unlocking a lock.tryLock with timeout

Question

I have this code

Lock lock = new ReentrantLock();

void someMethod() {
    try {
        if (lock.tryLock(120, TimeUnit.SECONDS)) {
            // do stuff
        } else {
            // timed out 
            throw new RuntimeException("timeout");
        }
    } finally {
        lock.unlock();
    }
}

This works fine except when when it times out. Since the thread that timesout does not own the lock, IllegalMonitorStateException is thrown. There is no isHeldByCurrentThread in Lock interface. If I dont want to cast to ReentrantLock, I have to use something ugly

...
} finally {
    if (lock.tryLock()) {
        lock.unlock();
        lock.unlock(); // twice because tryLock is called twice
    }
}

or

...
} finally {
    if ( !timeout ) {
        lock.unlock();
    }
}

Any better options ? Thanks

Answer 1

Only unlock if you acquired the lock:

if (lock.tryLock(120, TimeUnit.SECONDS)) {
  try {
    // Do stuff.
  } finally {
    lock.unlock();
  }
}

Note that:

• There is an example like this in the Javadoc;
• This idiom doesn't have anything to do with using the timeout: you would not want to unlock the lock of a call to lock() were interrupted. You should always acquire the lock outside, but immediately before, the try.