10 Replies Latest reply on Jun 28, 2005 4:09 PM by Tolga Tarhan

    EJB 3.0 Thread Safety

    Tolga Tarhan Newbie

      In the JBoss EJB3 tutorial on "Lookup of EJBs" from within servlets, etc, you mention using a trick that looks up the EJB reference in the init() method of the servlet, and then stores that reference for use from within the servlet.

      That's a good solution, but doesn't the EJB specification prohibit the use of the same reference from multiple threads? If I understand everything correctly, the servlet container will only use one instance of the servlet from multiple threads. That means that methods in the EJB session bean reference may be called from multiple threads at the same time.... I haven't looked for what the EJB3 says about thread safety yet, but I know the EJB 2.0 specification specifically prohibited this in section 7.11.8.

      Lastly, if this is now valid in EJB3 (which would be great), does using the same reference mean that all of the calls from the servlet threads to the EJB SB will be serialized? That'd be a bad performance hit for the servlets... also I imagine the whole story is completely different if we start talking about stateful SBs.

        • 1. Re: EJB 3.0 Thread Safety
          Bill Burke Master

          injecting a stateful bean is not really that useful, because, as you said, stateful beans cannot have concurrent access and you will get an exception. Stateless beans should be fine to store a reference.

          • 2. Re: EJB 3.0 Thread Safety
            Tolga Tarhan Newbie

            Just to clarify, the older specifications do not make the distinction between a stateful and stateless session bean -- they make it sound like you can never have two threads calling the same session bean reference at the same time.

            I agree that it makes no sense, but that's what it says. Will 3.0 relax this restriction (the current 3.0 draft has no mention of this)? If it is in fact allowed for stateless session beans, does JBoss serialize calls to the same instance, or are the concurrent calls simply assigned to different instances of the bean to handle (which would be preferable)?

            Here's the quote from section 7.12.10 in the EJB 2.1 specification (identical to section 7.11.8 in the 2.0 specification):


            The container must ensure that only one thread can be executing an instance at any time. If a client request arrives for an instance while the instance is executing another request, the container may throw the java.rmi.RemoteException to the second request if the client is a remote client, or the javax.ejb.EJBException if the client is a local client.

            Note that a session object is intended to support only a single client. Therefore, it would be an application error if two clients attempted to invoke the same session object.

            One implication of this rule is that an application cannot make loopback calls to a session bean instance.


            • 3. Re: EJB 3.0 Thread Safety
              Tolga Tarhan Newbie

              FYI -- I've asked this question to the EJB 3.0 EG. I'll post their reply here.

              Here's the email:


              My question relates to session bean thread safety in EJB 3.0 ? that is, multiple threads using the same EJB reference to access the bean. The 2.0 and 2.1 specifications specifically required the container to throw an exception in this case (see section 7.12.10 of the EJB 2.1 specification). While this makes perfect sense for stateful session beans, its usefulness is less convincing for stateless session beans (but the specification made no distinction).

              The EJB 3.0 specification doesn?t make any reference to thread safety for stateless session beans (it does require the container to synchronize calls to stateful session beans). Does this imply that, as the creators of the specification, you intend for the EJB 3.0 container to assign each concurrent call to the EJB reference to a different instance of the bean? This would essentially just treat concurrent calls to the same reference as if they had been made to difference references and put them through the normal EJB lifecycle.

              This would be the optimal solution, since a client could simply lookup the bean?s reference once, and then use that over-and-over from wherever that bean?s functionality is needed without regard for synchronization issues. This is especially useful if EJB session bean injection is ever allowed into servlets ? and in the meantime, it is useful to lookup the session bean once in the init() method of the servlet.

              Any information on your plans as it relates to session bean thread safety / synchronization would be appreciated.


              • 4. Re: EJB 3.0 Thread Safety
                Tolga Tarhan Newbie

                Hate to bring this up again, but with the release of the PR draft, the EJB 3.0 specification now prohibits this as well -- you can't safely use a single EJB session bean reference from multiple threads, which means looking up a Session bean from init() in a servlet wont work.

                There is a footnote that makes an exception, but says clients can't depend on it.

                Bill -- any feedback on what we should do? Should we lookup SB references on each hit to the servlet? Seems costly at best.

                • 5. Re: EJB 3.0 Thread Safety
                  Bernard de Terwangne Newbie

                  I've had to face this problem with Preview 5. I'm actually using a reference for each end user and storing it into the HTTPSession instead of the ServletContext. This is not enough to prevent concurent calls anyway so that I had to place a synchronizing Java Bean in front the stateful beans to prevent concurent calls. I did not do it for stateless beans and had no problems.

                  It is indeed important to know if calls to stateless beans on a on a single reference have to be synchronized. On the other hand I read that some J2EE servers like WebLogic can be configured to synchronize calls on session beans. This would greatly simplifies client software development but is useless for me if just some app servers can do it. Especially if JBoss does not.

                  Just a user point of view

                  Bernard

                  • 6. Re: EJB 3.0 Thread Safety
                    Tolga Tarhan Newbie

                    Actually, it kinda stinks even if they synchronize the calls on stateless beans -- you don't want all of your servlets to be tied up waiting for synchronization of a single SLSB. Imagine 50 web clients all waiting to use that one bean...

                    Bill -- please tell us what happens when we make multiple calls to the same SLSB reference... is it sync'd on a single SLSB, or does the EJB container just pool each request to a different instance?

                    • 8. Re: EJB 3.0 Thread Safety
                      Tolga Tarhan Newbie

                      Does the same apply for @Remote interfaces?

                      • 9. Re: EJB 3.0 Thread Safety
                        Kabir Khan Master

                        Yes,

                        No matter what interface is used they end up in the StatelessInstanceInterceptor which calls get on the pool. The default pool implementation is the ThreadLocalPool

                        • 10. Re: EJB 3.0 Thread Safety
                          Tolga Tarhan Newbie

                          Excellent implementation as far as usability goes for us users, but doesn't this break the specification, which requires you to either serialize the calls or throw an exception if two calls are made against the same EJB SB reference?