Java中的同步嵌套集合

Question

I'm trying to create a thread-safe data structure in java, along the lines of the following:

public class A {

    ConcurrentHashMap<String, Set<String>> subscriptions

    private void addSubscription(String server, String client) {
        Set<String> clients = subscriptions.get(server);
        if (clients == null) {
            clients = Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>());
            subscriptions.put(server, agents);
        }
        clients.add(client);
    }

    private synchronized void removeSubscription(String server, String client) {
        Set<String> clients = subscriptions.get(server);
        if (clients != null) {
            clients.remove(client);
            if (clients.isEmpty()) {
                subscriptions.remove(server, agents);
            }
        }
    }
}

However, it appears that I need to add additional synchronization (I'm guessing something to protect accessing the Sets). Is there a better collection to use here, or do I just have to add the appropriate synchronization to this?

Answer 1

What makes you think you need additional synchronization? I don't see why that would be the case. One thing I would change is that addSubscription should check if the set for the given server is absent, and add it atomically. This way you avoid a race condition when two threads are adding a client to the same server:

Set<String> newClients = Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>());
Set<String> clients = subscriptions.putIfAbsent(server, newClients);
if(clients == null) clients = newClients;
clients.add(client);

Also, I would not remove the set from map after it becomes empty. Otherwise there is another race condition: someone might have added a new client just after you checked the size and discovered that it was empty, and you are going to throw it out. Just let the empty set stay there, there isn't much harm in that.