The details are still a bit murky but it seems to work now.

The additional module-info.class in the root of the JAR want be a problem for an pre JDK 9 because it's not used there anyway.

If you wanted to use JDK 9 specific stuff for let's say performance improvements, then a multi release JAR comes into play. There you can overwrite/add some classes using the JDK 9 specific stuff and put them into the same JAR. If that is not needed at the moment we do not need a multi release JAR at all.

Thanks for the offer. BTW, the modularizaton part is done.

To be more precise, I am trying to have a single jar file which will be seen as modular on Java 9 and as a regular jar in older JVMs, with module-info.class ignored.

Quoting from The State of the Module System:

A modular JAR file can be used as a module, in which case its
module-info.class file is taken to contain the module’s
declaration. It can, alternatively, be placed on the ordinary class
path, in which case its module-info.class file is ignored. Modular JAR
files allow the maintainer of a library to ship a single artifact that
works both as a module, on Java SE 9 and later, and as a regular JAR
file on the class path, on all releases. We expect that
implementations of Java SE 9 which include a jar tool will enhance
that tool to make it easy to create modular JAR files.

This is the problem I am struggling with.

I made already made a couple of module info files in the past, there I would be able to help. Since I do not have a lot of experiences with maven itself, there I'm not a big help.

There is seems to be a serious gap between the theory and actual tooling. Frankly, I am quite disgusted with the whole affair.

Unfortunately I do not have any bigger experiences with maven but I tried local the approach as David M. Lloyd described here, but with not much success.

Sounds good. Would you like to take a shot?

I am having a hard time configuring maven for the build...

We could basically add a separate source directory to hold the JDK 9 module info and create a Multi-Release JAR. supporting pre Java 9 using old-sytle service files as Java 9 and later as a fully complain module...