ANT war task- How to war only new /modified files

How do i modify ant war task so when everytime it is called, instead of building whole application, it builds only new files or modified files. I tried to use update, duplicate attributes and they do not seem helpful开发者_如何转开发.

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try

modified 

tag

<war ... >
  <fileset dir="${local.dir}">
    <modified>
      <param name="cache.cachefile" value="localdev.cache"/>
    </modified>
  </fileset>
</war>