Tuan,
I know. It is difficult both to explain and to understand these things in
plain English. That's why they invented UML ..  ;-)

"RMIClass" = "Delegator" 
implements MessageListener
has Engine
- that's the object that acts upon asynchronously received messages
- that's the object that you want to be able to re-configure in order to
     get messages from different topics
- has a method to change the topic (done by some "trigger" as I called it)
- runs outside EJB container as a standalone application
- delegates the business work to an engine using the Engine reference

"Engine"
- provides public method work() to be called by the Delegator
- does its work synchronously to the Delegator.onMessage()  - 
    thus  asynchronously from to the message sender's point of view.

If  "Engine" is an EJB, or some other J2EE component,
then the Delegator is a J2EE client.  This is what I actually meant 
when I said that the Delegator is basically your own Startup class.
These days (J2EE 1.3), J2EE clients use RMI-IIOP - 
I think that is why you called the Delegatr an  RMI class.

That's how I can understand the concept, and this should work.
Tell me if I am wrong!
Cheers, Rene