Unlock on ReentrantLock without IllegalMonitorStateException

Question

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?

Solution 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?

Solution 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.

Solution 3

ReentrantLock throws this exception according to this logic:

if (Thread.currentThread() != getExclusiveOwnerThread()) {
  throw new IllegalMonitorStateException();
}

So the solution is to check if the same thread is unlocking:

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

Comments

[Brett]

But you probably wouldn't want to.

[john16384]

Even though the docs may state that it is typically used for debugging and testing (although it also states it can be used to ensure the lock is used in a non-reentrant manner), its function is still well documented, and as such you should be able to rely on it. I think it is definitely a cleaner solution for cases where you need to unlock halfway a block of code (that calculated half a dozen variables declared inside it), but still want the safety of a finally block if an exception occured before the unlock was reached.