<H2>Class Evolution</H2>

<P>This is a demonstration of the class evolution mechanism
implemented with Javassist.  This mechanism enables a Java program to
reload an existing class file.  Although the reloaded class file is
not applied to exiting objects (the old class file is used for those
objects), it is effective in newly created objects.

<P>Since the reloading is transparently executed, no programming
convention is needed.  However, there are some limitations on possible
changes of the class definition.  For example, the new class definition
must have the same set of methods as the old one.  These limitations are
necessary for keeping the type system consistent.


<H3><a href="java.html">Run WebPage.show()</a></H3>

<P>The web server creates a new <code>WebPage</code> object and
calls <code>show()</code> on that object.  This method works as
if it is a CGI script or a servlet and you will see the html file
produced by this method on your browser.

<H3><a href="update.html">Change WebPage.class</a></H3>

<P>The web server overwrites class file <code>WebPage.class</code>
on the local disk.  Then it signals that <code>WebPage.class</code>
must be reloaded into the JVM.  If you run <code>WebPage.show()</code>
again, you will see a different page on your browser.

<H3>Source files</H3>

<P>Web server: <A HREF="DemoServer.java"><code>DemoServer.java</code></A>

<P>WebPage: <A HREF="WebPage.java"><code>WebPage.java</code></A> and
another  <A HREF="sample/evolve/WebPage.java"><code>WebPage.java</code></A>

<P>Class loader: <A HREF="DemoLoader.java"><code>DemoLoader.java</code></A>,
	<A HREF="Evolution.java"><code>Evolution.java</code></A>, and
	<A HREF="VersionManager.java"><code>VersionManager.java</code></A>.