1<H2>Class Evolution</H2> 2 3<P>This is a demonstration of the class evolution mechanism 4implemented with Javassist. This mechanism enables a Java program to 5reload an existing class file. Although the reloaded class file is 6not applied to exiting objects (the old class file is used for those 7objects), it is effective in newly created objects. 8 9<P>Since the reloading is transparently executed, no programming 10convention is needed. However, there are some limitations on possible 11changes of the class definition. For example, the new class definition 12must have the same set of methods as the old one. These limitations are 13necessary 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 19calls <code>show()</code> on that object. This method works as 20if it is a CGI script or a servlet and you will see the html file 21produced 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> 26on the local disk. Then it signals that <code>WebPage.class</code> 27must be reloaded into the JVM. If you run <code>WebPage.show()</code> 28again, 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> and 35another <A HREF="sample/evolve/WebPage.java"><code>WebPage.java</code></A> 36 37<P>Class loader: <A HREF="DemoLoader.java"><code>DemoLoader.java</code></A>, 38 <A HREF="Evolution.java"><code>Evolution.java</code></A>, and 39 <A HREF="VersionManager.java"><code>VersionManager.java</code></A>. 40