T - the bean typepublic interface FactoryBean<T>
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).
BeanFactory, 
org.springframework.aop.framework.ProxyFactoryBean, 
org.springframework.jndi.JndiObjectFactoryBean| Modifier and Type | Field and Description | 
|---|---|
| static String | OBJECT_TYPE_ATTRIBUTEThe name of an attribute that can be
  seton aBeanDefinitionso that
 factory beans can signal their object type when it can't be deduced from
 the factory bean class. | 
| Modifier and Type | Method and Description | 
|---|---|
| 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  nullif not known in advance. | 
| 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)? | 
static final String OBJECT_TYPE_ATTRIBUTE
set on a
 BeanDefinition so that
 factory beans can signal their object type when it can't be deduced from
 the factory bean class.@Nullable T getObject() throws Exception
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.
null)Exception - in case of creation errorsFactoryBeanNotInitializedException@Nullable Class<?> getObjectType()
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.
null if not known at the time of the callListableBeanFactory.getBeansOfType(java.lang.Class<T>)default boolean isSingleton()
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.
getObject(), 
SmartFactoryBean.isPrototype()