Is it safe to call unlock() on a lock which might have already been unlocked?

Question

In this example:

private ReentrantReadWriteLock mLock = new ReentrantReadWriteLock();

public void method(boolean condition) {
    try {
        mLock.writelock.lock()

        if (condition) {
            mLock.writelock.unlock();
        }
    } finally {
        mLock.writelock.unlock();
    }
}

What happens when mLock.writelock.unlock() is called in the finally block if the unlock in the if statement has been executed?

Is this code safe or do I need to perform some check on whether there is a lock in place before attempting to unlock?

[What does the javadoc say?](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/ReentrantReadWriteLock.WriteLock.html#unlock--) — Sotirios Delimanolis, Sep 16 '15 at 14:52
You spent more time in forming this question than it wouldve taken to try it yourself. — John Vint, Sep 16 '15 at 14:53

Subhrajyoti Majumder · Answer 1 · 2015-09-16T15:01:42.207

It seems very straight forward as per documentation, and it should throw IllegalMonitorStateException while re-executing ReentrantReadWriteLock.WriteLock.html#unlock.

Attempts to release this lock. If the current thread is the holder of this lock then the hold count is decremented. If the hold count is now zero then the lock is released. If the current thread is not the holder of this lock then IllegalMonitorStateException is thrown.

Documentation Link

Enforce condition on finally unlock execution also which would escape from exception.

Is it safe to call unlock() on a lock which might have already been unlocked?

1 Answers1