Saxon, custom extension element, XTDE:unknown extension instruction - xslt

Question

When I tried to use a custom extension element with Saxon, I got an error saying XTDE:unknown extension instruction in my XSL file. I asked this on Saxon mailing list, but haven't yet received a response, so I decided to ask here. In order to be helpful, here is the whole content from the mailing list:
from: sky
I just start using Saxon. After go through some documentations, I still found it hard to write my own custom extension instruction. I have read "writing XSLT extension instructions", and the example provided in the package net.sf.saxon.option.sql. But I'm still a little confusing: the document says,
A subclass of SimpleExpression should implement the methods getImplementationMethod() and getExpressionType(), and depending on the value returned by getImplementationMethod(), should implement one of the methods evaluateItem(), iterate(), or process().
However, there is only call() method implemented in the sql example. I'm new to XML/XSLT, hence find it hard to understand how to write my own extension elements.
Is there a tutorial of some kind which explains writing extension elements in more detail?(I have Google but found no luck, the best I can find is with older Saxon version that has different implementation). Or maybe I should go through some other XML/XSLT intermediate first?
Thanks in advance
from Michael Kay
You're right, implementing extension instructions is not easy. That's partly because the APIs are quite complex, partly because the documentation is poor, and partly because the code that would help you understand it is not open source. The underlying reason for this is that not many people have attempted to do this, so there has been little feedback that would lead to improvement over the years.
I would encourage you to ask yourself seriously whether this is something you really want to do badly enough to cope with the difficulties.
The documentation extract you cite appears to be out of date. The "Callable" interface was a relatively recent addition, and the documentation has not caught up. Implementing the call() method is enough.
Michael Kay
Saxonica
from sky
Thanks for reply.
I'm replacing Xalan with Saxon, so there are extension instructions written for Xalan that need to be rewritten. I think it would be better if I rewrite Xalan extension elements into Saxon extension functions, however, I want to give extension element a try before making the choice. Right now I have a problem with extension element. I tried to write a simple extension element, but it failed to run with "XTDE 1450: Unknown extension instruction". Here is what my code looks like:
//Config.java
ProfessionalConfiguration config = new ProfessionalConfiguration();
config.setExtensionElementNamespace("degx", "DegElementFactory");
//DegElementFactory.java
if(localname.equals("value-of")) return DegxValueOf.class;
//version.xsl
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:degx="http://DegElementFactory" extension-element-prefixes="degx">
...
<degx:value-of select="alpha 1"/>
...
</xsl:stylesheet>
I run Config.class first, and then running sf.net.saxon.Transform, I got the error above. My guess is I configured extension element namespace the wrong way. I have Saxon PE with evaluate license install correctly, because I got message about expired days after running Transform.
In summary, I have Config.class, DegElementFactory.class, DegxValueOf.class, all in my working directory, and I have add it to class path through -cp argument. Please help :)

Answer 1

Because I'm using net.sf.saxon.Transform from command line, the configuration class object cease to exist after the execution of Config.class. At first I thought config.setExtensionElementNamespace() would write to a configuration file somewhere. But it turns out wrong. So there are two ways to set extension element namespace:
from command line, supply -config:file argument. e.g.
net.sf.saxon.Transform -config:config.xml -s:source.xml -xsl:transform.xsl -o:result.out
invoking XSLT from application, instance a configuration class and execute setExtensionElementNamespace() method.
#Martin Honnen also pointed out another problem in comment, thanks!