Question

I have following code for a chat server application in Java –

public synchronized List<ChatMessage> getMessages(int messageNumber) {
    return messages.subList(messageNumber + 1, messages.size());
}

public synchronized int addMessage(ChatMessage c) {
    messages.add(c);
    return messages.size()-1;
}

I have following test code –

public static void main(String[] args) {
    final ChatRoom c = new ChatRoom();
    Thread user1 = new Thread(new Runnable() {
        public void run() {
            for(int i=0;i<1000;i++) {
                c.addMessage(new ChatMessage());
                c.getMessages(0);
            }
        }
    });
    Thread user2 = new Thread(new Runnable() {
        public void run() {
            for(int i=0;i<1000;i++) {
                c.addMessage(new ChatMessage());
                c.getMessages(0).size();
            }
        }
    });
    user1.start();
    user2.start();
}

I am getting a ConcurrentModificationException.

How is this possible?

Answer 1

How is this possible?

Your getMessages method just returns a view on the original list. It doesn’t create a copy of the list. So one thread is using a view on the list while another modifies the list – at that point, you get the exception.

From the docs for List.subList:

The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is structurally modified in any way other than via the returned list. (Structural modifications are those that change the size of this list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.)

It’s not clear what you’re really trying to achieve here, but fundamentally you can’t use subList to magically create a thread-safe list 🙂