Java Platform Module System

Alex Buckley alex.buckley at
Mon May 1 18:47:42 UTC 2017

On 4/30/2017 3:25 AM, Stephan Herrmann wrote:
> For the question at hand, this is what we learn from that improved
> reference:
>    "A readability graph is constructed"
> Now we only need a link to the specification that *defines* what is a
> readability graph and what is the meaning of "m1 reads m2".
> I assume, you want to add a further reference to the "Resolution"
> section of the package specification for java.lang.module?

Yes, the API spec for java.lang.module will be updated to define the 
readability relation. But instead of waiting for that, I recommend you 
watch because readability has been 
stable for a long time.

>      BTW: while pondering if the given package specification is sufficient,
>      I wonder if "requires transitive" should work through multiple levels:
>        M1
>        M2 requires transitive M1
>        M3 requires transitive M2
>        M4 requires M3
>      Does M4 read M1?

It does work through multiple levels, in order to support arbitrary 
amounts of refactoring: once you've released a module that someone else 
reuses (via 'requires'), then you've committed to your module's name and 
API but are free to refactor its content into deeper modules which your 
original module "reuses" (via 'requires transitive') for the benefit of 
consumers. There is no "re-exporting", just modules being made to read 
one another.

So, going top down (because resolution starts from a set of root 
modules) :- M4 requires and thus reads M3, and M3 requires transitive 
M2, so M4 reads M2. Since M4 reads M2, and M2 requires transitive M1, we 
have M4 reads M1.

>      Looking at 7.7.1:
>        "The requires keyword may be followed by the modifier transitive.
> This causes
>         any module which depends on the current module to have an
> implicitly declared
>         dependence on the module specified by the requires transitive
> directive."
>      Am I right in assuming that "depends" should cover explicitly and
> implicitly
>      declared dependences? Taking into consideration the subtlety about
> dependence
>      vs. dependency, may I suggest adding s.t. like
>        "A module M1 is said to depend on another module M2, if it has an
> explicitly
>         or implicitly declared dependence on M2."
>      (this also makes "depends" a technical term rather than the general
> (fuzzy)
>       English word).

I understand the point; when we clarify the API spec for readability, 
I'll make sure the JLS usage of "depends" is explicitly aligned.

> Revisiting other references to "Java Platform Module System" inside JLS,
> what about the two occurrences in the body of 7.7:
> - One reference is used to discriminate "dependence" from "dependency":
>    From a quick scan, I believe this sentence:
>      "Generally, the  rules  of  the  Java  programming  language  are
>       more  interested  in  dependences  than dependencies."
>    can probably be made stronger:
>      "The rules of the Java programming language are not interested in
>       dependencies, only in dependences.".
>    Or perhaps the paragraph about dependencies could be removed entirely.
>    If this interpretation is wrong, another reference to detailed
> specification
>    would be needed. Perhaps it is only JLS, that is agnostic to
> dependencies,
>    whereas the API specification part indeed uses this concept?

This text is an informative note distinguishing the "dependence" 
expressed rather statically in the Language, from the "dependency" 
module determined rather dynamically by the JPMS. I see no reason to 
change it.

> - Another reference links "automatic modules" into JLS and will probably
>    link to ModuleFinder.of(Path...), right?

This text is also an informative note. Automatic modules are discovered 
through ModuleFinder.of, sure, and they appear in other places in the 
java.lang.module API too -- but none of that is the point of the note. 
The point of the note is that the developer doesn't specify 'requires' 
any differently for an automatic module than for an explicit module.


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