Java 9: Possible to have 2 modules with same name on module path

Question

Is it possible to have 2 modules with the exact same name (but with slightly different contents) on the module path?

As far as I can tell, the Java 9 compiler does n

Answer 1

No

It is not possible to have two modules of the same name in the same directory on the module path. The official documentation is not putting that information in a particularly prominent spot - it's the Javadoc of ModuleFinder::of that gives it away:

It is an error if a directory contains more than one module with the same name.

I've created a small demo project for the module system and it covers that case by creating two versions of the same module...

jar --create
    --file mods/monitor.observer.beta-1.0.jar
    --module-version 1.0
    -C classes/monitor.observer.beta .
jar --create
    --file mods/monitor.observer.beta-2.0.jar
    --module-version 2.0
    -C classes/monitor.observer.beta .

... and then referencing the folder in the next compilation ...

javac
    --module-path mods
    -d classes/monitor.statistics
    $(find monitor.statistics -name '*.java')

... which as expected leads to the following error message:

error: duplicate module on application module path
module in monitor.observer.beta
1 error

Note that I said in the same directory. Across directories multiple modules are possible.

Yes

The module system only enforces uniqueness within directories. Once again from ModuleFinder::of (emphasis mine):

The module finder locates modules by searching each directory, exploded module, or packaged module in array index order. It finds the first occurrence of a module with a given name and ignores other modules of that name that appear later in the sequence.

That makes it possible to have the same module in different directories.