app.rb

require 'rubygems'
require 'sinatra'

get '/' do
  "Hello from Sinatra running on Java!"
end

appengine-web.xml

<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
    <application>YOUR-APPLICATION-ID</application>
    <version>1</version>
    <static-files />
    <resource-files />
    <sessions-enabled>false</sessions-enabled>
    <system-properties>
      <property name="jruby.management.enabled" value="false" />
      <property name="os.arch" value="" />
      <property name="jruby.compile.mode" value="JIT"/> <!-- JIT|FORCE|OFF -->
      <property name="jruby.compile.fastest" value="true"/>
      <property name="jruby.compile.frameless" value="true"/>
      <property name="jruby.compile.positionless" value="true"/>
      <property name="jruby.compile.threadless" value="false"/>
      <property name="jruby.compile.fastops" value="false"/>
      <property name="jruby.compile.fastcase" value="false"/>
      <property name="jruby.compile.chainsize" value="500"/>
      <property name="jruby.compile.lazyHandles" value="false"/>
      <property name="jruby.compile.peephole" value="true"/>
   </system-properties>
</appengine-web-app>

config.ru

require 'rubygems'
require 'sinatra'

root_dir = File.dirname(__FILE__)

set :environment, :development
set :root,        root_dir
set :app_file,    File.join(root_dir, 'app.rb')
disable :run

require 'app'

run Sinatra::Application

warble.rb

Warbler::Config.new do |config|
  config.staging_dir = 'app'
  config.gems = ['sinatra']
  config.dirs = []
  config.gem_dependencies = true
 
  config.webxml.booter = :rack
 
  config.webxml.jruby.min.runtimes = 1
  config.webxml.jruby.max.runtimes = 1
  config.webxml.jruby.init.serial = true
 
  config.java_libs = []
end