Summary

We have seen how coupling occurs between systems, based on API, on communication style, and on environment. One of the best reasons to adopt JMS is to build loosely coupled systems that are independent, flexible, and tolerant of changes. We explored how XML can contribute further to building loosley coupled systems. XML brings standards, vendor and system independence, internationalization, and flexibility to messaging systems. Consideration must be given to issue like parsing overhead, bandwidth utilization, and how to effectively package non-XML data. We have considered three different models of accessing the data in XML-based messages: through the DOM; using SAX; and finally using data binding, which is still in its infancy, but promises to greatly simplify how developers work with XML messages.

We then looked at what is involved in writing applications that use both JMS and XML together. This is an area that remains immature, with no real standards to guide developers; however, based on what we know about XML messaging in the web services arena, we can identify some best practices. Using these, we developed a simple XML router that reads messagres from an incoming queue, and routes them to an outgoing queue based on the message content. This example also gave us a basic framework for doing simple stylesheet transforms on message. As graphical XML translation tools mature, allowing automatic generation of stylesheets, message translations like this will become quite common.

In the final chapter, we will look at one area where JMS and messaging could be of considerable potential mobile messaging.