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Rack for JRuby and Java appservers


JRuby-Rack is a lightweight adapter for the Java servlet environment that allows any Rack-based application to run unmodified in a Java servlet container. JRuby-Rack supports Rails as well as any Rack-compatible Ruby web framework.

For more information on Rack, visit http://rack.rubyforge.org.

Build Status

Getting Started

The easiest way to use JRuby-Rack is to get Warbler. Warbler depends on the latest version of JRuby-Rack and ensures it gets placed in your WAR file when it gets built.

If you’re assembling your own WAR using other means, you can install the jruby-rack gem. It provides a method to locate the jar file:

require 'fileutils'
require 'jruby-rack'
FileUtils.cp JRubyJars.jruby_rack_jar_path, '.'

Otherwise you’ll need to download the latest jar release, drop it into the WEB-INF/lib directory and configure the RackFilter in your application’s web.xml. Example web.xml snippets are as follows.

For Rails

Here’s sample web.xml configuration for Rails. Note the environment and min/max runtime parameters. For multi-threaded Rails with a single runtime, set min/max both to 1. Otherwise, define the size of the runtime pool as you wish.


  <!-- optional filter configuration init-params : -->
    <param-value>true</param-value> <!-- true (default), false or buffer -->
    <param-value>true</param-value> <!-- true (default), false -->
    <param-value>false</param-value> <!-- true, false (default) -->


For Other Rack Applications

Here’s a sample web.xml configuration for a Rack application. The main difference is that JRuby-Rack looks for a “rackup” file named config.ru in WEB-INF/config.ru or WEB-INF/*/config.ru.



If you don’t have a config.ru or don’t want to include it in your web app, you can embed it in web.xml as follows (using Sinatra as an example).

Be sure to escape angle-brackets for XML !

    require 'rubygems'
    gem 'sinatra', '~&gt; 1.3'
    require './lib/app'
    set :run, false
    set :environment, :production
    run Sinatra::Application


Servlet Filter

JRuby-Rack’s main mode of operation is as a servlet filter. This allows requests for static content to pass through and be served by the application server. Dynamic requests only happen for URLs that don’t have a corresponding file, much like many Ruby applications expect. The application can also be configured to dispatch through a servlet instead of a filter if it suits your environment better.

Servlet environment integration

  • Servlet context is accessible to any application through the Rack environment variable java.servlet_context as well as the $servlet_context global.
  • Servlet request object is available in the Rack environment via the key java.servlet_request.
  • Servlet request attributes are passed through to the Rack environment.
  • Rack environment variables and headers can be overridden by servlet request attributes.
  • Java servlet sessions are available as a session store for Rails. Session attributes with String keys and String, numeric, boolean, or java object values are automatically copied to the servlet session for you.


Several aspects of Rails are automatically set up for you.

  • The Rails controller setting ActionController::Base.relative_url_root is set for you automatically according to the context root where your webapp is deployed.
  • Rails.logger output is redirected to the application server log.
  • Page caching and asset directories are configured appropriately.

JRuby Runtime Management

JRuby runtime management and pooling is done automatically by the framework. In the case of Rails, runtimes are pooled. For other Rack applications, currently, a single runtime is created and shared for every request.

JRuby-Rack Configuration

JRuby-Rack can be configured by setting these key value pairs either as context init parameters in web.xml or as VM-wide system properties.

  • rackup: Rackup script for configuring how the Rack application is mounted. Required for Rack-based applications other than Rails. Can be omitted if a config.ru is included in the application root.
  • jruby.min.runtimes: For non-threadsafe Rails applications using a runtime pool, specify an integer minimum number of runtimes to hold in the pool.
  • jruby.max.runtimes: For non-threadsafe Rails applications, an integer maximum number of runtimes to keep in the pool.
  • jruby.init.serial: When using runtime pooling, indicate that the runtime pool should be created serially in the foreground rather than spawning background threads. For environments where creating threads is not permitted.
  • jruby.compat.version: Set to “1.8” or “1.9” to make JRuby run a specific version of Ruby.
  • public.root: Relative path to the location of your application’s static assets. Defaults to /.
  • gem.path: Relative path to the bundled gem repository. Defaults to /WEB-INF/gems.
  • rails.root: Root path to the location of the Rails application files. Defaults to /WEB-INF.
  • rails.env: Specify the Rails environment to run. Defaults to ‘production’.
  • rails.relative_url_append: Specify a path to be appended to the ActionController::Base.relative_url_root after the context path. Useful for running a rails app from the same war as an existing app, under a sub-path of the main servlet context root.
  • jruby.rack.logging: Specify the logging device to use. Defaults to servlet_context. See below.
  • jruby.rack.ignore.env: Clears out the ENV hash in each runtime to insulate the application from the environment.
  • jruby.rack.request.size.initial.bytes: Initial size for request body memory buffer, see also jruby.rack.request.size.maximum.bytes bellow.
  • jruby.rack.request.size.maximum.bytes: The maximum size for the request in memory buffer, if the body is larger than this it gets spooled to a tempfile.
  • jruby.rack.filter.adds.html: deprecated use addsHtmlToPathInfo filter config init parameter. The default behavior for Rails and many other Ruby applications is to add an .html extension to the resource and attempt to handle it before serving a dynamic request on the original URI. However, this behavior may confuse other servlets in your application that have a wildcard mapping. Defaults to true.
  • jruby.rack.filter.verify.resource.exists: deprecated use verifiesHtmlResource filter config init parameter. If jruby.rack.filter.adds.html is true, then this setting, when true, adds an additional check using ServletContext#getResource to verify that the .html resource exists. Default is false. (Note that apparently some servers may not implement getResource in the way that is expected here, so in that case this setting won’t matter.)


