Trying to override old definition of task javac Error

最近在看pentaho-ce-5.1的源代码，用ant进行编辑的时候总是出错

Trying to override old definition of task javac Error

修改以前的内容

<?xml version="1.0" ?>
<project name="HelloWorld" default="compress">
    <presetdef name="javac">
            <javac includeantruntime="false" />
    </presetdef>
    <target name="init">
        <mkdir dir="build/classes" />
        <mkdir dir="dist" />
    </target>
    <target name="compile" depends="init">
        <javac srcdir="src" destdir="build/classes"/>
    </target> 
    <target name="compress" depends="compile">
            <jar destfile="dist/sample.jar" basedir="build/classes" />
    </target>
    <target name="execute" depends="compile">
        <java classname="src" classpath="build/classes" />
    </target>
    <target name="clean">
    </target>
</project>

修改以后的内容

<presetdef name="my.javac">
            <javac includeantruntime="false" />
    </presetdef>

看到了，主要是将javac改成了my.javac