How to debug JWS/JNLP

The easiest way found, that works since 1.5:

  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

The JAVAWSTRACENATIVE flag is optional, and its purpose is solely to show in a splash window the command that is being used to launch your JVM.

After this, you can attach your favourite IDE of choice.