How to acquire a lock by a key

Question

What is the best way to prevent concurrent update of one record in a key-value set without locking the entire set? Semantically, I\'m looking for some kind of locking by a k

Answer 1

private static final Set lockedKeys = new HashSet<>();

private void lock(String key) throws InterruptedException {
    synchronized (lockedKeys) {
        while (!lockedKeys.add(key)) {
            lockedKeys.wait();
        }
    }
}

private void unlock(String key) {
    synchronized (lockedKeys) {
        lockedKeys.remove(key);
        lockedKeys.notifyAll();
    }
}

public void doSynchronously(String key) throws InterruptedException {
    try {
        lock(key);

        //Do what you need with your key.
        //For different keys this part is executed in parallel.
        //For equal keys this part is executed synchronously.

    } finally {
        unlock(key);
    }
}

try-finally - is very important - you must guarantee to unlock waiting threads after your operation even if your operation threw exception.