On 17/10/2016 08:32, Peter Levart wrote:


Do we need an --exclude-modules (in addition to --add-modules) option on javac, java and jlink commands?

--exclude-modules would be different to --limit-modules. If some module requires module M and there is no module M on the module path or it is not observable because it was not mentioned in the --limit-modules option, then an exception is raised. OTOH if some module X requires module M and module M is mentioned in the --exclude-modules option, then such requires is silently ignored in hope that module X will not actually need types from module M.
The module declaration is intended to be authoritative and so we have to trust module author when they declare that the module `requires M`. So my view is that options such as --exclude-modules that would have the effect of dropping requires puts us on the road to anarchy.

That said, I do see Robert's concern that there might be orphaned `requires` clauses in some modules. My module started using the preferences API but later the implementation changed to use something else. I neglected to remove the `requires java.prefs` from the module declaration and the result is that my module cannot compile against or run on a run-time image that doesn't include this module. Static analysis tools might help here, as might the IDE. We are used to IDEs highlighting unused `import` statements and in time then I expect they will do the same for apparently unused `requires` clauses in module-info.java. If the usage is purely reflective then the module author might need to put a comment on the `requires` clause to avoid other maintainers from removing it (a bit like "// used by javadoc" in comments today when an import is for an unqualified reference in the javadoc).

Another part to Robert's mail is the case where something is making use of types in modules that it doesn't depend on. Assuming these are static references then they will be caught at compile-time. This is big improvement compared to today's class path.

A more general comment is that module authors will need to learn a few new things about compatibility and refactoring. One example is changing `requires transitive M` to `requires M` is an incompatible change. Another is splitting a module (several sub-cases) where the module author will need to add `requires transitive` to avoid breaking consumers. There are lots of opportunities here for authoritative books and documentation to help module authors do this right.


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