It is safe to use same classes from same modules in different layers

[Johannes Spangenberg]

> Lets consider the following situation:
>
> Bootlayer (module Core) is a main application
> - child layer A (module A, module Foo) is a plugin A
> - child layer B (module B, module Foo) is a plugin B.

If I understand you correctly, you would add module Foo to the module 
path of both child layers? In this case, both layers would have access 
to a class called Foo, but it would not be the same class in both 
layers. This makes sense if you consider that both layers might just as 
well depend on two different versions of module Foo, so they cannot 
share the same loaded classes. As a result, your code would run into a 
ClassCastException.

> and in plugin B I do
> var foo = (Foo) coreService.getInterPluginObject();

However, note that the JPMS is not limited to a single parent layer. You 
could add a separate layer containing Foo, which could be an additional 
parent for both, layer A and B. Depending on your context, plugin B may 
also declare a dependency on plugin A, and therefore get child layer A 
as a parent. I don't think that I can give you a proper recommendation 
as I don't know much about your project. (Also not that I am NOT 
affiliated with the OpenJDK project, I am just a fellow subscriber to 
this mailing list.)