Interface to be implemented by objects used within a BeanFactory that are themselves factories. If a bean implements this interface, it is used as a factory, not directly as a bean.
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 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.
This interface is heavily used within the framework, for example for the AOP ProxyFactoryBean or JndiObjectFactoryBean. It can be used for application components, but this is not common outside of infrastructure code.
可以自己創建FactoryBean，這個時候的類就要當做工廠類來使用。在spring的配置文件中<ref local="factoryBean"/>調用的是getObject()方法，返回是object對象。