Get started with these tutorials:
.warfile with Spring and Maven.
If you don't use a traditional Ruby application server like Mongrel, you can use a Java application server. To deploy to a Java app server, you can use the tool Warbler to bundle your Rails application in a Java Web Application Archive (.war file). (Warbler is a minimal, flexible, Ruby-like way to create a .war file.) Once you have a .war file, you can deploy to any Java app server using its war deployment mechanism.
Some links to information on various Java appllication servers:
The Glassfish gem by Vivek Pandey enables you to easily deploy a JRuby on Rails application to the Glassfish v3 modularized Java application server. The Glassfish gem wraps the essential technologies in 3 MB and allows you to run your application using a traditional approach, as if you were running Mongrel, Rack, and so on. A brief tutorial can be found here:
JRuby on Rails with Glassfish Gem.