I wanted to post some follow-up information, in case anyone had any additional 
thoughts.  This appears to be a bug in multiple versions of JBoss AS, including 
apparently 5.0, where any time a stateless session bean is injected into an MDB 
a new instance of that stateless session bean is created, but never removed 
from the thread pool.  Therefore, if an MDB makes 500 calls to a SLSB, the 
container will end up with 500 instances of that SLSB which never get removed.  
This could potentially be the source of a major memory leak.

So, after some digging, the following information pointed to the ability to 
force the SLSB's into a pool of a specific maximum size:
http://www.jboss.org/jbossejb3/docs/reference/build/reference/en/html/session-bean-config.html

Which worked, for the specific SLSB which you annotated to use that pool, but 
unfortunately each subsequent SLSB that was called from that initial SLSB would 
still display the original behavior.  Now, adding that annotation to every 
single bean individually down the stack did not seem like a great idea either, 
although it could be done if required.  But the idea of controlling the pool 
was interesting, which lead me to research this more, and I came up with the 
following information:

https://jira.jboss.org/jira/browse/JBAS-5345 (BUG Report!)
http://www.jboss.org/index.html?module=bb&op=viewtopic&t=132763
http://www.jboss.org/index.html?module=bb&op=viewtopic&t=132331

Long story short:

- EJB3 annotations are intercepted using JBoss AOP and are configured in 
"default/deploy/ejb3-aop-interceptors.xml".  So, instead of annotating each 
SLSB individually, we can provide a blanket solution at this point to all 
SLSB's.

- ThreadlocalPool isn't very strict, apparently, but StrictMaxPool appears to 
give us the behavior we want

- As noted by wolfc in the first forum thread...It would appear that JBossMQ 
does not use a ThreadPool, but instantiates threads on the fly. Thus the 
ThreadlocalPool will keep on creating instances to match.  That makes the use 
of StrictMaxPool mandatory. Which is the behavior we are seeing.

To put all of this to the test, all of the Stateless Bean domain annotation 
expressions in ejb3-aop-interceptors.xml where changed from:

@org.jboss.annotation.ejb.PoolClass 
(value=org.jboss.ejb3.ThreadlocalPool.class, maxSize=30, timeout=10000)

to

@org.jboss.annotation.ejb.PoolClass (value=org.jboss.ejb3.StrictMaxPool.class, 
maxSize=30, timeout=10000)

Note: The MDB domain annotation expressions use StrictMaxPool by default!

This appears to solve the problem, but will require more monitoring in the 
coming weeks.  Any other thoughts on this issue?  Concerns about contention in 
the StrictMaxPool?  Thoughts on another, better solution?

Thanks!

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