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 ProxyFactoryBean or the
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).
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.
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.
the type of object that this FactoryBean creates,
or null if not known at the time of the call
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.