Adam, I am really enjoying your book and am starting to use the patterns in modernising our Java app here.

We are implementing an events framework internal to the app that uses a JMS topic. My initial design for dispatching messages was to have a single MDB that subscribes to all event messages, which then uses a factory to generate an implementation of an action class according to the message contents.

But doing that makes it difficult to use EJBs as action classes (you have sold me on the value of stateless session beans!) so I have changed the design to use a different MDB for each action. In the new design, each MDB specifies a JMS message selector when it subscribes to the topic and it has its own action class (EJB) injected into it. (Obviously the JMS properties need to include enough information for the message selector to discriminate between messages.)

Does that sound like a good design? Are MDBs as inexpensive to use as session beans in Java EE 5?

Thanks in advance for any advice!
Michael