Monitor Mechanics
Doug Lea - 2nd Edition

• Implementing guarded methods
• for each condition that needs to be waited on, write a guarded wait loop that causes the current thread to block if the guard condition is false
• ensure that every method causing state changes that affect the truth value of any waited-for condition notifies threads waiting for state changes, causing them to wake up and recheck their guard conditions
• each object has a lock and also has a "wait set" (which is maintained internally by the JVM) that is manipulated by wait, notify, notifyAll, and Thread.interrupt.
• wait actions:
• if the current thread has been interrupted, then the method exits immediately, throwing an InterruptedException. otherwise the current thread is blocked.
• the JVM places the thread in the internal wait set
• the synchronization lock is released, but all other locks are retained by the thread.
• notify actions:
• an arbitrary thread is removed (by the JVM) from the internal wait set
• the chosen thread must re-obtain the synchronization lock for the target object, which will always cause it to block at least until the thread calling notify releases the lock.
• the chosen thread is then resumed from the point of its wait
• notifyAll actions
• removes all threads from the wait set (simultaneously), but each thread still has to acquire the lock
• Interrupt actions
• if Thread.interrupt is invoked for a thread suspended on a wait, the same notify mechanics apply, except that after re-acquiring the lock, the method throws an InterruptedException and the thread's interruption status is set to false.
• Timed Waits
• when wait takes an argument of how much time can elapse before the wait is aborted, there is no status indication differentiating waits that return via notifications vs. time-outs.
• a time wait may resume at an arbitrary amount of time after the time-out has expired
• Thread.sleep(long msecs) uses a timed wait but does not tie up the current object's synchronization lock
• Interrupted Waits
• a wait will be broken if the waiting thread has been interrupted.
• enables blocked threads to respond to thread cancellation
• most common response is to rethrow interruptedException to denote failure to the client