I’d heard about jigsaw but not looked into it yet. It seems like a great thing but only operates at the class level. I don’t see an obvious way to export different fields or methods of a class via different modules, which some might want for testing. Perhaps one could make a class method package-scope, have it implement a package-scope interface from a sub-package, then export the sub-package from a separate module and/or export it only to test modules? Not sure that works, though.
Of course, you can usually refactor to avoid having to call some otherwise private/protected methods for testing.