Why is my RMI Server Throwing a `ClassNotFoundException`?

RMI ServerException: Troubleshooting ClassNotFoundException

When attempting to start an RMI server, you may encounter the following exception:

java.rmi.ServerException: RemoteException occurred in server thread (ClassNotFoundException)

This exception indicates that the server is unable to find a class or interface referenced in the remote method's signature or dependencies. There are several possible causes for this error:

1. Registry Classpath Issue

The RMI Registry, where the server binds its objects, may not have access to the required classes. Ensure that the registry's classpath includes all necessary JARs or directories.

2. Server Classpath Issue

The server must have access to the classes and interfaces mentioned in the remote method signature and its dependencies. Check if the server's classpath is configured correctly.

3. Dynamic Stubs

If dynamic stubs are being used, the Registry or server may still require access to the remote interface or its dependencies. Verify that the classpath includes these classes.

4. Mismatched Class Versions

Ensure that the classes on the client and server side have compatible versions. Different versions can lead to ClassNotFoundException errors.

5. Codebase Feature

Using the codebase feature allows clients to locate classes on the server's side. However, this may require additional configuration in the client's environment.

Solutions:

1. Update Registry Classpath: Start the Registry with the appropriate classpath.
2. Fix Server Classpath: Configure the server's classpath to include the required classes.
3. Disable Dynamic Stubs: Revert to using static stubs to avoid classpath issues for the remote interface.
4. Ensure Class Compatibility: Verify that classes on both sides have matching versions.
5. Consider Codebase Feature: Configure the codebase property for deployment purposes.

By addressing these potential causes, you can successfully start your RMI server and resolve the ClassNotFoundException error.

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