How to set JAVA HOME in Ant build.xml

I have the following build.xml file and i want to set JAVA HOME path but i don't know where to write and what to write. Does anyone wants to help me?

<target name="clean">
    <delete dir="build"/>
</target>

<target name="compile">
    <mkdir dir="build/classes"/>
    <javac srcdir="src" destdir="build/classes"/>
</target>

<target name="jar">
    <mkdir dir="build/jar"/>
    <jar destfile="mapreduce.jar" basedir="build/classes">
        <manifest>
            <attribute name="Main-Class" value="Master"/>
        </manifest>
    </jar>
</target>

<target name="run">
    <java jar="mapreduce.jar" fork="true"/>
</target>