sample/rmi/webdemo.html - platform/external/javassist - Git at Google

 <html>
 <body>
 <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>

 </body>
 </html>
	<html>
	<body>
	<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>

	</body>
	</html>