Hi,
Being a newcomer to EJB (and this mailing list), I may be raising a
question that has already been threshed out previously. My apologies
if that is the case. I will re-search the archives for that discussion
(an initial look does not seem to have thrown up any previous threads
on this specific aspect of EJB-JMS integration).
The backbone of the application I am working on is essentially
asynchronous. JMS offers everything that is needed to do the job,
persistent messages, publish/subscribe model etc. We do however want
to present a synch interface to our clients in some cases and here
session beans seems to fit the bill. Our data model also seems to fit
nicely into the use of Entity beans.
In such a case, we would want our session bean to send out an asynch
message (on a Topic) and wait for a response. The problem is that we
have been discouraged (or forbidden?) from implementing the
MessageListener interface in our session bean, because the session bean
can be passivated any time and would not be invoked on the onMessage().
Or restating it as a concepts violation, the session bean in EJB1.1 is
a component for the "synchronous" domain and should not have
an asynch/callback nature applied on it.
To get around this restriction, we developed a helper class which
implements the MessageListener interface. The session bean does the
following:
synchronize(helperObject)
{
helperObject.start(); // To switch on listening for messages in
the JMS session in helperObject
helperObject.wait(); // handle InterruptedException
helperObject.stop(); // To switch off listening for messages in
the JMS session in helperObject
// process the message, which is stored in the helperObject
}
The helperObject meanwhile, in its onMessage() method:
synchronize(this)
{
// process message received as input to onMessage()
notifyAll();
}
Note that we ensure that the message listening is switched on only
when the session bean is actually waiting for a response.
This appears to work. But I am concerned that I may be missing
some subtle (or not so subtle) problems with such an implementation.
There doesn't seem to be much treatment on this topic either in books
or in the JMS/EJB specification (all they seem to say is that the issue
will be addressed in EJB2.0).
Are there any holes that I cant see?
Regards and Thanks,
Proneel.
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