Re: DOM L3 XPATH implementation

[Joseph Kesselman <ke...@us.ibm.com>]

On Wednesday, 09/18/2002 at 08:33 AST, Elena Litani <el...@ca.ibm.com> 
wrote:
> Vivek Pandey wrote:
> > I have started implementation work of DOM L3 XPATH 1.0
> > in my xerces 2.1 workspace.
> 
> To my knowledge Xalan already implements this API. So I am wondering if
> we should implement it in Xerces...?

Well, Xalan is certainly intending to implement it, and I think we've got 
a prototype. Obviously our version wouldn't put the factory on the 
Document node; that needs modification of the DOM. One possible approach 
would be to have Xerces set up to check if Xalan is avaialble and leverage 
it if so.

Currently Xalan's performance against DOMs is not wonderful; when we 
switched back to DTM we did so in a bit of a hurry and the DOM2DTM layer 
is somewhat inefficient as a result, especially if you want to run 
multiple XPaths against a DOM and change the DOM between those calls. I'm 
currently looking at whether we can improve that. 

(Our biggest hassle is the difficulty of associating a DOM Object 
reference with a DTM Node-Handle integer. My prototype's currently 
leveraging Xerces' prototype of DOM3 named userData -- not blindingly 
fast, unfortunately. May try a DOM-subclassing solution next, after I've 
got the framework running.)

> > * XPathEvaluator implementation will be in a separate class and will 
not
> > be implemented by w3c Document implementation class CoreDocumentImpl.

That's basically the approach Xalan is taking in its prototype. 
XPathEvaluator was specifically designed so it could be implemented by the 
Document node (in which case you can say the DOM actually supports this 
feature) or as a stand-alone (in which case the DOM does NOT support the 
feature, since obtaining the evaluator requires custom code... but the 
rest of the user code should be mostly compatable).

> > * Using org.apache.xpath packe in xalan for xpath parsing. is this
> > ok?
> 
> I am not sure what do you mean? You are implementing this code in
> Xerces, so how can you use the Xalan package??

This may be my suggestion above of "Check whether the Xalan package is 
available -- eg via reflection -- and leverage it if so; if not, return 
feature-not-supported."

> As a general rule you should not add ANY fields to NodeImpl.

As I say, I'd _really_ like to add an integer node-ID for my own 
project... but I'm looking at userData and subclassing as possible 
solutions, and trying to design my code so it can plug in the appropriate 
module for whatever solution is available.

______________________________________
Joe Kesselman  / IBM Research