Explanation/Reference:
The Lifecycle of a Stateless Session Bean
The EJB container typically creates and maintains a pool of stateless session beans, beginning the stateless session bean's lifecycle. The container performs any dependency injection and then invokes the method annotated @PostConstruct, if it exists. The bean is now ready to have its business methods invoked by a client.
At the end of the lifecycle, the EJB container calls the method annotated @PreDestroy, if it exists. The bean's instance is then ready for garbage collection.
Lifecycle of a Stateless Session Bean:

Note: An enterprise bean goes through various stages during its lifetime, or lifecycle. Each type of enterprise bean (stateful session, stateless session, singleton session, or message-driven) has a different lifecycle.
Reference: http://docs.oracle.com/javaee/6/tutorial/doc/giplj.html