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

This is how; i did it. And yes I agree if two different strings shares the same hashcode will end up with acquiring the same lock.

class LockByKey {
    ObjectForString objHolder = new ObjectForString(100);
    public void lockThenWorkForKey (String key) {
        synchronized(objHolder.valueOf(key)){
            //DoSomeWork
        }
    }
}

public final class ObjectForString {

    private final Object[] cache;
    private final int cacheSize;
    final int mask;

    public ObjectForString(int size) {
        // Find power-of-two sizes best matching arguments
        int ssize = 1;
        while (ssize < size) {
            ssize <<= 1;
        }

        mask = ssize - 1;
        cache = new Object[ssize];
        cacheSize = ssize;
        //build the Cache
        for (int i = 0; i < cacheSize; i++) {
            this.cache[i] = new Object();
        }
    }

    public Object valueOf(String key) {
        int index = key.hashCode();
        return cache[index & mask];
    }
}