FULL PRODUCT VERSION :
java version "9"
Java(TM) SE Runtime Environment (build 9+175)
Java HotSpot(TM) Server VM (build 9+175, mixed mode, emulated-client, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

A DESCRIPTION OF THE PROBLEM :
The Jigsaw JDK modularity project introduces a flag --illegal-access=debug which is provided to debug runtime reflection issues when running with JDK 9. This flag is not functional under Java Web Start.

Possibly a duplicate of JDK-8182697

REGRESSION. Last worked in version 9

ADDITIONAL REGRESSION INFORMATION:
java version "9"
Java(TM) SE Runtime Environment (build 9+175)
Java HotSpot(TM) Server VM (build 9+175, mixed mode, emulated-client, sharing)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Compile the given source code
2. Create a JAR file with the resulting class file, and the given manifest file
3. Deploy to a web server, with the given JNLP file
4. On the client, add the server to the Exception Site List (Java Control Panel->Web Settings->Exception Site List)
5. Add the parameter "--illegal-access=debug" to the Runtime Parameters (Java Control Panel->Desktop Settings->JDK 9->Runtime Parameters"
6. Turn on tracing, logging and the console (Java Control Panel->Advanced->Enable tracing/Enable logging/Show console)
7. Run the Web Start app on the client

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The application should run with the following output in the console

Testing stderr
WARNING: Illegal reflective access by OpenModuleTest (jar://something.jar) to field java.util.TreeMap.comparator
        at OpenModuleTest.main(OpenModuleTest.java:10)
OK
ACTUAL -
The application runs, without debug text

Testing stderr
OK

REPRODUCIBILITY :
This bug can be reproduced always.