Java 8 Reentrant Lock and Condition results in IllegalMonitorStateException: current thread is not owner

Question

I have already searched this error in here, but I think that my piece of code looks correct:

1. I gain the lock outside the try..finally
2. I have an unlock in the finally section
3. I only tried to wait on the condition inside the lock.
4. I even print if the current lock is held by this thread and it returns true.

This is an excerpt of the code, if I tried to run the code I get a java.lang.IllegalMonitorStateException: current thread is not owner. The error is in the cond.wait() method.

public void takeARest() {
    lock.lock();
    try {
        while (disembark < totalPassengers) {
            System.err.printf("Held by %s%n",lock.isHeldByCurrentThread());
            cond.wait();
        }
    } catch (InterruptedException e) {
        e.printStackTrace();
    } finally {
        lock.unlock();
    }
}

Any ideas?

Answer 1

For that you want Condition.await().

Object.wait() is a different method that requires to hold the monitor of the object (synchornized(cond){} around the call)

So:

public void takeARest() {
    lock.lock();
    try {
        while (disembark < totalPassengers) {
            System.err.printf("Held by %s%n",lock.isHeldByCurrentThread());
            cond.await();
        }
    } catch (InterruptedException e) {
        e.printStackTrace();
    } finally {
        lock.unlock();
    }
}