Building and Deploying an EAR
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An EAR file provides a convenient way to bundle up all pieces of a J2EE application. This article, the first of two parts, will show you how to build one and explain its various pieces. It is excerpted from chapter 3 of the book
JBoss at Work: A Practical Guide, written by Tom Marrs and Scott Davis (O'Reilly; ISBN: 0596007345). Copyright © 2006 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.
In the last chapter we introduced you to web applications, but the web tier is just one part of the J2EE spectrum. In this chapter, we’ll expand the JAW application from a simple WAR file into a full-fledged EAR.
We’ll explore the different parts of an EAR file. We’ll build a Common JAR containing classes that can be shared across all tiers of the application. Finally, we’ll play with various Ant and XDoclet tasks to create our EAR and dynamically generate the deployment descriptors JBoss needs.
WARs Versus EARs The WAR file is a convenient way to bundle up all pieces of a web application. All servlet containers know how to deploy a WAR file—they expand the bundle, look for the WEB-INF directory, and read the web.xml found there for further deployment instructions.
The EAR file provides the same type of functionality for a full-fledged J2EE application. JBoss expands the EAR, finds the required deployment descriptors, and proceeds from there.
An EAR is like a carton of eggs—it keeps everything organized. While the carton doesn’t add any direct value to your omelet, it makes getting the eggs home from the store so easy that you wouldn’t think about transporting eggs any other way.
Each egg in your EAR carton is a specific piece of the J2EE puzzle. These eggs (or JARs) come in three basic varieties called “modules”:
Web module
A WAR file containing presentation tier components
EJB module
An EJB JAR file containing the middle-tier components
(EJBs, MDBs, etc.)
Java module
A regular JAR file containing classes and libraries that
are shared across the entire application. An
application client JAR and a common JAR are two
examples of Java modules.
An EAR can contain at least one of any of these modules. By the same token, any of them can be safely omitted if they aren’t needed. Figure 3-1 shows the structure of an EAR file.
Figure 3-1. EAR file structure
Next: Application.xml >>
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 This article is excerpted from chapter 3 of the book JBoss at Work: A Practical Guide, written by Tom Marrs and Scott Davis (O'Reilly; ISBN: 0596007345). Check it out today at your favorite bookstore. Buy this book now.
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