Saxon

I've added some tracing and I'm seeing

Loader.getClass() org.jdom2.Document
XPath exception created: net.sf.saxon.trans.XPathException.<init> from net.sf.saxon.trans.DynamicLoader.getClass from net.sf.saxon.Configuration.getClass
Loader.getClass() org.dom4j.Document
XPath exception created: net.sf.saxon.trans.XPathException.<init> from net.sf.saxon.trans.DynamicLoader.getClass from net.sf.saxon.Configuration.getClass

The code is trying to determine whether JDOM2 and DOM4J are on the classpath by loading relevant classes. I don't think there is any way to do this without throwing and catching an exception. However, wrapping that exception in an XPathException seems unnecessary. And do we really need to know in advance whether JDOM2 and DOM4J are available? Could we arrange things instead so the failure only occurs if they are actually used?

I'm puzzled that the trace doesn't also show calls for the Axiom and XOM object models - I see those calls when running in the debugger.

Should add org.w3.dom.Document and com.saxonica.ee.domino.DominoNode to knownClasses to avoid the dynamic load.

The mechanism for registering object models by seeing what can be loaded is used for two purposes:

(a) for the JAXP XPath API, to decide whether newInstance() should work for a given object model URI. (In practice, this mechanism is probably used only by Saxon - most JAXP implementations probably work only with DOM).

(b) in various APIs (e.g when handling the result of a reflexive extension function call) we're presented with an object of a given class, and we need to know which object model (if any) can handle objects of this class. We will obviously never be presented with a class that isn't on the class path. So perhaps we should build in the mapping from class names (or package names) in known object models to the Saxon support module and only do the dynamic loading when the need arises? We still need the registration mechanism to support third-party plugins for object models that Saxon doesn't support natively, but we don't need to use this for the built-in models such as DOM4J and JDOM2.

(But is it worth the trouble...?)