org.springframework.beans.factory

## 接口 FactoryBean<T>

- 所有已知子接口:
    SmartFactoryBean<T>
  所有已知实现类:
    AbstractFactoryBean, AbstractServiceLoaderBasedFactoryBean, FieldRetrievingFactoryBean, ListFactoryBean, MapFactoryBean, MethodInvokingFactoryBean, ObjectFactoryCreatingFactoryBean, PropertiesFactoryBean, PropertyPathFactoryBean, ProviderCreatingFactoryBean, ServiceFactoryBean, ServiceListFactoryBean, ServiceLoaderFactoryBean, ServiceLocatorFactoryBean, SetFactoryBean, YamlMapFactoryBean, YamlPropertiesFactoryBean

      public interface FactoryBean<T>

  Interface to be implemented by objects used within a `BeanFactory` which are themselves factories for individual objects. If a bean implements this interface, it is used as a factory for an object to expose, not directly as a bean instance that will be exposed itself.

  NB: A bean that implements this interface cannot be used as a normal bean. A FactoryBean is defined in a bean style, but the object exposed for bean references (`getObject()`) is always the object that it creates.

  FactoryBeans can support singletons and prototypes, and can either create objects lazily on demand or eagerly on startup. The `SmartFactoryBean` interface allows for exposing more fine-grained behavioral metadata.

  This interface is heavily used within the framework itself, for example for the AOP `org.springframework.aop.framework.ProxyFactoryBean` or the `org.springframework.jndi.JndiObjectFactoryBean`. It can be used for custom components as well; however, this is only common for infrastructure code.

  `FactoryBean` is a programmatic contract. Implementations are not supposed to rely on annotation-driven injection or other reflective facilities. `getObjectType()` `getObject()` invocations may arrive early in the bootstrap process, even ahead of any post-processor setup. If you need access other beans, implement `BeanFactoryAware` and obtain them programmatically.

  Finally, FactoryBean objects participate in the containing BeanFactory's synchronization of bean creation. There is usually no need for internal synchronization other than for purposes of lazy initialization within the FactoryBean itself (or the like).

  从以下版本开始:
    08.03.2003
  作者:
    Rod Johnson, Juergen Hoeller
  另请参阅:
    `BeanFactory`, `org.springframework.aop.framework.ProxyFactoryBean`, `org.springframework.jndi.JndiObjectFactoryBean`

- - ### 方法概要

    所有方法 实例方法 抽象方法 默认方法
    限定符和类型 | 方法和说明
    --- | ---
    `T` | `getObject()` Return an instance (possibly shared or independent) of the object managed by this factory.
    `Class<?>` | `getObjectType()` Return the type of object that this FactoryBean creates, or `null` if not known in advance.
    `default boolean` | `isSingleton()` Is the object managed by this factory a singleton?

- - ### 方法详细资料

    - #### getObject

          @Nullable
          T getObject()
                          throws Exception

      Return an instance (possibly shared or independent) of the object managed by this factory.

      As with a `BeanFactory`, this allows support for both the Singleton and Prototype design pattern.

      If this FactoryBean is not fully initialized yet at the time of the call (for example because it is involved in a circular reference), throw a corresponding `FactoryBeanNotInitializedException`.

      As of Spring 2.0, FactoryBeans are allowed to return `null` objects. The factory will consider this as normal value to be used; it will not throw a FactoryBeanNotInitializedException in this case anymore. FactoryBean implementations are encouraged to throw FactoryBeanNotInitializedException themselves now, as appropriate.

      返回:
        an instance of the bean (can be `null`)
      抛出:
        `Exception` - in case of creation errors
      另请参阅:
        `FactoryBeanNotInitializedException`

    - #### getObjectType

          @Nullable
          Class<?> getObjectType()

      Return the type of object that this FactoryBean creates, or `null` if not known in advance.

      This allows one to check for specific types of beans without instantiating objects, for example on autowiring.

      In the case of implementations that are creating a singleton object, this method should try to avoid singleton creation as far as possible; it should rather estimate the type in advance. For prototypes, returning a meaningful type here is advisable too.

      This method can be called before this FactoryBean has been fully initialized. It must not rely on state created during initialization; of course, it can still use such state if available.

      NOTE: Autowiring will simply ignore FactoryBeans that return `null` here. Therefore it is highly recommended to implement this method properly, using the current state of the FactoryBean.

      返回:
        the type of object that this FactoryBean creates, or `null` if not known at the time of the call
      另请参阅:
        `ListableBeanFactory.getBeansOfType(java.lang.Class<T>)`

    - #### isSingleton

          default boolean isSingleton()

      Is the object managed by this factory a singleton? That is, will `getObject()` always return the same object (a reference that can be cached)?

      NOTE: If a FactoryBean indicates to hold a singleton object, the object returned from `getObject()` might get cached by the owning BeanFactory. Hence, do not return `true` unless the FactoryBean always exposes the same reference.

      The singleton status of the FactoryBean itself will generally be provided by the owning BeanFactory; usually, it has to be defined as singleton there.

      NOTE: This method returning `false` does not necessarily indicate that returned objects are independent instances. An implementation of the extended `SmartFactoryBean` interface may explicitly indicate independent instances through its `SmartFactoryBean.isPrototype()` method. Plain `FactoryBean` implementations which do not implement this extended interface are simply assumed to always return independent instances if the `isSingleton()` implementation returns `false`.

      The default implementation returns `true`, since a `FactoryBean` typically manages a singleton instance.

      返回:
        whether the exposed object is a singleton
      另请参阅:
        `getObject()`, `SmartFactoryBean.isPrototype()`