Warbler is a gem to make a .war file out of a Rails project. The intent is to provide a minimal, flexible, ruby-like way to bundle up all of your application files for deployment to a Java application server.
Warbler provides a sane set of out-of-the box defaults that should allow most Rails applications without external gem dependencies (aside from Rails itself) to assemble and Just Work.
Warbler bundles JRuby and the JRuby-Rack servlet adapter for dispatching requests to your application inside the java application server, and assembles all jar files in WARBLER_HOME/lib/*.jar into your application. No external dependencies are downloaded.