Is there an ANT task for watching a directory for changes?

Question

It sounds a little far fetched to me, but is there an ANT task for watching a directory for changes and then running a particular ANT class when the directory changes?

Answer 1

You might be able to use the Waitfor task to achieve what you want. It blocks until one or more conditions (such as the presence of a particular file) become true.

Answer 2

If files can only be added to or changed in the watched directory, then you can use this simple OutOfDate task from antcontrib.

<property name="watched-dir.flagfile"
  location="MUST be not in watched dir"/>
<outofdate>
  <sourcefiles>
    <fileset dir="watched-dir"/>
  </sourcefiles>
  <targetfiles>
    <pathelement location="${watched-dir.flagfile}"/>        
  </targetfiles>
  <sequential>

    <!--Tasks when something changes go here-->

    <touch file="${watched-dir.flagfile}"/>
  </sequential>
</outofdate>

If files can disappear from the watched-dir, then you have more complicated problem, that you can solve by creating shadow directory structure of the watched dir and checking if its consistent with the watched-dir. This task is more complex, but I'll give you a script to create a shadow directory, as it is not straight forward:

<property name="TALK" value="true"/>
<property name="shadow-dir"
  location="MUST be not in watched dir"/>

<touch
  mkdirs="true"
  verbose="${TALK}"
>
  <fileset dir="watched-dir">
    <patterns/>
    <type type="file"/>
  </fileset>

  <!-- That's the tricky globmapper to make touch task work -->
  <globmapper from="*" to="${shadow-dir}/*"/>
</touch>

<!--
  Due to how touch task with mapped fileset is implemented, it 
  truncates file access times down to a milliseconds, so if you 
  would have used outofdate task on shadow dir it would always 
  show that something is out of date.

  Because of that, touching all files in ${shadow-dir} again fixes
  that chicken and egg problem.
-->
<touch verbose="${TALK}">
  <fileset dir="${shadow-dir}"/>
</touch>

With shadow directory created, I'll leave the task of checking directory consistency as an exercise for the reader.

Answer 3

You can combine the apply task with a fileset selector

<apply executable="somecommand" parallel="false">
  <srcfile/>
  <fileset dir="${watch.dir}">
    <modified/>
  </fileset>
</apply>

The fileset will check the files against a stored MD5 checksum for changes. You'll need to put ANT into a loop in order to repeatedly run this check. this is easy to do in Unix:

while true
> do
> ant
> sleep 300
> done