Application architecture
Programming and Deployment Platform
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Java

Java was chosen for several reasons. First it offers a simple, object-oriented and reflective language. But its main strength lies in the J2EE platform (see Figure 3.1).

Figure 3.1: The Java 2 platform

The Java 2 Platform exists in four versions, each targeted at a different hardware environment. The Java Card, is the platform for smart card applications. The Micro Edition (J2ME) is aimed at the embedded devices market (phones, cars,...). The Standard Edition (J2SE) is provided for building desktop applications. Finally, the Enterprise Edition (J2EE) is aimed at the server-side enterprise applications market.

Among the various APIs offered by J2EE are the Servlet API, and the Enterprise Java Bean (EJB) component framework. These form the foundations on which XMLbroker server is plugged into its software environment. J2EE represents however only a set of specifications, fixed by the Java Community Process (JCP). These are then implemented in J2EE application servers.
This project initially started using IBM WebSphere 3.5 application server. Websphere however presented some deployment issues, due to its partial compliance to the J2EE specifications. The JBoss application server--an open-source implementation of J2EE, was adopted instead.

We concluded Chapter by the proposition that components collaborating through XML adapters could be the building blocks of a new generation of distributed applications. This view is matured in this chapter, by introducing the Enterprise Java Bean component architecture. In our model for exposing business services to the Web (see Section ), each service is implemented as one or several components (a.k.a. business objects). As formulated in the last section, the availability of the EJB component architecture is the primary reason for our choice of the J2EE platform. Whereas the detailed study of the EJB component model is not the subject of this report, some elements are worth noting:

• each EJB component is executed within a module of the application server known as an EJB container
• components only provide the business behaviour of a service
• they delegate the system behaviour, that is; the implementation of the transparency layers, resource management and transaction management to the EJB container and other modules of the application server.