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From David Blevins <>
Subject Re: Connector, Message Inflow Dispatch Problem.
Date Fri, 20 Nov 2009 18:13:49 GMT

On Nov 20, 2009, at 7:30 AM, thabach wrote:

> We are implementing a JCA resource adapter

Ooh, sounds like a fun project.  You can do unbelievably cool and  
*portable* things with RAs.

If you're doing anything REST based or generic, could be a great  
contribution.  There are like a million connectors I'd love to write.

> and are experiencing a strange
> problem regarding dispatching incoming messages to the MDB pool. We  
> have
> implemented the standard mechanism for dispatching to the endpoint  
> as shown
> in this snippet (according to the J2EE Connector Architecture Spec -
> November 2003, section 12.5.1) :
> OurMessageListener endpoint =
> (OurMessageListener)_messageEndpointFactory.createEndpoint(null);
> endpoint.onMessage(message);
> We have exactly one acceptor thread receiving incoming messages  
> which will
> then hit the above code.  The problem we are experiencing is that  
> after ten
> messages we receive the following:
> javax.resource.spi.UnavailableException: Only 10 instances can be  
> created
> And indeed we notice it uses each MDB instance exactly once until it  
> runs
> out. It seems as if MDB's are not being released back into the pool.

The missing part of the puzzle is the MessageEndpoint interface.  Cast  
your instance of OurMessageListener to MessageEndpoint and call the  
release() method.  Also there is no pool, the createEndpoint will  
physically create an instance of the MDB (injection, @PostConstruct,  
and such) and the release method will destroy it (@PreDestroy).   It's  
the RA that does the pooling and the InstanceLimit property of the MDB  
container is basically there to prevent mistakes like this from going  
into production.

You'll probably immediately notice the beforeDelivery and  
afterDelivery methods which the RA should call before invoking the  
"onMessage" method.

Check out this test case.  It has just about everything you need to  
know to write your own RA.  It should save you a ton of time.


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