Deadlocks resulting from threads being created in wrong group

Name: gm110360 Date: 04/08/2004


FULL PRODUCT VERSION :
Java(TM) Plug-in: Version 1.4.2_03
Using JRE version 1.4.2_03 Java HotSpot(TM) Client VM

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
The Plug-in creates a thread group for an applet codebase, including
an event dispatch thread, plus a thread for each applet to run the
lifecycle methods. So when one of those methods invokes
EventQueue.invokeAndWait(), the event goes on the applet's queue
and is serviced by the applet dispatch thread. It is therefore
single-threaded with respect to GUI events in the applet.

But when a thread is created to service an incoming event, e.g. from
a script function, it is created in the main thread group. Events therefore
go on the base event queue, which breaks the single thread model
and conflict with GUI events.

Such threads should surely be created in the applet thread group.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
The problem involves an applet which executes visible effects from
both Swing GUI events and script events.

It is always reproducible in the sense that it is structurally wrong.
A threaded debugger shows which event queue is used in each
case.

Consequent problems (usually deadlocks) are intrinsically hard
to reproduce. However, in our complex web application, sustained use
will generally yield a problem within a few minutes.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
All events submitted by EventQueue.invokeAndWait() should be
dispatched by the applet group's dispatch thread.
ACTUAL -
GUI-originated events go on the applet group's dispatch thread, but
script-originated events go on the main group's dispatch thread.
Deadlocks between the two threads can be seen on a debugger.

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
As a workaround, we are using a worker thread in the applet thread
group, to find the correct dispatch queue. Events are transferred
to this thread by a mechanism similar to that of
EventQueue.invokeAndWait(); this thread then invokes that method.
(Incident Review ID: 244954)
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