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From Jeremy Boynes <jer...@boynes.com>
Subject Re: xalan usage in taglibs
Date Mon, 27 Nov 2017 00:12:01 GMT
> On Nov 26, 2017, at 2:17 PM, Matthew Broadhead <matthew.broadhead@nbmlaw.co.uk>
wrote:
> 
> So to run the tests I could reverse the changes of the commit and then update to javax.xml.*
and run tests?
> 
> I am still struggling a bit to understand exactly what is happening wrt VariableStack
and how I can change over to XPathVariableResolver.  And also don't see a way to replace XBoolean,
XNumber, XString etc...
> 
> I will keep trying things.   Can I come back to you with specific queries?

I did experiment with a pure JAXP based solution - there’s a patch <https://bz.apache.org/bugzilla/attachment.cgi?id=26445&action=diff>
attached to #27717 that might be a place to start from. You may need to roll back a bit to
get it to apply. 

From what I remember, the performance problem stemmed from the evaluation of XPath inside
the loop, with both Xalan and JAXP (which used a shaded version of Xalan) reinitializing the
DTM each time leading to N * N scaling. The fix was to use Xalan’s API directly to convert
the document’s DOM to a DTM once and then apply the XPath against the DTM each time leading
to 1 * N scaling. 

The downside is that there was a direct dependency on Xalan 2.7.1. I think we tried using
2.7.2 but there was some other problem with that I can’t find at the moment. Given the stability
of Xalan/Xerces, what’s causing the conflict with FOP?
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