Java Enterprise Best Practices

Separate Components and Create Shared Libraries

J2EE programmers usually tend to write one big application, with one JAR file containing all EJBs, because it is easy to develop this way. This is bad when it comes to maintenance of a deployed application because a bug fix in one EJB would mean a redeployment of the whole application.

It’s not hard to see the benefits of packaging EJBs into related groups. Aside from easing redeployment, this also facilities maintenance of the source code and deployment descriptors. Reusing different parts of the system is also easier if you can split up and package the application into separate components.

Frequently, classes and interfaces are shared among components and layers of the system—usually domain objects, utility classes, and various service interfaces. These can easily be packaged into shared libraries and used by other components. We use a shared library by putting it into the main EAR file, and then referencing the library in the manifest file of each JAR file that needs to use it. Here is an example:

Manifest-Version: 1.0
Created-By: Apache Ant 1.5
Class-Path: SharedLibrary1.jar log4j.jar

You can use the same method to specify interdependencies among EJB JARs as well.