The most likely explanation is that you have a problem in your dependencies. In order for a class in one module to reference a class/interface defined in another module, the following must be true:

These rules are pretty straightforward and it is easy in most cases to verify that dependencies are set up correctly. If you receive a ClassNotFoundException or NoClassDefFoundError at runtime, the stack trace will generally lead you to the problem.

However, there are some cases where you will receive a ClassNotFoundException or NoClassDefFoundError at runtime, but finding which modules need to declare dependencies on one another is more difficult because the stacktrace does not directly identify the code involved. This occurs most frequently when you have library modules (composed of JAR files which were compiled outside of the platform). Although the dependencies were satisfied (by setting the classpath as needed) when the libraries were compiled, the developer may not have correctly set these dependencies in the platform application which uses them.

In this case, you can often locate the problem by rebuilding the suite and paying close attention to the output generated by the verify-class-linkage task. For example:

This tells us that the module which provides SomeImplementation needs to declare a dependency on the module which provides SomeInterface.

For more background, see link:.