Compiling Java 9 (take 2)

Alex Buckley alex.buckley at
Thu Jan 12 19:23:05 UTC 2017

On 1/12/2017 3:44 AM, Stephan Herrmann wrote:
> On 01/10/2017 11:24 PM, Alex Buckley wrote:
>> On 1/10/2017 12:20 PM, Stephan Herrmann wrote:
>>> Can a module export a package even if no compilation units of that
>>> package
>>> are associated with the current module? The export might only affect
>>> types
>>> exported from some other required module. Is that allowed?
>> There is no re-exporting of another module's packages. Per
>> lang-vm.html, "It is a compile-time error if the package specified by
>> exports is not observable in the current module."
>> Then: "If the package specified by exports is observable in the
>> current module, then at least one compilation unit containing a
>> declaration of the package must be observable in the current module,
>> or a compile-time error occurs."
>> "observable in the current module" needs to be reworded, but I hope
>> you can see the intent.
> Yes and no. I'm still not sure how to implement this but it helps
> to raise new questions: what's the layering of concepts "observable"
> and "associated to a module"?
>  From 7.3 I understand:
> - observability is determined by the host system (so JLS doesn't have
> the rules)
> - if a compilation unit is observable it can be associated to a module
> Now you mention a paragraph where "observable" can only be answered
> relative to a module, so we first need to know the association to modules
> before we can answer observability. This sounds cyclic to me.
> Or is "observable" a different concept than "observable in the current
> module"?

As you say, observability is the lowest level predicate, then 
associated-with is the next level up, then visible-to, then 
accessible-to. The lang-vm.html paragraph is not meant to introduce any 
new concepts -- it was written before the JLS text, so it uses the 
traditional notion of observability and has not yet been reworded to use 
the associated-with relation (we need to have cu's+packages associated 
with a module for the rule to enforce "in the _current_ module").


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