org.springframework.context.annotation

## 注释类型 Bean

-     @Target(value={METHOD,ANNOTATION_TYPE})
       @Retention(value=RUNTIME)
       @Documented
      public @interface Bean

  Indicates that a method produces a bean to be managed by the Spring container.

  ### Overview

  The names and semantics of the attributes to this annotation are intentionally similar to those of the `<bean/>` element in the Spring XML schema. For example:

           @Bean
           public MyBean myBean() {
               // instantiate and configure MyBean obj
               return obj;
           }

  ### Bean Names

  While a `name()` attribute is available, the default strategy for determining the name of a bean is to use the name of the `@Bean` method. This is convenient and intuitive, but if explicit naming is desired, the `name` attribute (or its alias `value`) may be used. Also note that `name` accepts an array of Strings, allowing for multiple names (i.e. a primary bean name plus one or more aliases) for a single bean.

           @Bean({"b1", "b2"}) // bean available as 'b1' and 'b2', but not 'myBean'
           public MyBean myBean() {
               // instantiate and configure MyBean obj
               return obj;
           }

  ### Profile, Scope, Lazy, DependsOn, Primary, Order

  Note that the `@Bean` annotation does not provide attributes for profile, scope, lazy, depends-on or primary. Rather, it should be used in conjunction with `@Scope`, `@Lazy`, `@DependsOn` and `@Primary` annotations to declare those semantics. For example:

           @Bean
           @Profile("production")
           @Scope("prototype")
           public MyBean myBean() {
               // instantiate and configure MyBean obj
               return obj;
           }

  The semantics of the above-mentioned annotations match their use at the component class level: `@Profile` allows for selective inclusion of certain beans. `@Scope` changes the bean's scope from singleton to the specified scope. `@Lazy` only has an actual effect in case of the default singleton scope. `@DependsOn` enforces the creation of specific other beans before this bean will be created, in addition to any dependencies that the bean expressed through direct references, which is typically helpful for singleton startup. `@Primary` is a mechanism to resolve ambiguity at the injection point level if a single target component needs to be injected but several beans match by type.

  Additionally, `@Bean` methods may also declare qualifier annotations and `@Order` values, to be taken into account during injection point resolution just like corresponding annotations on the corresponding component classes but potentially being very individual per bean definition (in case of multiple definitions with the same bean class). Qualifiers narrow the set of candidates after the initial type match; order values determine the order of resolved elements in case of collection injection points (with several target beans matching by type and qualifier).

  NOTE: `@Order` values may influence priorities at injection points, but please be aware that they do not influence singleton startup order which is an orthogonal concern determined by dependency relationships and `@DependsOn` declarations as mentioned above. Also, `Priority` is not available at this level since it cannot be declared on methods; its semantics can be modeled through `@Order` values in combination with `@Primary` on a single bean per type.

  ### `@Bean` Methods in `@Configuration` Classes

  Typically, `@Bean` methods are declared within `@Configuration` classes. In this case, bean methods may reference other `@Bean` methods in the same class by calling them directly. This ensures that references between beans are strongly typed and navigable. Such so-called 'inter-bean references' are guaranteed to respect scoping and AOP semantics, just like `getBean()` lookups would. These are the semantics known from the original 'Spring JavaConfig' project which require CGLIB subclassing of each such configuration class at runtime. As a consequence, `@Configuration` classes and their factory methods must not be marked as final or private in this mode. For example:

       @Configuration
       public class AppConfig {

           @Bean
           public FooService fooService() {
               return new FooService(fooRepository());
           }

           @Bean
           public FooRepository fooRepository() {
               return new JdbcFooRepository(dataSource());
           }

           // ...
       }

  ### `@Bean` Lite Mode

  `@Bean` methods may also be declared within classes that are not annotated with `@Configuration`. For example, bean methods may be declared in a `@Component` class or even in a plain old class. In such cases, a `@Bean` method will get processed in a so-called 'lite' mode.

  Bean methods in lite mode will be treated as plain factory methods by the container (similar to `factory-method` declarations in XML), with scoping and lifecycle callbacks properly applied. The containing class remains unmodified in this case, and there are no unusual constraints for the containing class or the factory methods.

  In contrast to the semantics for bean methods in `@Configuration` classes, 'inter-bean references' are not supported in lite mode. Instead, when one `@Bean`-method invokes another `@Bean`-method in lite mode, the invocation is a standard Java method invocation; Spring does not intercept the invocation via a CGLIB proxy. This is analogous to inter-`@Transactional` method calls where in proxy mode, Spring does not intercept the invocation — Spring does so only in AspectJ mode.

  For example:

       @Component
       public class Calculator {
           public int sum(int a, int b) {
               return a+b;
           }

           @Bean
           public MyBean myBean() {
               return new MyBean();
           }
       }

  ### Bootstrapping

  See the @`Configuration` javadoc for further details including how to bootstrap the container using `AnnotationConfigApplicationContext` and friends.

