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/Users/lyon/j4p/src/javassist/sample/rmi/webdemo.html
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1 <html>
2 <body>
3 <h2>Remote Method Invocation</h2>
4
5 <P>Javassist enables an applet to access a remote object as if it is a
6 local object. The applet can communicate through a socket with the
7 host that executes the web server distributing that applet. However,
8 the applet cannot directly call a method on an object if the object is
9 on a remote host. The <code>javassist.rmi</code> package provides
10 a mechanism for the applet to transparently access the remote object.
11 The rules that the applet must be subject to are simpler than the
12 standard Java RMI.
13
14 <h3>1. Sample applet</h3>
15
16 <P>The applet showing below is a simple number counter.
17 If you press the button, the number is increased by one.
18 An interesting feature of this applet is that the object
19 recording the current number is contained by the web server
20 written in Java. The applet must access the object through a socket
21 to obtain the current number.
22
23 <p><center>
24 <applet codebase="http://localhost:5001"
25 code="sample.rmi.CountApplet" width=200 height=200>
26 <param name=name value="counter">
27 <param name=button value="+1">
28 </applet>
29 </center>
30
31 <p>However, the program of the applet does not need to directly handle
32 a socket. The <code>ObjectImporter</code> provided by Javassist deals
33 with all the awkward programming.
34 Look at the lines shown with red:
35
36 <p><b>Figure 1: Applet</b>
37
38 <pre>
39 <font color="red">import javassist.rmi.ObjectImporter;</font>
40
41 public class CountApplet extends Applet implements ActionListener {
42 private Font font;
43 <font color="red">private ObjectImporter importer;
44 private Counter counter;</font>
45 private AlertDialog dialog;
46 private String message;
47
48 public void init() {
49 font = new Font("SansSerif", Font.ITALIC, 40);
50 Button b = new Button(getParameter("button"));
51 b.addActionListener(this);
52 add(b);
53 <font color="red">importer = new ObjectImporter(this);</font>
54 dialog = new AlertDialog();
55 message = "???";
56 }
57
58 public void start() {
59 String counterName = getParameter("name");
60 <font color="red">counter = (Counter)importer.getObject(counterName);</font>
61 message = Integer.toString(<font color="red">counter.get()</font>);
62 }
63
64 /* The method called when the button is pressed.
65 */
66 public void actionPerformed(ActionEvent e) {
67 message = Integer.toString(<font color="red">counter.increase()</font>);
68 repaint();
69 }
70
71 public void paint(Graphics g) {
72 g.setFont(font);
73 g.drawRect(50, 50, 100, 100);
74 g.setColor(Color.blue);
75 g.drawString(message, 60, 120);
76 }
77 }
78 </pre>
79
80 <p>A <code>Counter</code> object running on a remote host
81 maintains the counter number. To access this object, the applet first
82 calls <code>getObject()</code> on an <code>ObjectImporter</code>
83 to obtain a reference to the object. The parameter is the name associated
84 with the object by the web server. Once the reference is obtained, it
85 is delt with as if it is a reference to a local object.
86 For example, <code>counter.get()</code> and <code>counter.increase()</code>
87 call methods on the remote object.
88
89 <p>The definition of the <code>Counter</code> class is also
90 straightforward:
91
92 <p><b>Figure 2: Remote object</b>
93
94 <pre>
95 public class Counter {
96 private int count = 0;
97
98 public int get() {
99 return count;
100 }
101
102 public int increase() {
103 count += 1;
104 return count;
105 }
106 }
107 </pre>
108
109 <p>Note that the <code>javassist.rmi</code> package does not require
110 the <code>Counter</code> class to be an interface unlike the Java RMI,
111 with which <code>Counter</code> must be an interface and it must be
112 implemented by another class.
113
114 <p>To make the <code>Counter</code> object available from the applet,
115 it must be registered with the web server. A <code>AppletServer</code>
116 object is a simple webserver that can distribute <code>.html</code> files
117 and <code>.class</code> files (Java applets).
118
119 <p><b>Figure 3: Server-side program</b>
120
121 <pre>
122 public class MyWebServer {
123 public static void main(String[] args) throws IOException, CannotCompileException
124 {
125 AppletServer web = new AppletServer(args[0]);
126 <font color="red">web.exportObject("counter", new Counter());</font>
127 web.run();
128 }
129 }
130 </pre>
131
132 <p>The <code>exportObject()</code> method registers a remote object
133 with the <code>AppletServer</code> object. In the example above,
134 a <code>Counter</code> object is registered. The applet can access
135 the object with the name "counter". The web server starts the service
136 if the <code>run()</code> method is called.
137
138 <p><br>
139
140 <h3>2. Features</h3>
141
142 The remote method invocation mechanism provided by Javassist has the
143 following features:
144
145 <ul>
146 <li><b>Regular Java syntax:</b><br>
147 The applet can call a method on a remote object with regular
148 Java syntax.
149 <p>
150
151 <li><b>No special naming convention:</b><br>
152 The applet can use the same class name as the server-side program.
153 The reference object to a remote <code>Foo</code> object is
154 also represented by the class <code>Foo</code>.
155 Unlike other similar
156 systems, it is not represented by a different class such as
157 <code>ProxyFoo</code> or an interface implemented by
158 <code>Foo</code>.
159 <p>
160
161 <li><b>No extra compiler:</b><br>
162 All the programs, both the applet and the server-side program,
163 are compiled by the regular Java compiler. No external compiler
164 is needed.
165 </ul>
166
167 <p> With the Java RMI or Voyager, the applet programmer must define
168 an interface for every remote object class and access the remote object
169 through that interface.
170 On the other hand, the <code>javassist.rmi</code> package does not
171 require the programmer to follow that programming convention.
172 It is suitable for writing simple distributed programs like applets.
173
174 <p><br>
175
176 <h3>3. Inside of the system</h3>
177
178 <p>A key idea of the implementation is that the applet and the server-side
179 program must use different versions of the class <code>Counter</code>.
180 The <code>Counter</code> object in the applet must work as a proxy
181 object, which transfers the method invocations to the <code>Counter</code>
182 object in the server-side program.
183
184 <p>With other systems like the Java RMI, the class of this proxy object is
185 produced by a special compiler such as <code>rmic</code>.
186 It must be manually maintained by the programmer.
187
188 <center><img src="inside.gif"></center>
189
190 <p>However, Javassist automatically generates the proxy class at
191 runtime so that the programmer does not have to be concerned about the
192 maintenance of the proxy class.
193 If the web browser running the applet
194 requests to load the <code>Counter</code> class, which is the class
195 of an exported object,
196 then the web server
197 transfers the version of <code>Counter</code> that Javassist generates
198 as a proxy class.
199
200 <p><br>
201
202 </body>
203 </html>
204