Gist for "Java Build Tools, JavaScript Package Managers and Task Runners (Part 1)" blog-post.

build.xml

<project name="JavaBuildToolsExample" default="jar" basedir="." xmlns:ivy="antlib:org.apache.ivy.ant">

    <!-- Ant properties: Sub-directories are resolved with respect to the "basedir" attribute above. -->
    <property name="src.dir" value="src"/>
    <!-- Renamed the build directory as "/build-ant" so that it is distinguishable from "/build", created by Gradle. -->
    <property name="build.dir" value="build-ant"/>
    <property name="classes.dir" value="${build.dir}/classes"/>
    <property name="jar.dir" value="${build.dir}/jar"/>
    <property name="lib.dir" value="lib" />

    <!-- "fileset" attribute defines a group of files, in this case, under the "lib" directory. -->
    <path id="lib.path.id">
        <fileset dir="${lib.dir}" />
    </path>

    <target name="clean">
        <delete dir="${build.dir}"/>
    </target>

    <!-- Retrieves dependencies using default parameters. This usually retrieves all the dependencies of the last resolve call to the "lib" directory. -->
    <target name="resolve">
        <ivy:retrieve />
    </target>

    <!-- The "compile" target depends on the "resolve" target above.  -->
    <target name="compile" depends="resolve">
        <mkdir dir="${classes.dir}"/>
        <javac srcdir="${src.dir}" destdir="${classes.dir}" classpathref="lib.path.id"/>
    </target>

    <!-- The "jar" target depends on the "compile" target, which in turn depends on "resolve" -->
    <target name="jar" depends="compile">
        <mkdir dir="${jar.dir}"/>
        <jar destfile="${jar.dir}/${ant.project.name}.jar" basedir="${classes.dir}"/>
    </target>

</project>