Why ReentrantLock is not working while synchronized work in the demo?

Question

I am trying to follow the ReentrantLock Example in Java, Difference between synchronized vs ReentrantLock kind of tutorial. And I have a demo started with -ea on as

public class ReentrantLockZero {
    private static ReentrantLock CountLock = new ReentrantLock();
    private static int count = 0;
    private static final int RESULT_COUNT = 10_000;

    public static void main(String... args) throws Exception {
        ThreadPoolExecutor threadPoolExecutor = getMyCachedThreadPool();
        for (int i = 0; i < RESULT_COUNT; ++i) {
            threadPoolExecutor.submit(ReentrantLockZero::getCount);
            threadPoolExecutor.submit(ReentrantLockZero::getCountUsingLock);
        }
        threadPoolExecutor.shutdown();
        threadPoolExecutor.awaitTermination(10, TimeUnit.SECONDS);
        assert count == RESULT_COUNT * 2;
    }

    private static synchronized int getCount() {
        count++;
        System.out.println(Thread.currentThread().getName() + " counting in synchronized: " + count);
        return count;
    }

    private static int getCountUsingLock() {
        CountLock.lock();
        try {
            count++;
            System.out.println(Thread.currentThread().getName() + " counting in lock: " + count);
            return count;
        } finally {
            CountLock.unlock();
        }
    }
}

When using ReentrantLock as the second method getCountUsingLock, I will get java.lang.AssertionError but when I commented them out to use synchronized, it would be okay.

Considering its ReentrantLock, I removed the CountLock defined in the class and using local lock as following, but it still not work.

private static int getCountUsingLock() {
    ReentrantLock countLock = new ReentrantLock();
    countLock.lock();
    try {
        count++;
        System.out.println(Thread.currentThread().getName() + " counting in lock: " + count);
        return count;
    } finally {
        countLock.unlock();
    }
}

What's the missed point here?

Any help will be appreciated ;)

score 2 · Accepted Answer · edited Mar 30 '19 at 12:42

Kind of fool of myself.

It's working like that because I was actually locking on different objects.

private static synchronized int getCount()

is equal to

private static synchronized (ReentrantLockZero.class) int getCount()

while new ReentrantLock(); is always a new object and there is no way to eliminate the race condition using different locks.

So fool of me, it's easily fixed by the following demos

public class ReentrantLockZero {
    private static ReentrantLock CountLock = new ReentrantLock();
    private static int synchronisedCount = 0;
    private static int lockedCount = 0;
    private static final int RESULT_COUNT = 10_000;

    public static void main(String... args) throws Exception {
        ThreadPoolExecutor threadPoolExecutor = getMyCachedThreadPool();
        for (int i = 0; i < RESULT_COUNT; ++i) {
            threadPoolExecutor.submit(ReentrantLockZero::getSynchronisedCount);
            threadPoolExecutor.submit(ReentrantLockZero::getCountUsingLock);
        }
        threadPoolExecutor.shutdown();
        threadPoolExecutor.awaitTermination(10, TimeUnit.SECONDS);
        assert synchronisedCount == RESULT_COUNT;
        assert lockedCount == RESULT_COUNT;
    }

    private static synchronized int getSynchronisedCount() {
        synchronisedCount++;
        System.out.println(Thread.currentThread().getName() + " counting in synchronized: " + synchronisedCount);
        return synchronisedCount;
    }

    private static int getCountUsingLock() {
        CountLock.lock();
        try {
            lockedCount++;
            System.out.println(Thread.currentThread().getName() + " counting in lock: " + lockedCount);
            return lockedCount;
        } finally {
            CountLock.unlock();
        }
    }
}

Why the synchronized work? Because then there is only one lock the two methods are locking on so the race condition resolved directly.

_{Kind of easily fooled by the tutorial; shame on me ;(}

"Kind of fool of myself" as we all do. It is called programming (0: — c0der, Mar 30 '19 at 05:54

Why ReentrantLock is not working while synchronized work in the demo?

1 Answers1