public interface FactoryBean<T>
BeanFactory
which are themselves factories. 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 application components as well; however,
this is not common outside of infrastructure code.
NOTE: 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
,
ProxyFactoryBean
,
JndiObjectFactoryBean
Modifier and Type | Method and Description |
---|---|
T |
getObject()
Return an instance (possibly shared or independent) of the object
managed by this factory.
|
java.lang.Class<?> |
getObjectType()
Return the type of object that this FactoryBean creates,
or
null if not known in advance. |
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)? |
T getObject() throws java.lang.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
)java.lang.Exception
- in case of creation errorsFactoryBeanNotInitializedException
java.lang.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>)
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
.
getObject()
,
SmartFactoryBean.isPrototype()