible. See the following chapter for a discussion of the Spring AOP APIs.

One of the central tenets of the Spring Framework is that of non-invasiveness. This is the idea that you should not be forced to introduce framework-specific classes and interfaces into your business or domain model. However, in some places, the Spring Framework does give you the option to introduce Spring Framework-specific dependencies into your codebase. The rationale in giving you such options is because, in certain scenarios, it might be just plain easier to read or code some specific piece of functionality in such a way. However, the Spring Framework (almost) always offers you the choice: You have the freedom to make an informed decision as to which option best suits your particular use case or scenario.

One such choice that is relevant to this chapter is that of which AOP framework (and which AOP style) to choose. You have the choice of AspectJ, Spring AOP, or both. You also have the choice of either the @AspectJ annotation-style approach or the Spring XML configuration-style approach. The fact that this chapter chooses to introduce the @AspectJ-style approach first should not be taken as an indication that the Spring team favors the @AspectJ annotation-style approach over the Spring XML configuration-style.

See Choosing which AOP Declaration Style to Use for a more complete discussion of the “whys and wherefores” of each style.

AOP Proxies

Spring AOP defaults to using standard JDK dynamic proxies for AOP proxies. This enables any interface (or set of interfaces) to be proxied.

Spring AOP can also use CGLIB proxies. This is necessary to proxy classes rather than interfaces. By default, CGLIB is used if a business object does not implement an interface. As it is good practice to program to interfaces rather than classes, business classes normally implement one or more business interfaces. It is possible to force the use of CGLIB, in those (hopefully rare) cases where you need to advise a method that is not declared on an interface or where you need to pass a proxied object to a method as a concrete type.

It is important to grasp the fact that Spring AOP is proxy-based. See Understanding AOP Proxies for a thorough examination of exactly what this implementation detail actually means.

@AspectJ support

@AspectJ refers to a style of declaring aspects as regular Java classes annotated with annotations. The @AspectJ style was introduced by the AspectJ project as part of the AspectJ 5 release. Spring interprets the same annotations as AspectJ 5, using a library supplied by AspectJ for pointcut parsing and matching. The AOP runtime is still pure Spring AOP, though, and there is no dependency on the AspectJ compiler or weaver.

Using the AspectJ compiler and weaver enables use of the full AspectJ language and is discussed in Using AspectJ with Spring Applications.

Enabling @AspectJ Support

To use @AspectJ aspects in a Spring configuration, you need to enable Spring support for configuring Spring AOP based on @AspectJ aspects and auto-proxying beans based on whether or not they are advised by those aspects. By auto-proxying, we mean that, if Spring determines that a bean is advised by one or more aspects, it automatically generates a proxy for that bean to intercept method invocations and ensures that advice is executed as needed.

The @AspectJ support can be enabled with XML- or Java-style configuration. In either case, you also need to ensure that AspectJ’s aspectjweaver.jar library is on the classpath of your application (version 1.8 or later). This library is available in the lib directory of an AspectJ distribution or from the Maven Central repository.

Enabling @AspectJ Support with Java Configuration

To enable @AspectJ support with Java @Configuration, add the @EnableAspectJAutoProxy annotation, as the following example shows:

Java
@Configuration
@EnableAspectJAutoProxy
public class AppConfig {

}
Kotlin
@Configuration
@EnableAspectJAutoProxy
class AppConfig
Enabling @AspectJ Support with XML Configuration

To enable @AspectJ support with XML-based configuration, use the aop:aspectj-autoproxy element, as the following example shows:

<aop:aspectj-autoproxy/>

This assumes that you use schema support as described in XML Schema-based configuration. See the AOP schema for how to import the tags in the aop namespace.

Declaring an Aspect

With @AspectJ support enabled, any bean defined in your application context with a class that is an @AspectJ aspect (has the @Aspect annotation) is automatically detected by Spring and used to configure Spring AOP. The next two examples show the minimal definition required for a not-very-useful aspect.

