Unlocking lock owned by another thread java

java locks reentrantlock
Flame_Phoenix picture Flame_Phoenix · May 10, 2013 · Viewed 13.9k times · Source

I have a LockManager that manages the locks of several threads. Sometimes the threads are bad boys, and I have to kill them and ask the LockManager to release all their locks. However, since I use ReentrantLock in java this is impossible, I can not unlock a lock owned by another thread.

I am forced to use Locks (cannot use semaphores, it is point of the homework). Is there any Java Lock implementation that allows me to unlock locks owned by other threads?

So far the options I considered are:

  • re-implementing ReentrantLock in a way that allows me to do this
  • Make some sort of mapping between Semaphores and ReentrantLocks

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Answer

Dale Wilson picture Dale Wilson · May 10, 2013

You've discovered a major reason why common wisdom says: Don't kill threads!

Locks are only one of the potential resource leaks that can happen if you forcibly kill a thread. Consider open files and sockets, etc.

Also consider that if you did manage to unlock the lock, there was a reason the lock was locked in the first place. For example, the thread may have partially updated a data structure, and allowing access to that structure from another thread is likely to cause strange and wondrous program failures that are difficult if not impossible to debug.

The best way to handle this situation is to ask the thread to go away. Add a "stop()" method to the object associated with the thread (you do have an object for each thread, don't you?) that sets a flag, and have the thread check this flag regularly and exit if it is set.

If your threads are misbehaving in a way that prevents them from checking the stop flag, then the correct approach is to fix the code so that it does not misbehave.

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