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.

WWW: http://caldersphere.rubyforge.org/warbler/