http://log.kares.org/2012/04/more-server-dependencies-with-jruby.html

0_Gemfile.rb

source 'https://rubygems.org'

gem 'rails', '~> 3.2.3'
# ...

platforms :jruby do
  gem 'trinidad', '~> 1.4', :require => false
  gem 'trinidad_mysql_dbpool_extension', :require => false
end

gem 'thin', :require => nil, :platforms => :ruby # for dev

1_Gemfile.rb

# ...
group :server do
  platforms :jruby do
    gem 'trinidad', :require => false
    gem 'trinidad_daemon_extension', :require => false
  end
end

2_deploy.rb

namespace :trinidad do
  task :start, :roles => :app do
    trinidad = "trinidad -e production -d #{current_path}"
    run "cd #{current_path} && jruby -S bundle exec #{trinidad}"
  end
end

3_deploy.rb

set :bundle_cmd, "jruby -S bundle"
set :bundle_without, [ :development, :test, :server ]