Maven by Example

3.4. Simple Project Object Model

Simple Project’s pom.xml file. 

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                             http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>org.sonatype.mavenbook.simple</groupId>
    <artifactId>simple</artifactId>
    <packaging>jar</packaging>
    <version>1.0-SNAPSHOT</version>
    <name>simple</name>
    <url>http://maven.apache.org</url>
    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
    </dependencies>
</project>

This pom.xml file is the most basic POM you will ever deal with for a Maven project, usually a POM file is considerably more complex: defining multiple dependencies and customizing plugin behavior. The first few elements—groupId, artifactId, packaging, version—are what is known as the Maven coordinates which uniquely identify a project. name and url are descriptive elements of the POM providing a human readable name and associating the project with a web site. The dependencies element defines a single, test-scoped dependency on a unit testing framework called JUnit. These topics will be further introduced in Section 3.5, “Core Concepts”, all you need to know, at this point, is that the pom.xml is the file that makes Maven go.

Maven always executes against an effective POM, a combination of settings from this project’s pom.xml, all parent POMs, a super-POM defined within Maven, user-defined settings, and active profiles. All projects ultimately extend the super-POM, which defines a set of sensible default configuration settings. While your project might have a relatively minimal pom.xml, the contents of your project’s POM are interpolated with the contents of all parent POMs, user settings, and any active profiles. To see this "effective" POM, run the following command in the simple project’s base directory.

$ mvn help:effective-pom

When you run this, you should see a much larger POM which exposes the default settings of Maven. This goal can come in handy if you are trying to debug a build and want to see how all of the current project’s ancestor POMs are contributing to the effective POM.