@SessionScoped
public class ConversationLocks implements Serializable {

  private final ConcurrentHashMap<String, Lock> conversationLocks = new ConcurrentHashMap<>();

  public Lock get(String cid) {
    ReentrantLock newLock = new ReentrantLock();
    Lock alreadyAvailableLock = conversationLocks.putIfAbsent(cid, newLock);
    return alreadyAvailableLock == null ? newLock : alreadyAvailableLock;
  }
}