I have an app that I am migrating from Ruby to JRuby (due to need for better Web Service Security support via Java). One of the gems I use is daemons to create a background
Since you cannot fork in JRuby you are basically left with having to refactor your daemon code so that the daemon can be executed as a standard foreground application then create a separate daemon launcher that will run and background the daemon. You can look at spoon and also check this gist which uses spoon to create a more elaborate launcher. Also jruby-jsvc provides the glue to use jsvc with a JRuby daemon.