11.1 Messaging in J2EE

Most discussions of messaging in the J2EE world focus on JMS, the Java Message Service API, which is a core component of the J2EE standard. JMS provides access to heavy-duty messaging software. A JMS-enabled application can take advantage of a range of services, such as delivery guarantees, quality of service contracts, and an easy-to-use Java interface.

The patterns in this chapter apply to JMS applications, but many of them can be used alongside another message transport mechanism: trusty old Internet email. Email has gotten relatively short shrift in most texts on enterprise software development, and we're hard pressed to see why; it's easy to use, and the infrastructure is nearly ubiquitous. There's even a handy J2EE API, JavaMail, designed for dealing with it. We look at most of the patterns in this chapter through the lenses of both JMS and JavaMail. Of course, JavaMail and JMS are hardly equivalent, and we'll point out some of the differences in the next few pages.

11.1.1 Asynchronous Messaging Versus Web Services

This chapter doesn't cover message-oriented web services, partially because messaging is such a broad subject. If you think about it, most of what an application does can be cast in terms of messaging: method calls, for instance, are messages from one class to another, with immediate receipt and confirmation of delivery.

For our purposes, there's one major difference between asynchronous messaging and web services: web services are synchronous. When you're remotely accessing an EJB session bean or other Remote Procedure Call interface, it's expected that the service you're looking for will be available and online, and a web service is the same way. Web services use XML messages that conform to the SOAP specification, but they deliver the message immediately or not at all. The SOAP protocol doesn't mandate a delivery mechanism, but web services do. (HTTP—that's where the "web" part comes from.) These messages, like method calls, are synchronous: they happen immediately, and both sides of the exchange must coordinate with each other.

In this chapter, we're focusing on asynchronous messaging: communication between multiple systems in something other than real time. In an asynchronous exchange, the sender dispatches the message and goes on with other work. The recipient may receive the message milliseconds, minutes, hours, or days later, depending on the transport mechanism, system design, network conditions, and so forth (sunspots have been known to play a role here).

Even though the core web services concept is synchronous, that doesn't mean web services can't be deployed in support of an asynchronous application: you just need to write a little more code. Once you've done that, the underlying principles and patterns start to look a lot like the ones in this chapter.