Did you know that too much coupling -- which, in development terms, is a measure of how dependent two entities are upon each other -- can make your enterprise software hard to manage? Learn how the Java Message Service (JMS) can help reduce dependencies between applications.
Coupling, in the development world, is a measure of how dependent two entities are upon each other. Programmers make decisions everyday (consciously or otherwise) that affect coupling: class coupling, API coupling, application coupling, etc.
Coupling between applications isn't necessarily bad, but too much coupling can make your enterprise software hard to manage. It can also reduce your development options when you're expanding software or writing new applications or services for your company. The Java Message Service (JMS) can help reduce dependencies between applications, while still allowing them to communicate with and take advantage of each other's resources.
JMS normally involves at least three components: two JMS clients and the JMS server. The two clients communicate with each other through the JMS server. JMS clients are regular applications that use the JMS API to send and receive messages.
The JMS server can be any application that implements the JMS specification. Some JMS servers are part of larger applications; others are solely dedicated to JMS tasks. There are many third-party commercial and some open source JMS servers available from which to choose.
Applications can choose to communicate with one another using JMS in one of two ways: JMS topics or JMS queues. Topics and queues differ in a few ways, but the most significant way is in their approach to delivery.
JMS topics receive messages from a JMS client and distribute those messages to all JMS clients that are registered as topic listeners. JMS queues, on the other hand, distribute messages to only one client, no matter how many are registered as queue listeners. If two or more clients are subscribed to a queue and a message is stored in the queue, only one of the clients will receive the message. The specification does not define which client will receive the message.
Whether you're using topics or queues, the type of data you pass in your messages is up to you. Some programmers send business objects through their messaging systems, while other programmers send XML messages as text to reduce coupling even more.
If you haven't used JMS before, perhaps it's because you've never had a reason. If you've been writing your own messaging systems, then, hopefully, you'll be able to take advantage of the JMS API.
JMS has been around for a few years now and has proven to be a practical technology. Check out this JMS tutorial, and then get out there and take a look for yourself.
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