Bringing up a Sinatra app. Thanks to Ola Bini for the split script.

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

require 'app'

run Sinatra::Application

split-jruby.sh

#!/bin/sh

rm -rf jruby-core.jar
rm -rf ruby-stdlib.jar
rm -rf tmp_unpack
mkdir tmp_unpack
cd tmp_unpack
jar xf ../jruby-complete.jar
cd ..
mkdir jruby-core
mv tmp_unpack/org jruby-core/
mv tmp_unpack/com jruby-core/
mv tmp_unpack/jline jruby-core/
mv tmp_unpack/jay jruby-core/
mv tmp_unpack/jruby jruby-core/
cd jruby-core
jar cf ../jruby-core.jar .
cd ../tmp_unpack
jar cf ../ruby-stdlib.jar .
cd ..
rm -rf jruby-core
rm -rf tmp_unpack
rm -rf jruby-complete.jar

warble.rb

# Disable automatic framework detection by uncommenting/setting to false
# Warbler.framework_detection = false

# Warbler web application assembly configuration file
Warbler::Config.new do |config|
  # Temporary directory where the application is staged
  # config.staging_dir = "tmp/war"

  # Application directories to be included in the webapp.
  config.dirs = %w(lib public views)

  # Additional files/directories to include, above those in config.dirs
  # config.includes = FileList["db"]
  config.includes = FileList["appengine-web.xml", "app.rb", "config.ru"]
  # Additional files/directories to exclude
  # config.excludes = FileList["lib/tasks/*"]

  # Additional Java .jar files to include.  Note that if .jar files are placed
  # in lib (and not otherwise excluded) then they need not be mentioned here.
  # JRuby and JRuby-Rack are pre-loaded in this list.  Be sure to include your
  # own versions if you directly set the value
  # config.java_libs += FileList["lib/java/*.jar"]

  # Loose Java classes and miscellaneous files to be placed in WEB-INF/classes.
  # config.java_classes = FileList["target/classes/**.*"]

  # One or more pathmaps defining how the java classes should be copied into
  # WEB-INF/classes. The example pathmap below accompanies the java_classes
  # configuration above. See http://rake.rubyforge.org/classes/String.html#M000017
  # for details of how to specify a pathmap.
  # config.pathmaps.java_classes << "%{target/classes/,}p"

  # Gems to be included. You need to tell Warbler which gems your application needs
  # so that they can be packaged in the war file.
  # The Rails gems are included by default unless the vendor/rails directory is present.
  # config.gems += ["activerecord-jdbcmysql-adapter", "jruby-openssl"]
  # config.gems << "tzinfo"
  config.gems = ['sinatra']

  # Uncomment this if you don't want to package rails gem.
  # config.gems -= ["rails"]

  # The most recent versions of gems are used.
  # You can specify versions of gems by using a hash assignment:
  # config.gems["rails"] = "2.0.2"

  # You can also use regexps or Gem::Dependency objects for flexibility or
  # fine-grained control.
  # config.gems << /^merb-/
  # config.gems << Gem::Dependency.new("merb-core", "= 0.9.3")

  # Include gem dependencies not mentioned specifically
  config.gem_dependencies = true

  # Files to be included in the root of the webapp.  Note that files in public
  # will have the leading 'public/' part of the path stripped during staging.
  # config.public_html = FileList["public/**/*", "doc/**/*"]

  # Pathmaps for controlling how public HTML files are copied into the .war
  # config.pathmaps.public_html = ["%{public/,}p"]

  # Name of the war file (without the .war) -- defaults to the basename
  # of RAILS_ROOT
  config.war_name = "test"

  # Name of the MANIFEST.MF template for the war file. Defaults to the
  # MANIFEST.MF normally generated by `jar cf`.
  # config.manifest_file = "config/MANIFEST.MF"

  # Value of RAILS_ENV for the webapp -- default as shown below
  # config.webxml.rails.env = ENV['RAILS_ENV'] || 'production'

  # Application booter to use, one of :rack, :rails, or :merb. (Default :rails)
  # config.webxml.booter = :rack
  config.webxml.booter = :rack

  # When using the :rack booter, "Rackup" script to use.
  # The script is evaluated in a Rack::Builder to load the application.
  # Examples:
  # config.webxml.rackup = %{require './lib/demo'; run Rack::Adapter::Camping.new(Demo)}
  # config.webxml.rackup = require 'cgi' && CGI::escapeHTML(File.read("config.ru"))

  # Control the pool of Rails runtimes. Leaving unspecified means
  # the pool will grow as needed to service requests. It is recommended
  # that you fix these values when running a production server!
  # config.webxml.jruby.min.runtimes = 2
  # config.webxml.jruby.max.runtimes = 4
  config.webxml.jruby.min.runtimes = 1
  config.webxml.jruby.max.runtimes = 1
  config.webxml.jruby.init.serial = true

  # JNDI data source name
  # config.webxml.jndi = 'jdbc/rails'
  config.java_libs = []
end