http://c25o7knygjm3m67jy27yuynvv4pkfi25naucscmh4ubq2ggiig3v57ad.onion/en/supporting-java-modules-in-guix.html
As I am not a Java developer, and I only care about building packages, the only
thing I need to know is that, whenever I see a module-info.java in the source
code, it requires Java 9 or later because it's using modules. To support this, I changed the JDK used for building the package, which is
easy to do in the arguments field of the package declaration: (arguments
`(#:jdk ,openjdk9; or later versions
...)))