Embed JRuby Rack applications in your Java container.

In a Maven project, add rack-servlet and jruby-complete to your pom.xml:

<dependencies>
  <dependency>
    <groupId>com.squareup.rack</groupId>
    <artifactId>rack-servlet</artifactId>
    <version>${rack.servlet.version}</version>
  </dependency>
  <dependency>
    <groupId>org.jruby</groupId>
    <artifactId>jruby-complete</artifactId>
    <version>${jruby.version}</version>
  </dependency>
</dependencies>

// Use JRuby to build your Rack application:
ScriptingContainer ruby = new ScriptingContainer();
IRubyObject application = ruby.parse("lambda { |env| [200, {}, ['Hello, World!']] }").run();

// Build a RackServlet with that Rack application:
Servlet servlet = new RackServlet(new JRubyRackApplication(application));

// Install that servlet in your container...
// (See our examples directory for concrete code.)

• Gems: You can use the gem-maven-plugin to put gems in your pom.xml on your test classpath. For production deployments, you'll need to be a little more clever.
• Frameworks: We're successfully running Sinatra applications on Rack Servlet. We've not yet tried Rails. At any rate, you'll need to make separate arrangements for any database connections you may need.
• Logging: rack.logger and rack.errors use slf4j, so that you can choose your logging backend. Log messages are written to com.squareup.rack.RackLogger and com.squareup.rack.RackErrors, respectively.

• Examples: Our examples directory has some good concrete code to get you started.
• Stack Overflow: Use the rack-servlet tag to ask any questions. We'll keep an eye on it.
• GitHub Issues: If you've found a bug, please file an issue.

Rack Servlet grew out of our desire to embed Rack applications in Square's Java service container, in order to take advantage of our sophisticated infrastructure.

There are other options in the "run Rack applications on the JVM" space, all of which are shaped somewhat differently:

• Kirk is a Rack server, like Mongrel or Unicorn.
• Warbler bundles your Rack application into a WAR file.
• jruby-rack is close in intent to Rack Servlet. It offers amazing support for instantiating and pooling your Rack application objects, though its custom embedding story is less clear.