有状态会话Bean与无状态会话Bean，实例变量困境

Question

I have a stateless session bean, but I want to add a reference to another bean in order to do some process. So if I add reference to another bean it shows up at instance level variable like this,

@Stateless
public class AccountFacade extends AbstractFacade<Account> implements AccountFacadeRemote         {
    @EJB
    private BillerFacadeRemote billerFacade;

    public AccountFacade() {
    super(Account.class);
    }

    ... all other methods...
}

Question

Now, by definition stateless bean should not have any instance level variables. So I am quite confuse about where to put this private BillerFacadeRemote billerFacade; ?

Answer 1

Your code is fine.

The @EJB annotation is injecting the bean into your class, and the application server manages its lifetime.

I recommend reading, or skimming the pretty long Java EE tutorial .

"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."

Answer 2

It's okay for a stateless beans to have instance variables that represent dependencies.

In fact, this is even encouraged. Without instance variables you could in many situations just use static method in a utility class instead.

What however is NOT encouraged, is have instance variables representing client observeable state. That is wrong, but dependencies like the entity manager, jms queues, JDBC connections, and thus other services that your stateless service delegates (part of) its work to, is absolutely okay.

Notice that the injections are true instance injections in instance variables. It are not class level (static) injections.