FilterChains and Filters

FilterChains can be compared to Unix pipes. Unix pipes add a great deal of flexibility to command line operations; for example, if you wanted to copy just those lines that contained the string blee from the first 10 lines of a file called foo to a file called bar, you could do:

cat foo | head -n10 | grep blee > bar

Something like this is not possible with the tasks and types that we have learned about thus far, and this is where the incredible usefulness of FilterChains becomes apparent. They emulate Unix pipes and provide a powerful dimension of file/stream manipulation for the tasks that support them.

FilterChain usage is quite straightforward: you pass the complex Phing type filterchain to a task that supports FilterChains and add individual filters to the FilterChain. In the course of executing the task, the filters are applied (in the order in which they appear in the XML) to the contents of the files that are being manipulated by your task.

<filterchain>
  <replacetokens>
    <token key="BC_PATH" value="${top.builddir}/"/>
    <token key="BC_PATH_USER" value="${top.builddir}/testsite/user/${lang}/"/>
  </replacetokens>

  <filterreader classname="phing.filters.TailFilter">
    <param name="lines" value="10"/>
  </filterreader>
</filterchain>

The code listing above shows you some example of how to use filter chains. For a complete reference see Core Types. This filter chain would replace all occurrences of BC_PATH and BC_PATH_USER with the values assigned to them in lines 4 and 5. Additionally, it will only return the last 10 lines of the files.

Notice above that FilterChain filters have a "shorthand" notation and a long, generic notation. Most filters can be described using both of these forms:

<replacetokens>
  <token key="BC_PATH" value="${top.builddir}/"/>
  <token key="BC_PATH_USER" value="${top.builddir}/testsite/user/${lang}/"/>
</replacetokens>

<!-- OR: -->

<filterreader classname="phing.filters.ReplaceTokens">
  <param type="token" name="BC_PATH" value="${top.builddir}/"/>
  <param type="token" name="BC_PATH" value="${top.builddir}/testsite/user/${lang}/"/>
</filterreader>

As the pipe concept in Unix, the filter concept is quite complex but powerful. To get a better understanding of different filters and how they can be used, take a look at any of the many uses of FilterChains in the build files for the binarycloud Bibliography project.