In the previous section we showed you how to create a profile that overrides default behavior for a specific target environment. In the previous build the default build was designed for development and the production profile exists to provide configuration for a production environment. What happens when you need to provide customizations based on variables like operating systems or JDK version? Maven provides a way to "activate" a profile for different environmental parameters, this is called profile activation.

Take the following example, assume that we have a Java library that has a specific feature only available in the Java 6 release: the Scripting Engine as defined in JSR-223. You’ve separated the portion of the library that deals with the scripting library into a separate Maven project, and you want people running Java 5 to be able to build the project without attempting to build the Java 6 specific library extension. You can do this by using a Maven profile that adds the script extension module to the build only when the build is running within a Java 6 JDK. First, let’s take a look at our project’s directory layout and how we want developers to build the system.

When someone runs mvn install with a Java 6 JDK, you want the build to include the simple-script project’s build, when they are running in Java 5, you would like to skip the simple-script project build. If you failed to skip the simple-script project build in Java 5, your build would fail because Java 5 does not have the ScriptEngine on the classpath. Let’s take a look at the library project’s pom.xml:

Dynamic Inclusion of Submodules Using Profile Activation. 

<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</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>
    <profiles>
        <profile>
            <id>jdk16</id>
            <activation> (1)
                    <jdk>1.6</jdk>
            </activation>
            <modules> (2)
                    <module>simple-script</module>
            </modules>
        </profile>
    </profiles>
</project>

If you run mvn install under Java 1.6, you will see Maven descending into the simple-script subdirectory to build the simple-script project. If you are running mvn install in Java 1.5, the build will not try to build the simple-script submodule. Exploring this activation configuration in more detail:

<project>
    ...
    <profiles>
        <profile>
            <id>dev</id>
            <activation>
                <activeByDefault>false</activeByDefault> (1)
                    <jdk>1.5</jdk> (2)
                        <os>
                            <name>Windows XP</name> (3)
                                <family>Windows</family>
                                <arch>x86</arch>
                                <version>5.1.2600</version>
                        </os>
                        <property>
                            <name>customProperty</name> (4)
                                <value>BLUE</value>
                        </property>
                        <file>
                            <exists>file2.properties</exists> (5)
                                <missing>file1.properties</missing>
                        </file>
            </activation>
            ...
        </profile>
    </profiles>
</project>

<project>
    ...
    <profiles>
        <profile>
            <id>development</id>
            <activation>
                <property>
                    <name>!environment.type</name>
                </property>
            </activation>
        </profile>
    </profiles>
</project>