My experience was a lot simpler, perhaps because I didn’t have the ‘install jruby’ requirement.
So all I did was install warbler and ran it in a rails app (created by calling ‘rails’ instead of jruby -S rails) and deployed the war file to tomcat. My spike was using AR with ruby mysql binding but it can be changed to use jdbc just like yours (need to add jdbc jar to WEB-INF/lib or somewhere else as long as it’s in classpath).

Cheers,
Andy