1    <H2>Class Evolution</H2> 
2     
3    <P>This is a demonstration of the class evolution mechanism 
4    implemented with Javassist.  This mechanism enables a Java program to 
5    reload an existing class file.  Although the reloaded class file is 
6    not applied to exiting objects (the old class file is used for those 
7    objects), it is effective in newly created objects. 
8     
9    <P>Since the reloading is transparently executed, no programming 
10   convention is needed.  However, there are some limitations on possible 
11   changes of the class definition.  For example, the new class definition 
12   must have the same set of methods as the old one.  These limitations are 
13   necessary for keeping the type system consistent. 
14    
15    
16   <H3><a href="java.html">Run WebPage.show()</a></H3> 
17    
18   <P>The web server creates a new <code>WebPage</code> object and 
19   calls <code>show()</code> on that object.  This method works as 
20   if it is a CGI script or a servlet and you will see the html file 
21   produced by this method on your browser. 
22    
23   <H3><a href="update.html">Change WebPage.class</a></H3> 
24    
25   <P>The web server overwrites class file <code>WebPage.class</code> 
26   on the local disk.  Then it signals that <code>WebPage.class</code> 
27   must be reloaded into the JVM.  If you run <code>WebPage.show()</code> 
28   again, you will see a different page on your browser. 
29    
30   <H3>Source files</H3> 
31    
32   <P>Web server: <A HREF="DemoServer.java"><code>DemoServer.java</code></A> 
33    
34   <P>WebPage: <A HREF="WebPage.java"><code>WebPage.java</code></A> 
35    
36   <P>Class loader: <A HREF="DemoLoader.java"><code>DemoLoader.java</code></A>, 
37       <A HREF="Evolution.java"><code>Evolution.java</code></A>, and 
38       <A HREF="VersionManager.java"><code>VersionManager.java</code></A>. 
39