There are often cases where you need to perform custom initialization of the Ruby environment before booting the application. You can create a file called META-INF/init.rb or WEB-INF/init.rb inside the war file for this purpose. These files, if found, will be evaluated before booting the Rack environment, allowing you to set environment variables, load scripts, etc.


JRuby-Rack sets up a delegate logger for Rails that sends logging output to javax.servlet.ServletContext#log by default. If you wish to use a different logging system, configure jruby.rack.logging as follows:

  • servlet_context (default): Sends log messages to the servlet context.
  • stdout: Sends log messages to the standard output stream System.out.
  • commons_logging: Sends log messages to Apache commons-logging. You still need to configure commons-logging with additional details.
  • slf4j: Sends log messages to SLF4J. Again, SLF4J configuration is left up to you.
  • log4j: Sends log messages to log4J. Again, Log4J configuration is left up to you.

For those loggers that require a specific named logger, set it in the jruby.rack.logging.name option.


Checkout the JRuby Rack code and cd to that directory.

git clone git://github.com/jruby/jruby-rack.git
cd jruby-rack

Ensure you have Maven installed. It is required for downloading jar artifacts that JRuby-Rack depends on.

You can choose to build with either Maven or Rake. Either of the following two will suffice.

mvn install
jruby -S rake

The generated jar should be located here: target/jruby-rack-*.jar.


Please use GitHub to file bugs, patches and pull requests.


For JRuby-Rack contributors, the release process goes something like the following:

  1. Ensure that release version is correct in pom.xml and mvn install runs clean.
  2. Ensure generated changes to src/main/ruby/jruby/rack/version.rb are checked in.
  3. Ensure History.txt is updated with latest release information.
  4. Tag current release in git: git tag <version>.
  5. Push commits and tag: git push origin master --tags
  6. Build gem: rake clean gem
  7. Push gem: gem push target/jruby-rack-*.gem
  8. Release jar to maven repository: mvn -DupdateReleaseInfo=true deploy
  9. Bump the version in pom.xml to next release version X.X.X.dev-SNAPSHOT, run mvn install, and commit the changes.

Rails Step-by-step

This example shows how to create and deploy a simple Rails app using the embedded Java database H2 to a WAR using Warbler and JRuby-Rack.

Install Rails and the ActiveRecord adapters + driver for the H2 database:

jruby -S gem install rails activerecord-jdbch2-adapter

Install Warbler:

jruby -S gem install warbler

Make the “Blog” application

jruby -S rails new blog
cd blog

Copy this configuration into config/database.yml:

  adapter: jdbch2
  database: db/development_h2_database

  adapter: jdbch2
  database: db/test_h2_database

  adapter: jdbch2
  database: db/production_h2_database

Add the following to your application’s Gemfile:

gem 'activerecord-jdbch2-adapter', :platform => :jruby

Generate a scaffold for a simple model of blog comments.

jruby script/rails generate scaffold comment name:string body:text

Run the database migration that was just created as part of the scaffold.

jruby -S rake db:migrate

Start your application on 3000 using WEBrick and make sure it works:

jruby script/rails server

Generate a production version of the H2 database for the application:

RAILS_ENV=production jruby -S rake db:migrate

Generate a custom Warbler WAR configuration for the blog application

jruby -S warble config

Edit config/warble.rb and add the following line after these comments:

# Additional files/directories to include, above those in config.dirs
# config.includes = FileList["db"]
config.includes = FileList["db/production_h2*"]

This will tell Warbler to include the just initialized production H2 database in the WAR.

Now generate the WAR file:

jruby -S warble war

This task generates the file: blog.war at the top level of the application as well as an exploded version of the war located at tmp/war.

The war should be ready to deploy to your Java application server.


  • All contributors! But also:
  • Dudley Flanders, for the Merb support
  • Robert Egglestone, for the original JRuby servlet integration project, Goldspike
  • Chris Neukirchen, for Rack
  • Sun Microsystems, for early project support
  • Engine Yard, for more recent support

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