如何锁定单个ConcurrentHashMap对象以进行修改而不会阻塞整个地图？

Question

Is there an easy way to lock on ConcurrentHashMap object during its modification without blocking whole map? As far as I know here is simple way in Java 8 - method competeIfPresent , but unfortunately I've got 1.7 java on project.

private Map<Long, AtomicInteger> idForCountMap;

public void decrement(Long id) {
    AtomicInteger count = idForCountMap.get(id);
    if (count != null){
    count.decrementAndGet();
    // do some additional operations
    }
}

Here is an example. How can I perform decrement opeperation as atomic one WITHOUT blocking whole map?

ps suppose it's quite common situation, but I could not find fine explained answer.

Answer 1

I think it will workout if you can bring a shared key for the mapped value, and acquire a lock on the key for the complete operation.

public void decrement(SharedKey id) {
synchronized(id){
     AtomicInteger count = idForCountMap.get(id);
     if (count != null){
        count.decrementAndGet();
        // do some additional operations
     }
}
}