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The Maven 2 POM demystified

The evolution of a project model

By Eric Redmond, JavaWorld.com, 05/29/06

Building a project is complex business. Due to the dozens of tasks required in converting your hodge-podge of files into a working program, there exist literally hundreds of tools that do everything from generating source code, to compiling, to testing, to distribution, to brewing your morning coffee (if you find one, dear reader, let me know). Many of these programs are excellent at what they do. Unfortunately, for those of us who manage large-scale build systems for a living, there is rarely much commonality; each program requires its own disparate installation and esoteric configuration. It has become an inevitable fact of our lives that the majority of build systems are custom built by hand-gluing these tools with several homebrew scripts (yeah, Ant scripts count).

More than another build tool, Maven is a build framework. It cleanly separates your code from configuration files, documentation, and dependencies. Maven is surprisingly flexible in letting users configure most aspects of their code, as well as in controlling the behavior of plug-ins, individual goals, and even the build lifecycle itself. Maven is the actual structure, and within these walls, your project dwells; it wants to be an accommodating host.

But the problem still remains: managing the work of thousands of custom build scripts within a single framework is tough and, to be done correctly, requires much information. Fortunately, the Maven 2 team has been quite successful. Learning from the mistakes of Maven 1, countless user requests, tweaking, and updating, Maven 2 is more powerful than ever. Unfortunately, with great power comes great configuration. In order for Maven 2 artifacts to be easily portable units, that complex configuration falls into a single file. Enter the Maven POM.

What is the POM?

POM stands for project object model. It is an XML representation of a Maven project held in a file named pom.xml. In the presence of Maven folks, speaking of a project is speaking in the philosophical sense, beyond a mere collection of files containing code. A project contains configuration files, as well as developers involved and roles they play, the defect tracking system, the organization and licenses, the URL where the project lives, the project's dependencies, and all the other little pieces that come into play to give code life. A project is a one-stop shop for all things related to it. In fact, in the Maven world, a project need not contain any code at all, merely a pom.xml. We will encounter a couple such types of projects later in the article.

A quick structural overview

The POM is large and complex, so breaking it into pieces eases digestion. For the purposes of this discussion, these pieces are regrouped into four logical units, as shown in Figure 1: POM relationships, project information, build settings, and build environment. We shall begin by discussing POM relationships.

Below is a listing of the elements directly under the POM's project element. Notice that modelVersion contains 4.0.0. That is currently the only supported POM version for Maven 2 and is always required. The Maven 4.0.0 XML schema definition is located at http://maven.apache.org/maven-v4_0_0.xsd. Its top-level elements are as follows: