Issue with qualified exports

[Alex Buckley]

On 7/20/2016 3:07 PM, Paul Benedict wrote:
> Currently I am writing a module that another team will consume. Let's just
> call these modules A and B. Module A must export its packages to Module B
> and B alone.
>
> For reasons beyond my control, I do not have access to Module B. However, I
> don't need to consume any types from B or use B in anyway -- just need to
> give package visibility to B. So the compiler is stopping me because it
> says "error: module not found". Yes, the compiler is right... but it's too
> right.
>
> What do you think of loosening the compiler restriction here? I don't see a
> reason why the export target must be known at this point.

We start by supposing that compile-time checking is good. Think of how 
the compiler checks your 'import' declarations in ordinary .java files. 
Even if you do a wildcard import because you're not sure which types 
you'll use, you still have the compiler checking that the package 
exists. For qualified exports, we assume that if you're friendly enough 
with the owner of B to add a qualified export to B, then you're friendly 
enough to have a copy of B available. As such, if you write 'exports ... 
to V' (yes, V, not B), then we aim to check that V exists. It doesn't, 
so you'll get an error, and you'll smack yourself for typing V rather 
than B.

A wrinkle in your scenario is that module B requires module A, so B must 
be compiled with A present. And, A must be compiled with B present, due 
to 'exports ... to B'. There's no circularity in the 'requires' clauses, 
but there is effectively a circularity in the module declarations more 
broadly. A certain amount of incremental craftsmanship will be necessary 
to allow this pair of modules to flower.

> PS: I don't know if the same thing is checked at runtime, but A should be
> able to be loaded without B, too.

Exports don't drive module resolution, of course, and there's nothing 
special for _qualified_ exports in the post-resolution consistency 
checks in java.lang.module.Configuration::resolveRequires. So, a 
"dangling" qualified export is OK at run time.

Alex