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<html> <body> <h2>Remote Method Invocation</h2> Javassist enables an applet to access a remote object as if it is a local object. The applet can communicate through a socket with the host that executes the web server distributing that applet. However, the applet cannot directly call a method on an object if the object is on a remote host. The <code>javassist.rmi</code> package provides a mechanism for the applet to transparently access the remote object. The rules that the applet must be subject to are simpler than the standard Java RMI. <h3>1. Sample applet</h3> The applet showing below is a simple number counter. If you press the button, the number is increased by one. An interesting feature of this applet is that the object recording the current number is contained by the web server written in Java. The applet must access the object through a socket to obtain the current number. <center> <applet codebase="http://localhost:5001" code="sample.rmi.CountApplet" width=200 height=200> <param name=name value="counter"> <param name=button value="+1"> </applet> </center> However, the program of the applet does not need to directly handle a socket. The <code>ObjectImporter</code> provided by Javassist deals with all the awkward programming. Look at the lines shown with red: Figure 1: Applet <pre> import javassist.rmi.ObjectImporter; public class CountApplet extends Applet implements ActionListener { private Font font; private ObjectImporter importer; private Counter counter; private AlertDialog dialog; private String message; public void init() { font = new Font("SansSerif", Font.ITALIC, 40); Button b = new Button(getParameter("button")); b.addActionListener(this); add(b); importer = new ObjectImporter(this); dialog = new AlertDialog(); message = "???"; } public void start() { String counterName = getParameter("name"); counter = (Counter)importer.getObject(counterName); message = Integer.toString(counter.get()); } /* The method called when the button is pressed. */ public void actionPerformed(ActionEvent e) { message = Integer.toString(counter.increase()); repaint(); } public void paint(Graphics g) { g.setFont(font); g.drawRect(50, 50, 100, 100); g.setColor(Color.blue); g.drawString(message, 60, 120); } } </pre> A <code>Counter</code> object running on a remote host maintains the counter number. To access this object, the applet first calls <code>getObject()</code> on an <code>ObjectImporter</code> to obtain a reference to the object. The parameter is the name associated with the object by the web server. Once the reference is obtained, it is delt with as if it is a reference to a local object. For example, <code>counter.get()</code> and <code>counter.increase()</code> call methods on the remote object. The definition of the <code>Counter</code> class is also straightforward: Figure 2: Remote object <pre> public class Counter { private int count = 0; public int get() { return count; } public int increase() { count += 1; return count; } } </pre> Note that the <code>javassist.rmi</code> package does not require the <code>Counter</code> class to be an interface unlike the Java RMI, with which <code>Counter</code> must be an interface and it must be implemented by another class. To make the <code>Counter</code> object available from the applet, it must be registered with the web server. A <code>AppletServer</code> object is a simple webserver that can distribute <code>.html</code> files and <code>.class</code> files (Java applets). Figure 3: Server-side program <pre> public class MyWebServer { public static void main(String[] args) throws IOException, CannotCompileException { AppletServer web = new AppletServer(args[0]); web.exportObject("counter", new Counter()); web.run(); } } </pre> The <code>exportObject()</code> method registers a remote object with the <code>AppletServer</code> object. In the example above, a <code>Counter</code> object is registered. The applet can access the object with the name "counter". The web server starts the service if the <code>run()</code> method is called. <h3>2. Features</h3> The remote method invocation mechanism provided by Javassist has the following features: <ul> <li>Regular Java syntax: The applet can call a method on a remote object with regular Java syntax. <li>No special naming convention: The applet can use the same class name as the server-side program. The reference object to a remote <code>Foo</code> object is also represented by the class <code>Foo</code>. Unlike other similar systems, it is not represented by a different class such as <code>ProxyFoo</code> or an interface implemented by <code>Foo</code>. <li>No extra compiler: All the programs, both the applet and the server-side program, are compiled by the regular Java compiler. No external compiler is needed. </ul> With the Java RMI or Voyager, the applet programmer must define an interface for every remote object class and access the remote object through that interface. On the other hand, the <code>javassist.rmi</code> package does not require the programmer to follow that programming convention. It is suitable for writing simple distributed programs like applets. <h3>3. Inside of the system</h3> A key idea of the implementation is that the applet and the server-side program must use different versions of the class <code>Counter</code>. The <code>Counter</code> object in the applet must work as a proxy object, which transfers the method invocations to the <code>Counter</code> object in the server-side program. With other systems like the Java RMI, the class of this proxy object is produced by a special compiler such as <code>rmic</code>. It must be manually maintained by the programmer. <center><img src="inside.gif"></center> However, Javassist automatically generates the proxy class at runtime so that the programmer does not have to be concerned about the maintenance of the proxy class. If the web browser running the applet requests to load the <code>Counter</code> class, which is the class of an exported object, then the web server transfers the version of <code>Counter</code> that Javassist generates as a proxy class. </body> </html>


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