  ### `BeanFactoryPostProcessor`-returning `@Bean` methods

  Special consideration must be taken for `@Bean` methods that return Spring `BeanFactoryPostProcessor` (`BFPP`) types. Because `BFPP` objects must be instantiated very early in the container lifecycle, they can interfere with processing of annotations such as `@Autowired`, `@Value`, and `@PostConstruct` within `@Configuration` classes. To avoid these lifecycle issues, mark `BFPP`-returning `@Bean` methods as `static`. For example:

           @Bean
           public static PropertySourcesPlaceholderConfigurer pspc() {
               // instantiate, configure and return pspc ...
           }

  By marking this method as `static`, it can be invoked without causing instantiation of its declaring `@Configuration` class, thus avoiding the above-mentioned lifecycle conflicts. Note however that `static` `@Bean` methods will not be enhanced for scoping and AOP semantics as mentioned above. This works out in `BFPP` cases, as they are not typically referenced by other `@Bean` methods. As a reminder, a WARN-level log message will be issued for any non-static `@Bean` methods having a return type assignable to `BeanFactoryPostProcessor`.
  从以下版本开始:
    3.0
  作者:
    Rod Johnson, Costin Leau, Chris Beams, Juergen Hoeller, Sam Brannen
  另请参阅:
    `Configuration`, `Scope`, `DependsOn`, `Lazy`, `Primary`, `Component`, `Autowired`, `Value`

- - ### 可选元素概要

    可选元素
    限定符和类型 | 可选元素和说明
    --- | ---
    `org.springframework.beans.factory.annotation.Autowire` | `autowire` Are dependencies to be injected via convention-based autowiring by name or type?
    `String` | `destroyMethod` The optional name of a method to call on the bean instance upon closing the application context, for example a `close()` method on a JDBC `DataSource` implementation, or a Hibernate `SessionFactory` object.
    `String` | `initMethod` The optional name of a method to call on the bean instance during initialization.
    `String[]` | `name` The name of this bean, or if several names, a primary bean name plus aliases.
    `String[]` | `value` Alias for `name()`.

- - ### 元素详细资料

    - #### value

          @AliasFor(value="name")
          public abstract String[] value

      Alias for `name()`.

      Intended to be used when no other attributes are needed, for example: `@Bean("customBeanName")`.

      从以下版本开始:
        4.3.3
      另请参阅:
        `name()`
      默认值:
        {}

  - - #### name

          @AliasFor(value="value")
          public abstract String[] name

      The name of this bean, or if several names, a primary bean name plus aliases.

      If left unspecified, the name of the bean is the name of the annotated method. If specified, the method name is ignored.

      The bean name and aliases may also be configured via the `value()` attribute if no other attributes are declared.

      另请参阅:
        `value()`
      默认值:
        {}

  - - #### autowire

          public abstract org.springframework.beans.factory.annotation.Autowire autowire

      Are dependencies to be injected via convention-based autowiring by name or type?

      Note that this autowire mode is just about externally driven autowiring based on bean property setter methods by convention, analogous to XML bean definitions.

      The default mode does allow for annotation-driven autowiring. "no" refers to externally driven autowiring only, not affecting any autowiring demands that the bean class itself expresses through annotations.

      另请参阅:
        `Autowire.BY_NAME`, `Autowire.BY_TYPE`
      默认值:
        org.springframework.beans.factory.annotation.Autowire.NO

  - - #### initMethod

          public abstract String initMethod

      The optional name of a method to call on the bean instance during initialization. Not commonly used, given that the method may be called programmatically directly within the body of a Bean-annotated method.

      The default value is `""`, indicating no init method to be called.

      另请参阅:
        `InitializingBean`, `ConfigurableApplicationContext.refresh()`
      默认值:
        ""

  - - #### destroyMethod

          public abstract String destroyMethod

      The optional name of a method to call on the bean instance upon closing the application context, for example a `close()` method on a JDBC `DataSource` implementation, or a Hibernate `SessionFactory` object. The method must have no arguments but may throw any exception.

      As a convenience to the user, the container will attempt to infer a destroy method against an object returned from the `@Bean` method. For example, given an `@Bean` method returning an Apache Commons DBCP `BasicDataSource`, the container will notice the `close()` method available on that object and automatically register it as the `destroyMethod`. This 'destroy method inference' is currently limited to detecting only public, no-arg methods named 'close' or 'shutdown'. The method may be declared at any level of the inheritance hierarchy and will be detected regardless of the return type of the `@Bean` method (i.e., detection occurs reflectively against the bean instance itself at creation time).

      To disable destroy method inference for a particular `@Bean`, specify an empty string as the value, e.g. `@Bean(destroyMethod="")`. Note that the `DisposableBean` callback interface will nevertheless get detected and the corresponding destroy method invoked: In other words, `destroyMethod=""` only affects custom close/shutdown methods and `Closeable`/`AutoCloseable` declared close methods.

      Note: Only invoked on beans whose lifecycle is under the full control of the factory, which is always the case for singletons but not guaranteed for any other scope.

      另请参阅:
        `DisposableBean`, `ConfigurableApplicationContext.close()`
      默认值:
        "(inferred)"