There is an inherent race condition between the DefaultKeyboardFocusManager and
the new architecture which fixed BugID 4051487. The DefaultKeyboardFocusManager
can in some situations attempt to send a message to an AppContext which has
already been disposed. Because the fix for 4051487 shuts down
EventDispatchThreads when they are not being used, the code will try to start
up a new EventDispatchThread in response to the sent message. However, since
the new Thread's AppContext's ThreadGroup has been destroyed, this operation
will fail. ThreadGroup.add explicitly checks to see if the ThreadGroup has
been destroyed, and throws an exception in that case.

To reproduce this problem, run any applet in appletviewer. The exception will
be thrown when closing appletviewer. I reproduced this bug while running one of
the Animator applets under a heavy processor load on Solaris. I also had several
applets running simulataneously under the same appletviewer process. However, I
suspect this bug could be reproduced in any circumstance on any platform.

The exception I saw looked like this:

Exception occurred during event dispatching:
java.lang.IllegalThreadStateException
        at java.lang.ThreadGroup.add(ThreadGroup.java:801)
        at java.lang.Thread.init(Thread.java:279)
        at java.lang.Thread.<init>(Thread.java:390)
        at java.awt.EventDispatchThread.<init>(EventDispatchThread.java:49)
        at java.awt.EventQueue$1.run(EventQueue.java:564)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.awt.EventQueue.initDispatchThread(EventQueue.java:561)
        at java.awt.EventQueue.wakeup(EventQueue.java:711)
        at sun.awt.SunToolkit.wakeupEventQueue(Native Method)
        at sun.awt.PostEventQueue.postEvent(SunToolkit.java:594)
        at sun.awt.SunToolkit.postEvent(SunToolkit.java:230)
        at java.awt.DefaultKeyboardFocusManager.sendMessage(DefaultKeyboardFocusManager.java:167)
        at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:508)
        at java.awt.Component.dispatchEventImpl(Component.java:2783)
        at java.awt.Container.dispatchEventImpl(Container.java:1309)
        at java.awt.Window.dispatchEventImpl(Window.java:1228)
        at java.awt.Component.dispatchEvent(Component.java:2764)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:423)
        at java.awt.SentEvent.dispatch(SentEvent.java:52)
        at java.awt.DefaultKeyboardFocusManager$DefaultKeyboardFocusManagerSentEvent.dispatch(DefaultKeyboardFocusManager.java:143)
        at java.awt.DefaultKeyboardFocusManager.sendMessage(DefaultKeyboardFocusManager.java:165)
        at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:236)
        at java.awt.Component.dispatchEventImpl(Component.java:2783)
        at java.awt.Container.dispatchEventImpl(Container.java:1309)
        at java.awt.Window.dispatchEventImpl(Window.java:1228)
        at java.awt.Component.dispatchEvent(Component.java:2764)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:423)
        at java.awt.SequencedEvent.dispatch(SequencedEvent.java:96)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:421)
        at java.awt.EventDispatchThread.pumpOneEvent(EventDispatchThread.java:136)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:122)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:118)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:98)