如何在ant jar文件中包含目录结构？

 <property name="src.dir" value="src"/>
 <property name="build.dir" value="build"/>
 <property name="jar.name" value="my_project.jar"/>

 <target name="init">
    <tstamp/>
    <mkdir dir="${build.dir}"/>
  </target>

 <target name="compile" depends="init">
    <javac srcdir="${src.dir}" destdir="${build.dir}">
     <classpath refid="master-classpath"/>
    </javac>
  </target>

 <target name="jar" depends="compile">
    <jar destfile="${jar.name}" basedir="${build.dir}">
     <manifest>
       <attribute name="Main-Class" value="com.example.Main"/>
      </manifest>
    </jar>
  </target>

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