How to debug JWS/JNLP 

The easiest way found, that works since 1.5:
<pre>
  set JAVAWS_TRACE_NATIVE=1
  set JAVAWS_VM_ARGS=-Xdebug 
      -Xnoagent -Djava.compiler=NONE
      -Xrunjdwp:transport=dt_socket,
            address=8989,server=y,suspend=n
  javaws http://server:port/descriptor.jnlp
</pre>
http://icoloma.blogspot.com/2005/06/how-to-debug-jwsjnlp.html

And somebody report problem of using it, not sure if sun respond to any of those - http://www.dynamicobjects.com/d2r/archives/002859.html