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<h2>Remote Method Invocation</h2>
<P>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.tools.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>
<P>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.
<p><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>
<p>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:
<p><b>Figure 1: Applet</b>
<pre>
<font color="red">import javassist.tools.rmi.ObjectImporter;</font>
public class CountApplet extends Applet implements ActionListener {
private Font font;
<font color="red">private ObjectImporter importer;
private Counter counter;</font>
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);
<font color="red">importer = new ObjectImporter(this);</font>
dialog = new AlertDialog();
message = "???";
}
public void start() {
String counterName = getParameter("name");
<font color="red">counter = (Counter)importer.getObject(counterName);</font>
message = Integer.toString(<font color="red">counter.get()</font>);
}
/* The method called when the button is pressed.
*/
public void actionPerformed(ActionEvent e) {
message = Integer.toString(<font color="red">counter.increase()</font>);
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>
<p>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.
<p>The definition of the <code>Counter</code> class is also
straightforward:
<p><b>Figure 2: Remote object</b>
<pre>
public class Counter {
private int count = 0;
public int get() {
return count;
}
public int increase() {
count += 1;
return count;
}
}
</pre>
<p>Note that the <code>javassist.tools.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.
<p>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).
<p><b>Figure 3: Server-side program</b>
<pre>
public class MyWebServer {
public static void main(String[] args) throws IOException, CannotCompileException
{
AppletServer web = new AppletServer(args[0]);
<font color="red">web.exportObject("counter", new Counter());</font>
web.run();
}
}
</pre>
<p>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.
<p><br>
<h3>2. Features</h3>
The remote method invocation mechanism provided by Javassist has the
following features:
<ul>
<li><b>Regular Java syntax:</b><br>
The applet can call a method on a remote object with regular
Java syntax.
<p>
<li><b>No special naming convention:</b><br>
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>.
<p>
<li><b>No extra compiler:</b><br>
All the programs, both the applet and the server-side program,
are compiled by the regular Java compiler. No external compiler
is needed.
</ul>
<p> 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.tools.rmi</code> package does not
require the programmer to follow that programming convention.
It is suitable for writing simple distributed programs like applets.
<p><br>
<h3>3. Inside of the system</h3>
<p>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.
<p>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>
<p>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.
<p><br>
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