Extension functions with a default/no-arg constructor can be loaded from the XML configuration file using this syntax:

<probe:addIn xmlns:probe='http://xmlprobe.com/200312'>
<probe:name>com.xmlprobe.QAHandler</probe:name>
<probe:feature>
<probe:name>http://xmlprobe.com/features/xpath-extension-function</probe:name>
<probe:value>[classname of function to be loaded]</probe:value>
<probe:alias>[name of XPath function]</probe:alias>
</probe:feature>
<!--...more features here...-->
</probe:config>
</probe:addIn>

The element <probe:alias> is not required. If omitted, the classname specified in <probe:value> is used as the registered name of the extension function.

Extension functions whose constructor takes arguments must be loaded from a subclass of com.xmlprobe.QAHandler. To do this, create an instance of the function and register it with the current org.jaxen.FunctionContext, accessible via QAHandler#functionContext(), e.g.:

MyExtensionFunction f = new MyExtensionFunction( arg1, arg2 ); //constructor
takes args
functionContext().registerFunction( namespaceURI, "my-ext-func", f );