In our imaginary project, the Ant "jar" target looks like:

<target name="jar" depends="compile">
    <jar jarfile="dist/example.jar">
        <fileset dir="build/main"/>
    </jar>
</target>

To use Jar Jar Links, we define a new task named "jarjar", and substitute it wherever we used the jar task. Because the JarJarTask class extends the normal Ant Jar task, you can use jarjar without any of its additional features, if you want:

<target name="jar" depends="compile">
    <taskdef name="jarjar" classname="com.tonicsystems.jarjar.JarJarTask"
        classpath="lib/jarjar.jar"/>
    <jarjar jarfile="dist/example.jar">
        <fileset dir="build/main"/>
    </jarjar>
</target>

Just like with the "jar" task, we can include the contents of another jar file using the "zipfileset" element. But simply including another projects classes is not good enough to avoid jar hell, since the class names remain unchanged and can still conflict with other versions.

To rename the classes, JarJarTask adds a new "rule" element. The rule takes a "pattern" attribute, which uses wildcards to match against class names, and a "result" attribute, which describes how to transform the matched names.

In this example we include classes from jaxen.jar and add a rule that changes any class name starting with "org.jaxen" to start with "org.example.jaxen" instead (in our imaginary world we control the example.org domain):

<target name="jar" depends="compile">
    <taskdef name="jarjar" classname="com.tonicsystems.jarjar.JarJarTask"
        classpath="lib/jarjar.jar"/>
    <jarjar jarfile="dist/example.jar">
        <fileset dir="build/main"/>
        <zipfileset src="lib/jaxen.jar"/>
        <rule pattern="org.jaxen.**" result="org.example.@1"/>
    </jarjar>
</target>

The ** in the pattern means to match against any valid package substring. To match against a single package component (by excluding dots (.) from the match), a single * may be used instead.

The @1 in the result is a reference to the ** portion of the rule. For every * or ** in the rule, a numbered reference is available for use in the result. References are numbered from left to right, starting with @1, then @2, and so on.

The special @0 reference refers to the entire class name.