This thread provides some useful info: http://www.nabble.com/JavaXPCOM-and-Handle-tf1318510.html

It has a reference to a web site with an example implementation:
http://nithril.free.fr/Xul/

The I uncompressed and recompressed the sample.rar so it is xul-sample.zip Given that the code is all taken originally from the eclipse atf project it seems valid to do so.
xul-sample.zip

xul-sample.zip (application/x-zip-compressed)