Another very good article with sample source code on How Java uses the producer/consumer model to handle images -- An insider's look . Read the article
Messaging lets a servlet delegate processing to a batch process either on the same machine or on a separate machine. The servlet creates a message and sends it to a queue. The servlet immediately completes and when the batch process is ready, it processes the message.
Messaging is therefore comprised of three main components:
A Producer creates messages and sends them to a Queue. The Producer could be something like a Servlet. A Queue stores the messages from the Produces and provides them to a Consumer when ready. The Queue is implemented by the messaging provider. A Consumer processes messages as they become available in the Queue. The Consumer is typically a bean implementing the MessageListener interface.
