In addition to the implicit properties provided by the POM, Maven Settings, environment variables, and the Java System properties, you have the ability to define your own arbitrary properties. Properties can be defined in a POM or in a Profile. The properties set in a POM or in a Maven Profile can be referenced just like any other property available throughout Maven. User-defined properties can be referenced in a POM, or they can be used to filter resources via the Maven Resource plugin. Here's an example of defining some arbitrary properties in a Maven POM.

<project>
  ...
  <properties>
    <arbitrary.property.a>This is some text</arbitrary.property.a>
    <hibernate.version>3.3.0.ga</hibernate.version>
  </properties>
  ...
  <dependencies>
    <dependency>
      <groupId>org.hibernate</groupId>
      <artifactId>hibernate</artifactId>
      <version>${hibernate.version}</version>
    </dependency>  
  </dependencies>
  ...
</project>

The previous example defines two properties: arbitrary.property.a and hibernate.version. The hibernate.version is referenced in a dependency declaration. Using the period character as a separator in property names is a standard practice throughout Maven POMs and Profiles. The next example shows you how to define a property in a profile from a Maven POM.

<project>
  ...
  <profiles>
    <profile>
      <id>some-profile</id>
      <properties>
        <arbitrary.property>This is some text</arbitrary.property>
      </properties>
    </profile>
  </profiles>
  ...
</project>

The previous example demonstrates the process of defining a user-defined property in a profile from a Maven POM. For more information about user-defined properties and profiles, see Chapter 5, Build Profiles.