In the book "EJB 3 in Action" (by Debu Panda, Reza Rahman and Derek Lane) I've found the following passage: "Keep in mind that you must not inject a stateful session bean into a stateless object, such as stateless session bean or servlet that may be shared by multiple concurrent clients (you should use JNDI in such cases instead)." Could someone please elaborate on that ?
I don't see how JNDI could help in such a case and why dependency injection is an issue. (I realize that using a stateful bean inside a stateless one is not very useful, since after each method call the stateless object "forgets the state".)
I wouldn't call a stateful bean from inside a stateless bean at all because as you noted it doesn't make sense. A servlet calling a stateful bean does make sense so I'm going to assume it works the same way.
A servlet is called by multiple clients. It is common to have only one instance of the servlet that processes all requests. However dependency injection would inject the stateful bean only once and multiple callers would use it. By looking up the stateful bean from JNDI each time, you are giving the container a clue that it should look for the appropriate one and not necessarily the last one used.
Interestingly, Spring solves this problem by introducing a scope for request specific objects.