FULL PRODUCT VERSION :
java version "1.9.0-ea"
Java(TM) SE Runtime Environment (build 1.9.0-ea-b15)
Java HotSpot(TM) Client VM (build 1.9.0-ea-b15, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

EXTRA RELEVANT SYSTEM CONFIGURATION :
I have the following JREs installed as User (copied from Java Runtime Environment Settings dialog in Java Control Panel)

1.9 1.9.0-ea http://java.sun.com/products/autodl/j2se C:\Program Files (x86)\Java\jre1.9.0\bin\javaw.exe true
1.8 1.8.0_20-ea http://java.sun.com/products/autodl/j2se C:\Program Files (x86)\Java\jre1.8.0_20\bin\javaw.exe false
1.8 1.8.0_05 http://java.sun.com/products/autodl/j2se C:\Program Files (x86)\Java\jre8\bin\javaw.exe true
1.7 1.7.0_55 http://java.sun.com/products/autodl/j2se C:\Program Files (x86)\Java\jre7\bin\javaw.exe true
1.7 1.7.0_21 http://java.sun.com/products/autodl/j2se C:\Program Files (x86)\Java\jdk1.7.0_21\jre\bin\javaw.exe true
1.6 1.6.0_45 http://java.sun.com/products/autodl/j2se C:\Program Files (x86)\Java\jre6\bin\javaw.exe true
1.5 1.5.0_06 http://java.sun.com/products/autodl/j2se C:\Program Files (x86)\Java\jre1.5.0_06\bin\javaw.exe true

And as System
1.9 1.9.0-ea http://java.sun.com/products/autodl/j2se C:\Program Files (x86)\Java\jre1.9.0\bin\javaw.exe true

A DESCRIPTION OF THE PROBLEM :
Launching web start applications no longer works with 1.9 EA, either by clicking a link in my browser or by running javaws.exe http://intranet/app.jnlp

REGRESSION. Last worked in version 8u5

ADDITIONAL REGRESSION INFORMATION:
java version "1.9.0-ea"
Java(TM) SE Runtime Environment (build 1.9.0-ea-b15)
Java HotSpot(TM) Client VM (build 1.9.0-ea-b15, mixed mode, sharing)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Create a small web start app
2. Install Java 9 EA
3. Try to run your app

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The web start process should launch after a short delay.
ACTUAL -
The cursor turns busy, then back to normal. No splash screen is shown. A Java process with "com.sun.deploy.panel.JreLocator" as its main class is launched but takes very little memory and 0% CPU. When killed, the web start process launches normally.

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Track down the errant process and kill it manually