web server - Borland Technical Publications

Chapter
20
20Message-Driven Beans and JMS
Chapter
Important
For documentation updates, go to www.borland.com/techpubs/bes.
JMS and EJB
According to the EJB 2.0 specification, there are no limitations on a bean acting as a
JMS message producer or synchronous consumer. It can use the regular JMS APIs to
send a message to a queue or publish to a topic. As long as you perform synchronous
style consumption of messages (that is, not based on javax.jms.MessageListener), then
there are no problems on the consumption side either. The complexity lies in wanting
the sending/receiving of the message to share the transaction context of some other
piece of work. We already know how to solve this problem using JMS and JTA in
conjunction. The EJBs demand no special treatment.
Since EJB method invocations are synchronous, some calls will have to wait until the
bean has completed its processing. This may include calling other beans, databases,
and so forth. This RMI behavior can be undesirable in many situations. For example,
you may just want to call the method and have it return before doing any heavy
processing, allowing the caller to proceed with other tasks in the meantime. Threading
in the client is an obvious way to achieve this, but it suffers from two problems:
■
■
the client's programming model is not a true asynchronous style
if the client is an EJB, threading is prohibited in its method implementations
Chapter 20: Message-Driven Beans and JMS 183