Take Control of Class Loading in Java  : Page 4

Loading Specific Class Versions
Now, you can use the container and restricted classloader to load components containing versioned classes from specified archives. Listing 2 shows how to instantiate instances of the component container and initialize them with the names of the configuration files for two different versions of a HelloWorld component. Each component version is then loaded and instantiated using the createComponent method of the ComponentContainer class.

Figure 3. Component Sequence: The figure shows the sequence the component container framework follows to create a component.

Calls to each instantiated component object produce results from the expected versions of each component. The sequence diagram in Figure 3 illustrates the steps taken by the framework to load and create a component:

Notice that calls to the RestrictedURLClassLoader class terminate before reaching an instance of the default classloader; thereby restricting class-searching to the URLs supplied to the RestrictedURLClassLoader instance. So, you've seen how to build a class loading component container framework that facilitates a self-contained context in which to define, version, and create Java components. Exploiting Java's class loading capabilities in this manner restricts class-loading to specified locations, leting you load different versions of classes simultaneously—both created and used in the same running JVM.

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