If you are getting exceptions like the one below with JRuby and Rails, read on for the solution

Rendering /tmp/neel/jruby-1.0.1/lib/ruby/gems/1.8/gems/actionpack-1.13.5/lib/
   action_controller/templates/rescues/layout.rhtml (500 Internal Error)
no marshal_dump is defined for class Java::JavaObject
/tmp/neel/jruby-1.0.1/lib/ruby/1.8/pstore.rb:349:in `dump'
/tmp/neel/jruby-1.0.1/lib/ruby/1.8/pstore.rb:327:in `transaction'
/tmp/neel/jruby-1.0.1/lib/ruby/1.8/cgi/session/pstore.rb:81:in `update'
/tmp/neel/jruby-1.0.1/lib/ruby/1.8/cgi/session/pstore.rb:88:in `close'
/tmp/neel/jruby-1.0.1/lib/ruby/1.8/cgi/session.rb:324:in `close'

The problem is that Rails is trying to store the Java object in the session store. The default session store is the filesystem, and it fails since it cannot marshal a Java Object. Setting the session store to use memory instead of the filesystem solves the problem.

In config/environment.rb set

config.action_controller.session_store = :memory_store