Unnamed module and duplicate package

[Alex Buckley]

On 3/9/2016 10:36 AM, Paul Benedict wrote:
>  From the doc:
> "If a package is defined in both a named module and the unnamed module then
> the package in the unnamed module is ignored. This preserves reliable
> configuration even in the face of the chaos of the class path, ensuring
> that every module still reads at most one module defining a given package.
> If, in our example above, a JAR file on the class path contains a class
> file named com/foo/bar/alpha/AlphaFactory.class then that file will never
> be loaded, since the com.foo.bar.alpha package is exported by the
> com.foo.bar module."
>
> I would like some clarification. Correct me if wrong, but I think this
> entire paragraph is really meant to be about the perspective from a
> modularized JAR? If a module has package A, and the unnamed module has
> package A, then of course the module's package A should win.
>
> However, if it is meant to be absolute language, then I disagree.
>
> The unnamed module should be coherent among itself. If the unnamed module
> has package B and relies on classes from package A, it should still be able
> to see its own package A. I don't think modules should be able to impact
> how the unnamed module sees itself. That's a surprising situation.

The unnamed module is not a root module during resolution. If your main 
class is in the unnamed module (i.e. you did java -jar MyApp.jar rather 
than java -m MyApp), then the module graph is created by resolving 
various root modules (what are they? separate discussion) and only then 
is the unnamed module hooked up to read every module in the graph.

Hope we're OK so far.

If some named module in the graph exports package A (more than one 
module exporting A? separate discussion), then since the unnamed module 
reads that named module, the unnamed module will access A.* types from 
that named module.

It's hard to imagine the unnamed module NOT accessing A.* types from 
that named module. Primarily, we need to avoid a split package situation 
where code in the unnamed module sometimes accesses A.* types from the 
named module and sometimes from the unnamed module.

You might say, OK, let code in the unnamed module exclusively access A.* 
in the unnamed module rather than exclusively access A.* in the named 
module. Then you have two problems:

1. There are issues for named modules in the same class loader as the 
unnamed module -- such named modules MUST get A.* from the named module 
rather than the unnamed module, and the class loading mechanism is 
incapable of switching based on accessor. It'll be common for named 
modules to exist in the same class loader as the unnamed module, as 
modular JARs on the modulepath and non-modular JARs on the classpath all 
end up in the application class loader (modular JARs as named modules; 
non-modular JARs jointly as the unnamed module).

2. While the module system is sure that package A exists in the named 
module, how would the module system possibly know that package A exists 
in the unnamed module? Scanning every class file in every non-modular 
JAR on the classpath at startup sounds bad.

Alex