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From Peter Donald <pe...@apache.org>
Subject Re: 1.1 compat ( was Re: Death of a mutant ...
Date Fri, 12 Jul 2002 05:20:40 GMT
At 11:20 PM 7/11/2002 -0500, you wrote:
>>I thought the idea was that ant2 will have those famous incompatible API/
>>build.xml changes ( or will just be the next release after 1.9 :-).
>Running existing Ant 1.x build files with Ant 2 could be implemented by a) 
>recognizing the document is not using the Ant 2 namespace, b) piping the 
>content through an XSLT processor.  No need to add a lot of code to 
>support backward compatibility.  Hopefully, the syntax will be close 
>enough that the transform would be fairly simple.  It would also give 
>people a tool to convert an Ant 1.x build file to Ant 2 build file.

That was the initial plan I believe. However in prototyping this it was 
found that it was easier to add special code to handle the differences. 
Along with the syntactical differences of ant1.x to ant2 there is also 
slightly semantic changes and unless you are aware of these changes your 
build wont work anyways. Consider the following;

<project ...>

   <path id="x" dir=".">
     <pathelement location="nonexistant/file.txt"/>
   </path>

   <target name="X">
   </target>

   <target name="Y">
     <property name="x.prop" refid="x"/>
     <echo message="x=${x.prop}"/>
   </target>

</project>

In Ant1.x you could run target X fine. However if you tried to run Y it 
would generate a

"Error: nonexistant/file.txt does not exist!" (or similar)

This is because Ant1.x delays evaluation of some constructs until they are 
referenced. Where as in Ant2 the idea was to unify all evaluation of "build 
model" to when execution occurs. So in Ant2, the <path/> data type would be 
evaluated imediately before any target was executed and thus would generate 
an exception regardless of target.

This becomes especially painful when you are dealing with things like 
filesets. You have to decide when the list of files is evaluated. Is it 
when declared, when it is first used or everytime it is referenced? In 
ANt1.x it is kinda haphazard (first time referenced unless it does not have 
any propertys to expand in which case it is declaration) and getting it to 
work consistently with Ant2 datatypes is a little painful (though possible).

Other build systems GNUMake (and CONS from what I hear) explicitly allow 
user to declare whether filesets are re-evaluated every reference or just 
once etc. I would love for Ant to get to the same level of clarity.

This is just one of the peculiarities of the way ant as it has grown up and 
one peculiarity that we would like to fix in Ant2 (but is incompatible with 
Ant1.x). Other changes that are incompatible ways of evaluating propertys. 
ie In Ant2 it is mostly decided that ${x} should throw an exception if x is 
undefined but this behaviour is different (and incompatible) with ant1.x.

These are just two issues that imediately come to mind when executing 
ant1.x files in ant2. While the syntax of ant1.x can be easily translated 
into ant2 and the Task API can easily be adapted between the two versions. 
There are still real issues that need special handling.

Hence why I believe that there will be no simple transformation of ant1 
build files to ant2 buildfiles. Or if there is the transformed buildfiles 
will have to have directives to ant runtime that indicate it should operate 
in certain ways. (ie One directive may indicate that unresolved proeprtys 
should not cause an exception to be thrown)

><!--  Ant 1   -->
><project>
>
><!--  Ant 2   -->
><project xmlns="http://jakarta.apache.org/ant/2.0">

Or even more simply...

<project> <!-- ant1.x -->
<project version="2.0"> <!-- ant2 -->


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