The first of the two example shows a regular bean definition in the application context that points to a bean class that has the @Aspect annotation:

<bean id="myAspect" class="org.xyz.NotVeryUsefulAspect">
    <!-- configure properties of the aspect here -->
</bean>

The second of the two examples shows the NotVeryUsefulAspect class definition, which is annotated with the org.aspectj.lang.annotation.Aspect annotation;

Java
package org.xyz;
import org.aspectj.lang.annotation.Aspect;

@Aspect
public class NotVeryUsefulAspect {

}
Kotlin
package org.xyz

import org.aspectj.lang.annotation.Aspect;

@Aspect
class NotVeryUsefulAspect

Aspects (classes annotated with @Aspect) can have methods and fields, the same as any other class. They can also contain pointcut, advice, and introduction (inter-type) declarations.

Autodetecting aspects through component scanning
You can register aspect classes as regular beans in your Spring XML configuration or autodetect them through classpath scanning — the same as any other Spring-managed bean. However, note that the @Aspect annotation is not sufficient for autodetection in the classpath. For that purpose, you need to add a separate @Component annotation (or, alternatively, a custom stereotype annotation that qualifies, as per the rules of Spring’s component scanner).
Advising aspects with other aspects?
In Spring AOP, aspects themselves cannot be the targets of advice from other aspects. The @Aspect annotation on a class marks it as an aspect and, hence, excludes it from auto-proxying.

Declaring a Pointcut

Pointcuts determine join points of interest and thus enable us to control when advice executes. Spring AOP only supports method execution join points for Spring beans, so you can think of a pointcut as matching the execution of methods on Spring beans. A pointcut declaration has two parts: a signature comprising a name and any parameters and a pointcut expression that determines exactly which method executions we are interested in. In the @AspectJ annotation-style of AOP, a pointcut signature is provided by a regular method definition, and the pointcut expression is indicated by using the @Pointcut annotation (the method serving as the pointcut signature must have a void return type).

An example may help make this distinction between a pointcut signature and a pointcut expression clear. The following example defines a pointcut named anyOldTransfer that matches the execution of any method named transfer:

Java
@Pointcut("execution(* transfer(..))") // the pointcut expression
private void anyOldTransfer() {} // the pointcut signature
Kotlin
@Pointcut("execution(* transfer(..))") // the pointcut expression
private fun anyOldTransfer() {} // the pointcut signature

The pointcut expression that forms the value of the @Pointcut annotation is a regular AspectJ 5 pointcut expression. For a full discussion of AspectJ’s pointcut language, see the AspectJ Programming Guide (and, for extensions, the AspectJ 5 Developer’s Notebook) or one of the books on AspectJ (such as Eclipse AspectJ, by Colyer et. al., or AspectJ in Action, by Ramnivas Laddad).

Supported Pointcut Designators

Spring AOP supports the following AspectJ pointcut designators (PCD) for use in pointcut expressions:

  • execution: For matching method execution join points. This is the primary pointcut designator to use when working with Spring AOP.

  • within: Limits matching to join points within certain types (the execution of a method declared within a matching type when using Spring AOP).

  • this: Limits matching to join points (the execution of methods when using Spring AOP) where the bean reference (Spring AOP proxy) is an instance of the given type.

  • target: Limits matching to join points (the execution of methods when using Spring AOP) where the target object (application object being proxied) is an instance of the given type.

  • args: Limits matching to join points (the execution of methods when using Spring AOP) where the arguments are instances of the given types.

  • @target: Limits matching to join points (the execution of methods when using Spring AOP) where the class of the executing object has an annotation of the given type.

  • @args: Limits matching to join points (the execution of methods when using Spring AOP) where the runtime type of the actual arguments passed have annotations of the given types.

  • @within: Limits matching to join points within types that have the given annotation (the execution of methods declared in types with the given annotation when using Spring AOP).

  • @annotation: Limits matching to join points where the subject of the join point (the method being executed in Spring AOP) has the given annotation.

