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.