Interesting thread bug

Question

Hi,
I'm developing a client/server whiteboard system for my term project. I've hit to an interesting threading bug. The very brief description of the goal is like this: A client has a method (getActiveUsers) to get the list of active(logged in) users from server. In order to accomplish this, it sends a command to server to receive the user list. In an undeterministic time frame, the server sends a message as a reply, which includes the user list. When the client gets the user list, it is expected to update its local user list. The code for these goes like this:
// The method that returns the list of active users
// in the system
public Vector getActiveUsers()
synchronized(activeUsers)
channel.send(new ListActiveUsers(username));
try
activeUsers.wait();
catch (InterruptedException e) {}
return activeUsers;
// The method that is fired when a new message
// is received (Observer pattern)
public void update(Message msg){
if (msg instanceof ActiveUsersList)
synchronized(activeUsers)
activeUsers=((ActiveUsersList)msg).activeUserList;
activeUsers.notifyAll();
return;
inform(msg);
the Error message is this:
java.lang.IllegalMonitorStateException: current thread not owner
     at java.lang.Object.notifyAll(Native Method)
     at domain.Whiteboard.update(Whiteboard.java:49)
     at services.messaging.Channel.inform(Channel.java:95)
How can a thread be not owner of an object when it enters a synchronized block for that object?
Thanks for any help.
Regards...

Answer 1

You synchronized on one object, referred to by your variable activeUsers, but then you reassigned activeUsers to refer to a different object and called notifyAll on that object. Remind yourself of the difference between variables and objects.