The ability to pass objects among hosts makes it extremely easy to program applications in which some classes are both clients and servers. For example, a method of a remote class R1 can be called with a parameter being a remote class R2. The remote class R1 will then be able to call any of the methods of the remote class R2.

A Conferencing Program
In this section I'll walk you through the complete code of a small conferencing program and show how it takes advantage of the facilities provided by RMI to achieve direct applet-to-applet communication. The program consists of two basic remote objects (implementing the "java.rmi.Remote" interface):

To explain briefly, each applet registers its reference in the server, which broadcasts these references back to all other applets. All applets now have a reference to all other applets, which enables them to communicate directly without having to access the server. (To prove this, the system allows an applet to shut down the server.) Each applet displays the list of other available applets, allows the user of the system to send messages to other users and displays incoming messages.

Defining the Remote Interfaces
The first step in writing an applet or an application using RMI is to define the remote interfaces to the remote objects in the application. A remote interface extends the interface java.rmi.Remote and declares all the methods that may be invoked remotely. The two remote interfaces of the conferencing program are the Talker and TalkerServer interfaces (see Listings 1 and 2). The Talker interface is used by the applet to receive notifications from the server and messages from other applets while the TalkerServer is used by the server to receive commands from the applet. The Talker interface is implemented by the TalkerImpl class and the TalkerServer interface is implemented by the TalkerServerImpl class.

Creating the Remote Server
The next step is to define the TalkerServerImpl class that extends java.rmi.UnicastRemoteObject. UnicastRemoteObject provides support for point-to-point object references using TCP streams. The TalkerServerImpl class (see Listing 3) has a constructor, a main method and an implementation for the methods declared in the TalkerServer interface. The main method starts the remote server process, installs a security manager, creates a registry on port 2006, creates a TalkerServer remote object and finally makes this object available via the registry. The implementation of all the methods is trivial. Register adds a client to a private vector and notifies every registered client that a new member has joined the group. UnRegister notifies every client that one of them has left the session and removes it from the vector. Lookup returns a copy of the vector, and shutdown terminates the server. Although these methods are remote, they differ from local methods only in throwing RemoteException.

Implementing the Talker Interface
TalkerImpl (see Listing 4) implements the Talker interface. The constructor saves references of the applet and the current user's name. The applet is used to update the user interface; the name is used by other users of the system. Remote users call the getName method to identify the current user and call the recvMsg method to send him messages. The add and remove methods are called by the server to update the system's list of users.

Creating the User Interface
The user interface (see Listing 5) consists of the following components displayed in an applet (Figure 1):

Although the code of the applet is longer than the code of the other classes, it is fairly straightforward. The init method initializes a connection with the server and contains the code necessary to lay down the components within the applet. An ActionListener is added to each button. StartListener initializes the list of persons already connected and registers the current user in the server; StopListener unregisters the user from the server. ShutdownListener allows the user to shut down the server to prove that once connections among users are established, the server is not needed anymore. A KeyListener is also added to the TextArea handling the messages to be sent. The initList, addTalker and removeTalker methods update the list of users. The addToMsg method displays an incoming message.

Installing the System
This system is based on an applet accessing a server and other applets with RMI. Therefore, it will work only with a browser supporting all of the features of JDK1.1. For this experiment I used HotJava running on several PCs while the server was running on a UNIX machine. The system is easy to configure; the only requirement is that the server must be located on the same host as the HTTP daemon. The HTML file is shown here:

<HTML>
<TITLE>Talk
<APPLET code="TalkerApplet.class" width=550 height=450>
</APPLET>
</HTML>

Conclusion
In this article, I have shown how to take advantage of the facilities provided by RMI to pass remote objects in remote calls to allow both bidirectional communication between the applets and the server and among the applets themselves. This short example also demonstrates that direct applet-to-applet communication is, in fact, possible.