Since jdk9 running sketches in JRubyArt generate WARNING: of illegal reflective access, that will be prohibited in future versions of jdk in line with a move to stronger encapsulation see here and here. Here is a table summarising those warnings.

Methods and fields that request reflective access by JRubyArt

The Object.finalize() method and file descriptor field are probably JRuby specific. I suspect access to Component.paramSring() method might also be required by processing. The last two are required by jogl and needed by P2D and P3D sketches. Here are the --add-opens to add to java_args.txt to suppress the warnings in JRubyArt and propane sketches. In practice many more fields and methods within the packages above are accessed but the following --add-opens open the whole package to reflective access. To display all reflective access warnings put --illegal-access=warn into java_args.txt. JRuby as an example also requests illegal access to java.lang.Object.clone() …, and to java.awt.Component.getAccessControlContext() … etc.

1. --add-opens java.base/java.lang=ALL-UNNAMED
2. --add-opens java.base/java.io=ALL-UNNAMED
3. --add-opens java.desktop/java.awt=ALL-UNNAMED
4. --add-opens java.desktop/sun.java2d.opengl=ALL-UNNAMED
5. --add-opens java.desktop/sun.awt=ALL-UNNAMED

It should be possible to replace ALL-UNNAMED with jruby.dist for the first 3, but that depends on the jruby.dist module being recognized (currently it is only an explicitly named automatic module ie has an AutomaticModuleName: entry in the Manifest in the jruby.jar). There is the possibility of doing a global fix in JRubyArt (not propane) by adding the --add-opens to the java_args: in ~/.jruby_art/config.yml

Update

Providing you have set JAVA_HOME there is the possibility that will work on PiCrate and propane of putting java_options in .jruby.java_opts file local to your sketch.