xml - ANT | Trying to override old definition of task javac Error -

i trying build jar in ant.below code generating jar file.i dont know why error("trying override old definition of task javac") happens.sometimes not generating jar.

<?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> 

the output follows:

buildfile: e:\gad project\project\gad\build.xml trying override old definition of task javac init:     [mkdir] created dir: e:\gad project\project\gad\dist compile: compress:       [jar] building jar: e:\gad project\project\gad\dist\sample.jar build successful total time: 1 second 

let me know how can rectify error msg.?

lets try this

<presetdef name="my.javac">             <javac includeantruntime="false" />     </presetdef>       <target name="compile" depends="init">         <my.javac srcdir="src" destdir="build/classes"/>     </target>