Problem with org.apache.xpath.XPathAPI, org.apache.xpath.NodeSet

[Werner Koch <we...@ipt.ch>]

Hi all,

I have a problem using the XPathAPI class, and at the moment, I am
completely stumped (didn't yet consult the sources too much).
Here is my case:
- I run transformations with xalan (file based xml/xslt).
- I have a sorting/restricting problem, so I wrote a small java extension.
- When I base my sorting of the nodeset I get from the extension call by
directly navigating the DOM, then I get the expected results (even here with
a problem when I cut away nodes of org.apache.xpath.NodeSet with
removeElementAt, because the nodeset in the XSLT will still show the
original, larger number; I could resolve this by returning a new NodeSet
where I just copied the wanted Nodes into).
- Because of generalization issues, I decided to base my sorting on a value
in my nodeset selected by an XPath, which I also receive from the call in
the stylesheeet. So I get a NodeSet from the stylesheet, take the Node from
this I like to handle, and try to XPathAPI.selectSingleNode(aNode, aPath)
select the wanted information needed for my sort. This unfortunately never
works for me, because the NodeIterator inside this call to XPathAPI stumbles
over a NullPointerException when it tries to return the first node by
executing nextNode(). This holds even true when I execute an XPath query of
'.' on an Element Node with a at least one TextNode attached.

  Node result = XPathAPI.selectSingleNode(nodeI, ".").nextNode();

As I said, the nodeI is an Element node with a Text Node attached (I do
something like my-node-set.item(i) to set nodeI, where my-node-set is the
one I got from the stylesheet).

If I try out other XPaths than '.', I even get 'Unknown XPath errors', so I
think, I am somehow completely off track.

I use Xalan 2.4.1 and Java 1.3.1_06-b01

Every help is very much appreciated!

Unfortunately, I mistakenly posted __THIS__ already on the xalan developers
list, on which I am even not subscribed, sorry for that.

Best regards,		Werner


Innovation Process Technology Inc.
Werner Koch
Aarbergergasse 56
CH - 3011 Bern

werner.koch@ipt.ch

Phone:  +41 31 311 12 23
Fax:    +41 31 311 12 24
Mobile: +41 78 648 56 44

http://www.ipt.ch

Re: Problem with org.apache.xpath.XPathAPI, org.apache.xpath.NodeSet

[Simon Kitching <si...@ecnetwork.co.nz>]

Hi Werner,

Am I right in believing that your extension method is doing the
following?
(a) taking a node from the source document
(b) trying to delete some of its child nodes
(c) returning the node as the output of the extension method

The XSLT model is one of taking a CONSTANT input tree and generating a
NEW output tree. This is particularly important in Xalan, where a
special high-performance DOM called the DTM is used, which is read-only.
Normally, DOM implementations can't be read-only but because this is
XSLT, and XSLT doesn't modify the input, this optimisation can be
performed.

When writing an extension method returning nodes, you should return NEW
nodes, not nodes from the input document. So rather than trying to
delete nodes, you should walk the tree of source nodes (starting at the
one passed to the extension method), making copies of the ones you want
in the output and ignoring the ones you don't want in the output. Then
return the NEW tree of nodes.

To create a new node, you can use a "dummy" DOM object; the nodes you
return don't need to be associated with any particular DOM. 

Note: I am not a Xalan expert; all the above is subject to correction by
the real experts....

Regards,

Simon

On Tue, 2003-02-18 at 21:20, Werner Koch wrote:
> Hi all,
> 
> I have a problem using the XPathAPI class, and at the moment, I am
> completely stumped (didn't yet consult the sources too much).
> Here is my case:
> - I run transformations with xalan (file based xml/xslt).
> - I have a sorting/restricting problem, so I wrote a small java extension.
> - When I base my sorting of the nodeset I get from the extension call by
> directly navigating the DOM, then I get the expected results (even here with
> a problem when I cut away nodes of org.apache.xpath.NodeSet with
> removeElementAt, because the nodeset in the XSLT will still show the
> original, larger number; I could resolve this by returning a new NodeSet
> where I just copied the wanted Nodes into).
> - Because of generalization issues, I decided to base my sorting on a value
> in my nodeset selected by an XPath, which I also receive from the call in
> the stylesheeet. So I get a NodeSet from the stylesheet, take the Node from
> this I like to handle, and try to XPathAPI.selectSingleNode(aNode, aPath)
> select the wanted information needed for my sort. This unfortunately never
> works for me, because the NodeIterator inside this call to XPathAPI stumbles
> over a NullPointerException when it tries to return the first node by
> executing nextNode(). This holds even true when I execute an XPath query of
> '.' on an Element Node with a at least one TextNode attached.
> 
>   Node result = XPathAPI.selectSingleNode(nodeI, ".").nextNode();
> 
> As I said, the nodeI is an Element node with a Text Node attached (I do
> something like my-node-set.item(i) to set nodeI, where my-node-set is the
> one I got from the stylesheet).
> 
> If I try out other XPaths than '.', I even get 'Unknown XPath errors', so I
> think, I am somehow completely off track.
> 
> I use Xalan 2.4.1 and Java 1.3.1_06-b01
> 
> Every help is very much appreciated!
> 
> Unfortunately, I mistakenly posted __THIS__ already on the xalan developers
> list, on which I am even not subscribed, sorry for that.
> 
> Best regards,		Werner
> 
> 
> Innovation Process Technology Inc.
> Werner Koch
> Aarbergergasse 56
> CH - 3011 Bern
> 
> werner.koch@ipt.ch
> 
> Phone:  +41 31 311 12 23
> Fax:    +41 31 311 12 24
> Mobile: +41 78 648 56 44
> 
> http://www.ipt.ch
> 
>