How Can I Effectively Unload Classes and Manage Multiple AppServer Class Versions in Java?

Unloading Class Loaders in Java

In Java, class unloading can only occur when the associated class loader is garbage collected. This implies that all references to the class and the class loader must be eliminated.

One potential solution to load classes from multiple AppServers is to implement a hierarchical class loading system. This system would involve:

• Creating a class loader dedicated to each jar file
• Creating a class loader for each AppServer that delegates class loading to specific jar class loaders

This approach allows you to load different versions of the jar file for each AppServer.

If OSGi is not a viable option for your project, consider implementing a custom class loader:

1. Create a JarClassLoader class to load classes from individual JAR files.
2. Create a MultiClassLoader class that extends ClassLoader. This class should maintain an array or list of JarClassLoader instances.
3. In the defineClass() method of MultiClassLoader, iterate through the internal class loaders until the class definition is found or a NoClassDefFoundException is thrown.
4. Provide accessor methods to add new JarClassLoader instances to the MultiClassLoader.

By instantiating a MultiClassLoader for each connection to the server, you can enable each server to use a different version of the same class. This approach has been successfully employed to load and unload classes containing user-defined scripts.

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