Chapter 9. Properties and Resource Filtering  TOC  9.2.2. Maven Settings Properties

9.2. Maven Properties

You can use Maven properties in a pom.xml file or in any resource that is being processed by the Maven Resource plugin's filtering features. A property is always surrounded by ${ and }. For example, to reference the project.version property, one would write: 

${project.version}

There some implicit properties available in any Maven project, these implicit properties are:

project.*

Maven Project Object Model (POM). You can use the project.* prefix to reference values in a Maven POM.

settings.*

Maven Settings. You use the settings.* prefix to reference values from your Maven Settings in ~/.m2/settings.xml.

env.*

Environment variables like PATH and M2_HOME can be referenced using the env.* prefix.

System Properties

Any property which can be retrieved from the System.getProperty() method can be referenced as a Maven property.

In addition to the implicit properties listed above, a Maven POM, Maven Settings, or a Maven Profile can define a set of arbitrary, user-defined properties. The following sections provide so detail on the various properties available in a Maven project.

9.2.1. Maven Project Properties

When a Maven Project Property is referenced, the property name is referencing a property of the Maven Project Object Model (POM). Specifically, you are referencing a property of the org.apache.maven.model.Model class which is being exposed as the implicit variable project. When you reference a property using this implicit variable, you are using simple dot notation to reference a bean property of the Model object. For example, when you reference ${project.version}, you are really invoking the getVersion() method on the instance of Model that is being exposed as project.

The POM is also represented in the pom.xml document present in all Maven projects. Anything in a Maven POM can be referenced with a property. A complete reference for the POM structure is available at http://maven.apache.org/ref/2.2.1/maven-model/maven.html. The following list shows some common property references from the Maven project.

project.groupId and project.version

Projects in a large, multi-module build often share the same groupId and version identifiers. When you are declaring interdependencies between two modules which share the same groupId and version, it is a good idea to use a property reference for both: 

<dependencies>
  <dependency>
    <groupId>${project.groupId}</groupId>
    <artifactId>sibling-project</artifactId>
    <version>${project.version}</version>
  </dependency>
</dependencies>
project.artifactId

A project's artifactId is often used as the name of a deliverable. For example, in a project with WAR packaging, you will want to generate a WAR file without the version identifiers. To do this, you would reference the project.artifactId in your POM file like this: 

<build>
  <finalName>${project.artifactId}</finalName>
</build>
project.name and project.description

The name and project description can often be useful properties to reference from documentation. Instead of having to worry that all of your site documents maintain the same short descriptions, you can just reference these properties.

project.build.*

If you are ever trying to reference output directories in Maven, you should never use a literal value like target/classes. Instead you should use property references to refer to these directories.

  • project.build.sourceDirectory

  • project.build.scriptSourceDirectory

  • project.build.testSourceDirectory

  • project.build.outputDirectory

  • project.build.testOutputDirectory

  • project.build.directory

sourceDirectory, scriptSourceDirectory, and testSourceDirectory provide access to the source directories for the project. outputDirectory and testOutputDirectory provide access to the directories where Maven is going to put bytecode or other build output. directory refers to the directory which contains all of these output directories.

project.baseUri

If you need a valid URI for your project's base directory, you can use the ${project.baseUri} property. If your project is stored in the directory /tmp/simple, ${project.baseUri} will resolve to file:/private/tmp/simple/.

Other Project Property references

There are hundreds of properties to reference in a POM. A complete reference for the POM structure is available at http://maven.apache.org/ref/2.2.1/maven-model/maven.html.

For a full list of properties available on the Maven Model object, take a look at the JavaDoc for the maven-model project here http://maven.apache.org/ref/2.2.1/maven-model/apidocs/index.html. Once you load this JavaDoc, take a look at the Model class. From this Model class JavaDoc, you should be able to navigate to the POM property you wish to reference. If you needed to reference the output directory of the build, you can use the Maven Model JavaDoc to see that the output directory is referenced via model.getBuild().getOutputDirectory(); this method call would be translated to the Maven property reference ${project.build.outputDirectory}.

For more information about the Maven Model module, the module which defines the structure of the POM, see the Maven Model project page at http://maven.apache.org/ref/2.2.1/maven-model.

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Chapter 9. Properties and Resource Filtering  Sponsored by Sonatype  9.2.2. Maven Settings Properties