Synchronization help in Java

Question

looking at http://download.eclipse.org/jetty/stable-7/xref/com/acme/ChatServlet.html, I don't seem to understand why there needs to be a synchronization block in a synchronized method, like so:

private synchronized void chat(HttpServletRequest request,HttpServletResponse response,String username,String message)
throws IOException
{
    Map<String,Member> room=_rooms.get(request.getPathInfo());
    if (room!=null)
    {
        // Post chat to all members
        for (Member m:room.values())
        {
            synchronized (m)
            {
                m._queue.add(username); // from
                m._queue.add(message);  // chat

                // wakeup member if polling
                if (m._continuation!=null)
                {
                    m._continuation.resume();
                    m._continuation=null;
                }
            }
        }
    }

Why does m need to be synchronized (again?) if the whole method is already thread-safe?

Thank you for any insight.

Answer 1

The synchronized method "chat(...)" synchronizes on it's instance object whereas the synchronized(m) synchronizes on the "m" object - so they are synchronizing on two different objects. Basically it's making sure that some other servlet object isn't messing with the same Member instance at the same time.

Answer 2

When whole method is synchronized the lock is obtained on the this object. But the synchronized block obtains lock only on the member currently being used in iteration.