Name: rmT116609 Date: 04/26/2004


FULL PRODUCT VERSION :
java version "1.5.0-beta"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0-beta-b32c)
Java HotSpot(TM) Client VM (build 1.5.0-beta-b32c, mixed mode)

also

java version "1.4.2_04"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_04-b05)
Java HotSpot(TM) Client VM (build 1.4.2_04-b05, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows 2000 [Version 5.00.2195]

A DESCRIPTION OF THE PROBLEM :
Calling setPriority() from another thread on a Thread object of a thread that has already exited results in a NullPointerException because the threads ThreadGroup is now null.

Thread.exit() sets the 'group' member to null, Thread.setPriority() dereferences the 'group' member without synchronizing or checking if 'group' is now null.

There is not an easy way to guard against this since exit() is private and setPriority() is final. Checking isAlive() before calling setPriority() does not work since the thread may exit during the call to setPriority().

The docs do not say it is illegal to call setPriority() on a finished Thread and I think the call should be a legal noop.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile and run the attached ThreadPriority class.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Expect nothing to happen, setPriority() should return normally and do nothing since the thread has exited.
ACTUAL -
NullPointerException thrown by setPriority

ERROR MESSAGES/STACK TRACES THAT OCCUR :
For JDK 1.5beta1:
Exception in thread "main" java.lang.NullPointerException
        at java.lang.Thread.setPriority(Thread.java:920)
        at ThreadPriority.main(ThreadPriority.java:6)

For JDK 1.4.2_04:
Exception in thread "main" java.lang.NullPointerException
        at java.lang.Thread.setPriority(Thread.java:875)
        at ThreadPriority.main(ThreadPriority.java:6)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public class ThreadPriority {
public static void main(String args[]) throws InterruptedException {
Thread t = new Thread();
t.start();
t.join();
t.setPriority(Thread.MIN_PRIORITY);
}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Catch and ignore the NullPointerException whenever calling setPriority
(Incident Review ID: 255671)
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