Other pointcut types

The full AspectJ pointcut language supports additional pointcut designators that are not supported in Spring: call, get, set, preinitialization, staticinitialization, initialization, handler, adviceexecution, withincode, cflow, cflowbelow, if, @this, and @withincode. Use of these pointcut designators in pointcut expressions interpreted by Spring AOP results in an IllegalArgumentException being thrown.

The set of pointcut designators supported by Spring AOP may be extended in future releases to support more of the AspectJ pointcut designators.

Because Spring AOP limits matching to only method execution join points, the preceding discussion of the pointcut designators gives a narrower definition than you can find in the AspectJ programming guide. In addition, AspectJ itself has type-based semantics and, at an execution join point, both this and target refer to the same object: the object executing the method. Spring AOP is a proxy-based system and differentiates between the proxy object itself (which is bound to this) and the target object behind the proxy (which is bound to target).

Due to the proxy-based nature of Spring’s AOP framework, calls within the target object are, by definition, not intercepted. For JDK proxies, only public interface method calls on the proxy can be intercepted. With CGLIB, public and protected method calls on the proxy are intercepted (and even package-visible methods, if necessary). However, common interactions through proxies should always be designed through public signatures.

Note that pointcut definitions are generally matched against any intercepted method. If a pointcut is strictly meant to be public-only, even in a CGLIB proxy scenario with potential non-public interactions through proxies, it needs to be defined accordingly.

If your interception needs include method calls or even constructors within the target class, consider the use of Spring-driven native AspectJ weaving instead of Spring’s proxy-based AOP framework. This constitutes a different mode of AOP usage with different characteristics, so be sure to make yourself familiar with weaving before making a decision.

Spring AOP also supports an additional PCD named bean. This PCD lets you limit the matching of join points to a particular named Spring bean or to a set of named Spring beans (when using wildcards). The bean PCD has the following form:

Java
bean(idOrNameOfBean)
Kotlin
bean(idOrNameOfBean)

The idOrNameOfBean token can be the name of any Spring bean. Limited wildcard support that uses the * character is provided, so, if you establish some naming conventions for your Spring beans, you can write a bean PCD expression to select them. As is the case with other pointcut designators, the bean PCD can be used with the && (and), || (or), and ! (negation) operators, too.

The bean PCD is supported only in Spring AOP and not in native AspectJ weaving. It is a Spring-specific extension to the standard PCDs that AspectJ defines and is, therefore, not available for aspects declared in the @Aspect model.

The bean PCD operates at the instance level (building on the Spring bean name concept) rather than at the type level only (to which weaving-based AOP is limited). Instance-based pointcut designators are a special capability of Spring’s proxy-based AOP framework and its close integration with the Spring bean factory, where it is natural and straightforward to identify specific beans by name.

Combining Pointcut Expressions

You can combine pointcut expressions by using &&, || and !. You can also refer to pointcut expressions by name. The following example shows three pointcut expressions:

Java
@Pointcut("execution(public * (..))")
private void anyPublicOperation() {} (1)

@Pointcut("within(com.xyz.someapp.trading..)")
private void inTrading() {} (2)

@Pointcut("anyPublicOperation() && inTrading()")
private void tradingOperation() {} (3)
1 anyPublicOperation matches if a method execution join point represents the execution of any public method.
2 inTrading matches if a method execution is in the trading module.
3 tradingOperation matches if a method execution represents any public method in the trading module.
Kotlin
@Pointcut("execution(public * (..))")
private fun anyPublicOperation() {} (1)

@Pointcut("within(com.xyz.someapp.trading..)")
private fun inTrading() {} (2)

@Pointcut("anyPublicOperation() && inTrading()")
private fun tradingOperation() {} (3)
1 anyPublicOperation matches if a method execution join point represents the execution of any public method.
2 inTrading matches if a method execution is in the trading module.
3 tradingOperation matches if a method execution represents any public method in the trading module.

It is a best practice to build more complex pointcut expressions out of smaller named components, as shown