On Nov 26, 2017, at 6:32 AM, Matthew Broadhead <matthew.broadh...@nbmlaw.co.uk> 
> Hi,
> I am currently evaluating whether the Xalan dependency can be dropped from 
> taglibs and replaced with javax.xml.*.
> The reason for this is a conflict which occurs when I have Apache Fop as a 
> dependency:
> java.lang.ClassCastException: org.apache.xml.dtm.ref.DTMManagerDefault cannot 
> be cast to org.apache.xml.dtm.DTMManager
>     at org.apache.xml.dtm.DTMManager.newInstance(DTMManager.java:137)
>     at org.apache.xpath.XPathContext.<init>(XPathContext.java:102)
>     at org.apache.xpath.XPathContext.<init>(XPathContext.java:349)
>     at org.apache.xpath.XPathContext.<init>(XPathContext.java:337)
>     at 
> org.apache.xalan.transformer.TransformerImpl.<init>(TransformerImpl.java:397)
>     at 
> org.apache.xalan.templates.StylesheetRoot.newTransformer(StylesheetRoot.java:200)
> In org.apache.taglibs.standard.tag.common.xml it seems like 
> org.apache.xpath.XPath could be replaced with javax.xml.xpath.XPathExpression.
> But the thing I don't understand is the method getContext in 
> org.apache.taglibs.standard.tag.common.xml.XalanUtil uses a VariableStack and 
> returns an XPathContext.  I am not sure how they fit into the SAX (?) model.
> Can anyone shed some light on the inner workings of this function?

This was added <https://svn.apache.org/viewvc?view=revision&revision=1054175> 
in 1.2.x to fix a performance problem (bug #27717 
<https://bz.apache.org/bugzilla/show_bug.cgi?id=27717>) with x:forEach. At the 
time, the default XPath implementation in javax.xml had the same n^2 behaviour 
as the previous implementation. There are some micro-benchmarks 
 still in the test class if you want to see if that’s still true.

Memory is a little fuzzy, but IIRC the JSTLVariableStack is used to make the 
JSP implicit variables (e.g. ${request.something}) resolvable in the transform. 
It’s the low-level equivalent of a custom XPathVariableResolver.

If you’re not using the XML tags, you may be able to simply drop the Xalan 
dependency from your application to avoid the conflict.

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