How can I debug applications under Java Web Start (JNLP)?

As for newer versions of Java (Java 8u20+ and Java 7u70+) i experienced that parameters like -Xrunjdwp cannot be passed directly nor using JAVAWS_VM_ARGS. Message Rejecting attempt to specify insecure property: -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000 started to show up in console output.

The only solution that worked for me was to pass those arguments into JAVA_TOOL_OPTIONS system variable.

It's quite the same like with any other Java process you want to debug remotely: You have to set up some arguments for the VM (-Xdebug -Xnoagent -Xrunjdwp:transport=dt_socket,server=n,suspend=y,address=12345 ) and then connect to the given port. In Java webstart 6.0 this can be done with the -J option, in earlier version via environment variable JAVAWS_VM_ARGS. See details here.

Start the JWS VM manually. This way you can provide the startup parameters to open the debug port. Here is a description, it goes like this:

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