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.
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.
The web server creates a new WebPage object and
calls show() 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.
The web server overwrites class file WebPage.class
on the local disk. Then it signals that WebPage.class
must be reloaded into the JVM. If you run WebPage.show()
again, you will see a different page on your browser.
Web server: DemoServer.java
WebPage: WebPage.java and
another WebPage.java
Class loader: DemoLoader.java,
Evolution.java, and
VersionManager.java.