Explanation/Reference:
While in the ready stage, the EJB container may decide to deactivate, or passivate, the bean by moving it from memory to secondary storage. (Typically, the EJB container uses a least-recently-used algorithm to select a bean for passivation.) The EJB container invokes the method annotated @PrePassivate, if any, immediately before passivating it. If a client invokes a business method on the bean while it is in the passive stage, the EJB container activates the bean, calls the method annotated @PostActivate, if any, and then moves it to the ready stage.
Note:
At the end of the lifecycle, the client invokes a method annotated @Remove, and the EJB container calls the method annotated @PreDestroy, if any. The bean's instance is then ready for garbage collection.
* When a stateful bean is passivated, the instance fields are read and then written to the secondary storage associated with the EJB object. When the stateful session bean has been successfully passivated, the instance is evicted from memory; it is destroyed.
* When a passivated bean instance times out or when a client invokes the method marked with @Remove, the container may destroy the bean. Before destroying, the container will invoke the method annotated with
@ PreDestroy.
Reference: The Java EE 6 Tutorial, The Lifecycles of Enterprise Beans