JRuby is a Ruby implementation that runs on the Java Virtual Machine (JVM). It allows Ruby code to leverage Java libraries and deployment options like WAR files. JRuby code is compiled to JVM bytecode, enabling features like multithreading and just-in-time compilation for performance. While C extensions and startup time are drawbacks, JRuby provides advantages for Java interoperability, threading, and deployment in Java environments.

1. JRuby
The Best of Java
and Ruby
by
Evgeny Rahman
Image source: https://github.com/jruby/collateral/ by Tony Price (tonyxprice)
JRuby is Copyright (c) 2007-2012 The JRuby project

2. A little bit about Ruby
• Created in 1993 by Yukihiro Matsumoto
(“Matz”).
• An object-oriented scripting language.
• Guiding philosophy - a language that feels
natural to programmers.
• Principle of least surprise.

3. Adoption
• First English mailing list appeared in 1999.
• “Programming Ruby” (aka “The Pickaxe
Book”) by Dave Thomas and Andy Hunt
released in 2000.
• Interest grew considerably in 2005 because of
the Rails framework.
• 225,000 Ruby projects on GitHub.

4. Examples
• The all-important Hello World:
puts "Hello World"
• Array iteration:
['a', 'b', 'c', 'd', 'e'].each { |x| puts x }
• Hash operations:
animal_sounds = { :dog => 'woof', :cat => 'meow'}
animal_sounds[:dog]
#=> 'woof'

5. Examples
• unless:
"Unicorns exist!" unless unicorns.empty?
• Blocks:
flights = flights.sort do |flight_a, flight_b|
flight_a.price <=> flight_b.price
end
• Loops:
100.times { |i| puts "The number is #{i}" }

6. JRuby
• Created by Jan Arne Petersen in 2001.
• Ruby implementation that runs on the JVM.
• Current core team of nine developers.
• Currently 1.7.0 is out in preview.
• Supports MRI 1.8.7 and 1.9.3.
• Code is JIT and AOT compiled into JVM
bytecode.

7. Advantages
• Java interop – use the Java ecosystem and
your existing libraries.
• Threading – real, JVM multithreading.
• InvokeDynamic in Java 7 – optimization in
performance for JRuby.
• Deployment – several options, including
familiar, Java ones.

8. Java interop – simple!
require 'java'
java_import 'java.lang.Math'
puts java.lang.Math::PI
#=> 3.14159265358979

9. Java interop – libraries
require 'java‘
require 'path/to/yourlib.jar'
Now if you do:
java_import "org.your.lib.YourClass"
Your class is available as
Java-style: org.your.lib.YourClass
Ruby-style: Java::YourLib::YourClass
and directly referenced as
YourClass
More information: https://github.com/jruby/jruby/wiki/CallingJavaFromJRuby

10. Caveats
• Gems with C extensions – use alternatives.
• JVM startup time – TDD, scripting.
• No compilation – more testing, more
discipline.
• Development-Deployment gap.

11. Deployment
• This was the missing link for Ruby in the enterprise *
• Warbler – create a WAR and deploy it to a Java EE container.
• jet.pack – package your JRuby rack app for Jetty.
• Trinidad – run Rails or Rack applications on embedded Tomcat container.
• Mizuno – jetty-powered server for JRuby in the style of Thin or WEBRick
• Torquebox – JBoss AS-based Ruby application platform
• Rack (Rails, Sinatra), JDBC, Daemons, Scheduled Jobs, Messaging,
Asynchronous Tasks
• “Deploying with JRuby” – Joe Kutner, The Pragmatic Bookshelf.
* My humble opinion

12. Community
• Really nice people!
• Core team is great, but there are also many
other contributors.
• Blogs, Twitter, IRC, Mailing Lists, Wiki, Issue
tracker.
• JRubyConf organized every year since 2009 –
this year’s was in Minneapolis, MN.
• Best place to start: jruby.org

13. Resources
• http://jruby.org/
• http://github.com/jruby/ - GitHub
• http://jira.codehaus.org/browse/JRUBY - issue
tracker
• http://kenai.com/projects/jruby/pages/Home -
old wiki
• https://github.com/jruby/jruby/wiki/ - new wiki
• #jruby IRC channel on FreeNode IRC
• @jruby on Twitter