I assume this arrangement will fail with ClassCastException as B can't cast a A-Foo to a B-Foo. If Core wants to provides a system-wide registry (which I think is what you are doing here) then it will need to be a Foo interface that is visible and accessible to all all users of that registry.
I understand you. However, this solution is not suitable for applications with plugins. As I've said, core application doesn't know what plugins do. Suppose - every plugin is developed by a separate company, so, it is not possible to provide their shared APIs in common parent layer that is a core application. Besides, all plugins are added to application the same way - one JPMS layer for one plugin with one parent that is a core application. As you understand it is absolutely impossible to create for every plugin a unique layer structure. Best regards, Pavel
