如何使用Ant任务将javadoc链接到Java API？

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

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

 <target name="javadoc" depends="init">
    <javadoc sourcepath="${src.dir}" destdir="${docs.dir}">
     <classpath refid="master-classpath"/>
      <link offline="true" href="http://docs.oracle.com/javase/8/docs/api/"/>
    </javadoc>
  </target>

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

ant javadoc