You will have to run the entire rails app in jruby. I would suggest you run your current app with https://github.com/jruby/jruby-lint to look for any conflicts. After you have your app jruby compliant, you can run rails via
jruby -S rails server
To require your jar file
require 'java'
require 'path/to/mycode.jar'
Always prepend any rails or rake commands with the jruby -S
command