Mutex that allows only one thread to proceed
other threads must wait until the ONE finishes.
The thread that "holds" the mutex (has the mutex access granted)
may reenter the mutex any number of times
(just increasing a "depth" of the locking).
If the priority thread enters waiting the mutex
then it will get serviced first once the current thread
leaves the mutex.
This method is intended to be called by the non-priority thread
that acquired the mutex to check whether there
is no priority thread (such as AWT event-notification thread)
If there is a priority thread waiting the non-priority thread
should attempt to stop its work as soon as possible and unlock
The method must *not* be called without first locking the hierarchy
(it is intentionally not synchronized).
Return the thread that holds a lock on this mutex.
This method is intended for diagnostic purposes only to determine
an intruder thread that entered the view hierarchy without obtaining
the lock first.
thread that currently holds a lock on the hierarchy or null
if there is currently no thread holding a lock on the hierarchy.