how do I add one more level of dependency

[Suu Quan]

I played with making this simple jar file
  <target name="utils" >
    <delete file="lib/utils.jar"/>
    <javac srcdir="utils" includes="*.java"/>
    <jar jarfile="${lib}/utils.jar"  basedir="utils"  includes="*.class"  />
  </target>

javac takes care of not recompiling if the .class is already uptodate.
but
how do I tell ant to not recompile a .java if lib/utils.jar is uptodate?

The way I'm doing it, it always recreates a new jar file (identical to the
old)

tia