ant/proposal/sandbox/filterreaders

Ant Filter Readers
==================

Usecase:
========
* Usage of filtering has become a common pattern in 
  Ant's tasks.  Filtering is being performed one way or
  another in tasks like <copy>, <move>, <fixcrlf>,
  <loadfile>, etc.

* There is no generic way to add custom filters
  currently to these tasks.  User has to either extend
  the task to add custom filter processing or add
  more attributes as needed to the task itself.
  
* If user is provided with a pluggable filtering
  mechanism, changes to built-in tasks can be mininized,
  while at the same time providing increased flexibility
  to the user.

Design:
======
* FilterChain is an ordered collection of 'AntFilterReader's and
  'java.io.FilterReader's

* Each AntFilterReader encloses the custom class representing 
  the actual java.io.FilterReader and contains configuration 
  parameters that may be used by the custom class if it
  implements the org.apache.tools.ant.types.Parameterizable
  interface.

* For ease of use, Ant's core filter readers can
  be used with a filter reader specific syntax also.
  
* Custom filter readers can be created easily even
  without using any of Ant's API - all one needs to
  do to create a custom filter is to extend 
  java.io.FilterReader.
  
* If the extended class also implements Parameterizable,
  operation parameters can be made available to the
  custom filter.

* Each FilterReader is piped through the other, if any, in
  the chain, in the order of declaration.

Example:
=======

<loadfile srcFile="foo" property="bar">
    <filterchain>
        <filterreader classname="org.apache.tools.ant.filters.StripLineComments">
            <param type="comment" value="//"/>
            <param type="comment" value="REM "/>
            <param type="comment" value="--"/>
        </filterreader>
        <filterreader classname="org.apache.tools.ant.filters.StripLineBreaks"/>
    </filterchain>
</loadfile>

The above example loads the contents of the file foo,
filters out the lines that begin with //, REM and --,
removes line breaks and then stores the result in
the property named bar.

Since StripLineComments and StripLineBreaks are built-in
Ant filter readers, the same can also be represented as:

<loadfile srcFile="foo" property="bar">
    <filterchain>
        <striplinecomments>
            <comment value="//"/>
            <comment value="REM "/>
            <comment value="--"/>
        </striplinecomments>
        <striplinebreaks/>
    </filterchain>
</loadfile>