Java Distributed Computing

A Shared Whiteboard

Now we’ll look at a collaborative application that’s not so simple—a shared whiteboard. There are more things to worry about in this application because, while we still need to keep track of agent identities and communications, the data shared between agents is a bit more complicated and can lead to performance problems pretty quickly.

First let’s build a simple shared whiteboard system based on our RMI collaborative classes. Like the chat example, our first version will use the standard mediator class, RMIMediatorImpl , because this version will only need the mediator to do the default routing of messages to the other agents. This initial whiteboard example will also need a new subclass of the RMICollaborator that acts as a whiteboard user. Actions that the user performs in the local whiteboard are broadcast to all other users so that their displays are updated properly.

Example 10.2 shows a WhiteboardUser class that subclasses RMICollaboratorImpl. It has a single constructor with four arguments: a name that the whiteboard user goes by, a color that is used when the user draws on the whiteboard, a host name for the mediator, and the mediator’s name. The user name, host, and mediator name are passed to the RMICollaboratorImpl constructor to establish the connection to the mediator, and to initialize the collaborator’s Identity. Once this is done, the color is added to the whiteboard user’s Identity. We include the user’s color in the Identity so other users ...