J2EE Design Patterns

Message Client Patterns

Once a message has been created and dispatched to an appropriate delivery channel, the next step is for the client to retrieve and process it. Retrieval mechanisms for messages need to balance speed of delivery, reliability, scalability, and ease of use. In this section, we’ll look at some patterns for handling messages in client applications. The Message Handle pattern shows how to decouple message processing code from message retrieval code. The Polling Consumer and Event-Driven Consumer patterns explain how to get messages into your application. Message Façade demonstrates using messaging to isolate your business logic. Competing Consumers shows how to speed up message processing in high volume applications, and Message Selector will help you build more efficient client-side message processors.

Message Handler Pattern

All message clients perform two functions: they retrieve and process a message. It’s possible to weave these two activities tightly together, but in more complex systems it can result in some pretty complex code. We want to isolate the latter function in a way that lets us share code across different message types. The Message Handler pattern abstracts the code responsible for receiving the actual message away from the code responsible for processing the message content.

This pattern allows us to change the method by which we receive a message without changing the code for processing the message. Changing message handling itself becomes ...