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.tools.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.tools.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.tools.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.tools.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
