Using ConcurrentHashMap, when is synchronizing necessary?

Question

I have a ConcurrentHashMap where I do the following:

sequences = new ConcurrentHashMap<Class<?>, AtomicLong>();

if(!sequences.containsKey(table)) {
    synchronized (sequences) {
        if(!sequences.containsKey(table))
            initializeHashMapKeyValue(table);
    }
}

My question is - is it unnecessary to make the extra

if(!sequences.containsKey(table))

Check inside the synschronized block so other threads wont initialize the same hashmap value?

Maybe the check is necessary and I am doing it wrong? It seems a bit silly what I'm doing, but I think it is necessary.

Answer 1

All operations on a ConcurrentHashMap are thread-safe, but thread-safe operations are not composable. You trying to make atomic a pair of operations: checking for something in the map and, in case it's not there, put something there (I assume). So the answer to your questions is yes, you need to check again, and your code looks ok.