Unlock on ReentrantLock without IllegalMonitorStateException

Question

Hi,

I have a piece of code (simplified):

if(reentrantLockObject.isLocked()) {
       reentrantLockObject.unlock();
}

where reentrantLockObject is java.util.concurrent.locks.ReentrantLock. Sometimes I get IllegalMonitorStateException. It seams that lock was released between check and unlock() call. How can I prevent this exception?

Answer 1

isLocked returns whether any thread holds the lock. I think you want isHeldByCurrentThread:

if (reentrantLockObject.isHeldByCurrentThread()) {
    reentrantLockObject.unlock();
}

Having said that, isHeldByCurrentThread is documented to be mainly for diagnostic purposes - it would be unusual for this piece of code to be the right approach. Can you explain why you think you need it?

Answer 2

You need to own the lock to be able to unlock it. reentrantLockObject.isLocked() only is true if some thread owns the lock, not necessarily you.

  reentrantLockObject.lock();
  try{

       // do stuff
  }finally{
         reentrantLockObject.unlock();
  }

Here the thread owns the lock so they are able to unlock it.