[Question] Accessibility of each modules

[Alan Bateman]

On 05/09/2017 12:01, KUBOTA Yuji wrote:
> Hi all,
>
> I have a question about accessibility of each modules: unnamed,
> automatic and named modules.
> * unnamed: the JAR compiled by java 8 and speficied by --class-path.
> * automatic: the JAR compiled by Java 8 and specified by --module-path.
> * named: the modular JAR.
For the modular JAR case then the term to use is "explicit module".

>
> I ran quick-start's example program on java 9+181 and 8u141, and
> confirmed the following results.
> * Named modules can call autonamed modules.
Yes, assuming the explicit module reads the automatic module. As an 
automatic module exports all packages then it means code in the explicit 
module can access all public members of public classes in the automatic 
module.

> * Automatic modules can *NOT* call named modules.
Automatic modules read all named modules (in the module graph), they 
also read all unnamed modules. I have not studied your github project 
but you should see that code in the automatic module can access all 
public members of public classes in packages exported by the explicit 
module (because it reads the explicit module).

> * We can run unnamed modules with only unnamed modules.
I'm not what you mean by this because you can mix the class path with 
named modules. When you run HelloWorld on the class path then you are 
doing this, although it might not be obvious.

If it helps then the code in an unnamed module an access all public 
members of public classes in all unnamed modules. It can also access all 
public members of public classes in the packages exported by named 
modules (automatic modules are named modules, they export all packages).

>
> I guessed that every modules can be accessed each other if
> `--illegal-access=permit` or/and `--add-exports` is given, but it does
> not seems to be fine.
The `--add-exports` option is for breaking encapsulation, it can be used 
to change a module so that it exports an otherwise concealed package to 
another named module or to all unnamed modules.

The `--illegal-access=<value>` option is specific to the standard and 
JDK modules.

> I guess that modules and classes (unnamed module) cannot be existed at
> the same time because `BuiltinClassLoader` seems to branch by
> `packageToModule` field in it.
No, that is just an implementation detail. The issue that was chosen not 
to tackle in JDK 9 is the issue of multiple modules in the boot layer 
with the same non-exported package. This topic has a lot of complexity 
and compatibility issues for (non-JDK) code. It will be looked in the 
future.

-Alan