Can I send Ant 'replace' task output to a new file?

Question

The Ant replace task does an in-place replacement without creating a new file.

The below snippet replaces tokens in any of the '*.xml' files with the corresponding values from the 'my.properties' file.

<replace dir="${projects.prj.dir}/config"
         replacefilterfile="${projects.prj.dir}/my.properties"
         includes="*.xml" summary="true" />

I want those files that had their tokens replaced to be created named after a pattern (e.g.) '*.xml.filtered', and keep the original files.

Is this possible in Ant with some smart combination of tasks?

Answer 1

There are a couple of ways to get close to what you want without copying to a temporary directory and copying back.

Filtersets

If the source files can be changed so that the parts to be replaced can be delimited with begin and end tokens, as in @date@ (@ is the default token, but it can be changed) then you can use the copy task with a globmapper and a filterset:

<copy todir="config">
  <fileset dir="config" includes="*.xml" />
  <globmapper from="*.xml" to="*.xml.filtered" />
  <filterset filtersfile="replace.properties" />
</copy>

If replace.properties contains FOO = bar, then any occurrence of @FOO@ in a source xml file file be replaced with bar in the target.

Note that the source and target directories are the same, the globmapper means the target files and named with the suffix .filtered. It's possible (and more usual) to copy files into a different target directory)

Filterchains

If the source file can't be changed to add begin and end tokens, a possible alternative would be to use a filterchain with one or more replacestring filters instead of the filterset:

<copy todir="config">
  <fileset dir="config" includes="*.xml" />
  <globmapper from="*.xml" to="*.xml.filtered" />
  <filterchain>
    <tokenfilter>
      <replacestring from="foo" to="bar" />
      <!-- extra replacestring elements here as required -->
    </tokenfilter>
  </filterchain>
</copy>

This will replace any occurrence of foo with bar, anywhere in the file, which is more like the behaviour of the replace task. Unfortunately this way means you need to include all your replacements in the build file itself, you can't have them in a separate properties file.

In both cases the copy task will only copy source files that are newer than the target files, so unnecessary work won't be done.

Copy then replace

A third possibility (that has just occured to me whilst writing up the other two) would be to perform the copy first to the renamed files, then run the replace task specifying the renamed files:

<copy todir="config">
  <fileset dir="config" includes="*.xml" />
  <globmapper from="*.xml" to="*.xml.filtered" />
</copy>
<replace dir="config" replacefilterfile="replace.properties" summary="true" 
  includes="*.xml.filtered" />

This might be the closest solution to the original requirement. The downside is that the replace task will be run each time on the renamed files. This could be a problem for some replacement patterns (admittedly they would be odd ones like foo=foofoo, but they would be okay with the first two methods) and you will be doing unnecessary work when the dependencies don't change.

Answer 2

The replace task doesn't observe dependencies, instead it carries out the replacement by writing a temporary file for each input file. If the temporary file is the same as the input file, it is discarded. A temporary file that differs from the input file is renamed to replace that input. This means all the files are processed, even if none of them need be - hence it can be inefficient.

The original solution to this question was to carry out a copy-replace-copy. The second copy isn't needed though, as a mapper can be used in the first. In the copy, dependencies can be used to restrict processing to just the files that have changed - by means of a depend selector in an explicit fileset:

<copy todir="${projects.prj.dir}">
  <fileset dir="${projects.prj.dir}">
    <include name="*.xml" />
    <depend targetdir="${projects.prj.dir}">
      <mapper type="glob" from="*.xml" to="*.xml.filtered" />
    </depend>
  </fileset>
  <mapper type="glob" from="*.xml" to="*.xml.filtered" />
</copy>

That will restrict the copy fileset to just those files that have changed. An alternative syntax for the mappers is:

<globmapper from="*.xml" to="*.xml.filtered" />

The simplest replace would then be:

<replace dir="${projects.prj.dir}"
         replacefilterfile="my.properties"
         includes="*.xml.filtered" />

That will still process all the files though, even if none of them need undergo replacements. The replace task has an implicit fileset and can operate on an explicit fileset, but unlike similar tasks the implicit fileset is not optional, hence to take advantage of selectors in an explicit fileset you must make the implicit one 'do nothing' - hence the .dummy file here:

<replace dir="${projects.prj.dir}"
         replacefilterfile="my.properties">
         includes=".dummy" />
  <fileset dir="${projects.prj.dir}" includes="*.xml.filtered">
    <not>
      <different targetdir="${projects.prj.dir}">
        <globmapper from="*.xml.filtered" to="*.xml" />
      </different>
    </not>
  </fileset>
</replace>

That will prevent the replace task from needlessly processing files that have previously undergone substitution. It doesn't, however, prevent processing of files that haven't changed and don't need substitution.

Beyond that, I'm not sure there is a way to 'code golf' this problem to reduce the number of steps to one. There isn't a multiple string replacement filter that can be used in a copy task to achieve the same affect as replace, which is a shame because that feels like it would be the right solution.

One other approach would be to generate the xml for a series of replace string filters and then have Ant execute that. But that will be more complex than the existing solution, and prone to problems with replacement strings that, if pasted into an xml fragment will result in something that can't be parsed.

Yet another approach would be to write a custom task or script task to do the work. If there are many files and the copy-replace solution is judged to be too slow, then this might be the way to go. But again, that approach is less simple than the existing solution.

If the requirement is to minimise the work done in the processing, rather than to come up with the shortest Ant solution, then this approach might do.

• Make a fileset containing a list of inputs that have changed.
• From that fileset create a comma-separated list of corresponding filtered files.
• Carry out the copy on the fileset.
• Carry out the replace on the comma-separated list.

A wrinkle here is that the implicit fileset in the replace task will fall back to processing everything if no files have changed. To overcome this we insert a dummy file name.

<fileset id="changed" dir="${projects.prj.dir}" includes="*.xml">
  <depend targetdir="${projects.prj.dir}">
    <globmapper from="*.xml" to="*.xml.filtered" />
  </depend>
</fileset>

<pathconvert property="replace.includes" refid="changed">
  <map from=".xml" to=".xml.filtered" />
</pathconvert>

<copy todir="${projects.prj.dir}" preservelastmodified="true">
  <fileset refid="changed" />
 <globmapper from="*.xml" to="*.xml.filtered" />
</copy>

<replace dir="${projects.prj.dir}"
         replacefilterfile="my.properties"
         includes=".dummy,${replace.includes}" summary="true" />