Do you have problems getting either the debugger or JConsole attached or connected to your Web Start client? We worked around this for way too long until we finally found the trick: environment variable JAVAWS_VM_ARGS.
JNLP allows for vm args to be passed to the Java runtime but there is a limited set of arguments that are allowed to be passed on. Anything not in that list of secure properties is ignored. You can find the list of secure properties in the Developers Guide.
As I said ealier, if you want to attach a debugger or JConsole to you Web Start application, then you need to use the JAVAWS_VM_ARGS parameter. For example, if I want to attached to my web start client for debugging purposes then I can do the following:
set JAVAWS_VM_ARGS=-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005
in my environment, start the client and then attach to the process using port 5005. The same goes for attaching to JConsole, set the properties you need in JAVAWS_VM_ARGS, so com.sun.management.jmxremote.* should all be set inside JAVAWS_VM_ARGS.
Hope this help!!!