JRuby, a Ruby interpretter written in Java and that runs on the Java platform has an upgraded release, 0.9, that now supports running Rails on JRuby.
If you are interested in looking at the benefits of a dynamic language and using Rails concepts on the Java platform, take a look at Grails. Grails uses Groovy scripting language which executes on the Java VM, and the Grails framework is based on a lot of the features included in Ruby on Rails. Take a look here – its worth a look.