Java™ Application Development on Linux - Dator

518Chapter 23Deploying EJBsautomate the building of many of the EJB files and deployment descriptors.Recall that Java comments can include special Javadoc tags, such as @authorand @param. Javadoc uses these tags to generate HTML files that are the documentationof your Java classes and methods based on the text associated withthese tags. XDoclet takes this a step further and defines tags like @ejb.beanand a few dozen more. Then, using the Javadoc mechanism, it can generate allthe various pieces required for an EJB. Used this way, you can write a singlesource file for your EJB, and have XDoclet generate the various home, remote,and local interfaces as well as the deployment descriptors.So why aren’t we all using XDoclet? It has been around for a few years andis gaining a following in the development community. We may be moving inthat direction, but it will take some time. It adds yet another layer to what isneeded to build an EJB application, albeit a layer that brings some simplification.Later releases of EJB specifications from Sun may subsume its EJB functionality.However, it is still very important to understand the pieces that gotogether to make an EJB application. One of the favorite quotes from XDocletin Action by Craig Walls and Norman Richards says it well: “Don’t generatewhat you don’t understand.”23.5ABSTRACTING LEGACY APPLICATIONSOne of the best uses of J2EE technology, particularly the EJB technology, is toprovide a single common interface to heterogenous systems. If an applicationprovides any sort of file, data, pipe, or network access to its data, you can wrapan EJB interface around it and make it available to an entire distributed network.This can be a powerful way to leverage investments in legacy systemswith modern multitier architectures.While it is commonplace for EJB applications to interface directly to a relationaldatabase back end, there is no requirement that such a system be theback end. IBM, for example, provides Java interfaces to their mainframe legacydata systems, such as CICS.23.6REVIEWWe’ve looked at the contents of an EAR file—not that you’ll need to be digginginside them or even building them by hand, but you’ll want to know what’sinside so as to understand what it takes to put one together. We took a look at