Slides to my talk at Richmond JUG + Central Virginia Ruby Enthusiasts' Group on June 14, 2011.

1. Using Java from Ruby
with JRuby IRB
Hiro Asari
Engine Yard
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2. Why JRuby?
• Interactive shell for looking around
(without writing lots of code)
• Testing frameworks
• Ruby on Rails
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3. Assumptions
• JRuby 1.6.2 (or 1.7.0.dev)
• Java 6 (5 or later; 7 support in the works)
• Ruby 1.8
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Ruby syntax will be introduced as we go along.
We do not use tricky syntax for this presentation

4. Starting IRB
jruby -S irb
http://jruby.org/tryjruby
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'-S' tells jruby to look for a script thus named in certain locations (in predefined order) and
execute it. (of course, the usual semantics re: scripts hold here.)
"tryjruby" has some limitations w.r.t. Java integration (obviously)

5. Loading Java support
require 'java'
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Like Java's "import"

6. Finding Java libraries
# shell
export CLASSPATH=…
jruby -J-cp <classpath> …
jruby -I lib …
# in ruby
$: << File.join(File.dirname(__FILE__),
'lib')
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Java way, or Ruby way
$: is the global for load paths where Ruby looks for libraries
__FILE__ is a shorthand for "this file"

7. Loading Java Library
require 'foo.jar'
$CLASSPATH << 'foo.jar'
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8. Importing Java class
Bazz = Java::foo.bar.Bazz
java_import 'foo.bar.Bazz'
java_import java.lang.System
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Ruby Class names are separated by namespaces, which is delimited by double colons.
Java's built-in classes don't need to be referred to via a String
Ruby class names are constants, the name of which must start with a capital letter.

9. Referring to Java class
bazz = Java::foo.bar.Bazz.new
bazz = Bazz.new # Bazz is already imported
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No parentheses are necessary for Ruby methods, unless ambiguous otherwise

10. Implementing Java interface
class Foo
include java.lang.Runnable
def run
puts "foo"
end
end
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JRuby works hard to make this simpler, but you get the idea

11. Calling static methods
Bazz.method(arg1, arg2)
java.lang.System.currentTimeMillis
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This is similar to calling Ruby's class methods

12. Accessing static fields
java.lang.System.out
java.util.logging.Level::INFO
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Here, "." and "::" are interchangeable

13. Type conversion
object.to_java(foo.bar.Bazz)
"Hello, Richmond!".to_java
5.to_java(java.lang.Double)
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For the most part, JRuby converts types for you behind the scenes, so you don’t need to
worry about this; but when you do, you can use this.
Cannot be used for casting to primitives. Sorry.

14. Invoking overloaded method
// you need a cast in Java…
java.lang.System.out.println((char)70)
java.lang.System.out.java_send :println,
[Java::char], 70
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:println is an example of a Ruby symbol. It is a uniquely identifiable object within the Ruby
runtime. "abc" and "abc" maybe two different Strings, but :abc is always unique.
Notice how we specify the Java primitive "char"

15. Complex example with Akka
http://akka.io
“ Akka is the event-driven, the nextand
generation
platform for
scalable
fault-tolerant architectures on the JVM ”
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16. Akka tutorial
Compute π using the Madhava-Leibniz series
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http://akka.io/docs/akka/1.1.2/intro/getting-started-first-java.html

17. π
= 4(1-1/3+1/5-1/7+1/9-1/11+1/13+…)
= 4(1-1/3+1/5-1/7)
+ 4(1/9-1/11+1/13-1/15)
+ 4(1/17-1/19+1/21-1/23)
+…
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http://en.wikipedia.org/wiki/Leibniz_formula_for_%CF%80
Does not converge very fast

18. How does it work?
Pi
Calculate
Work Work
Master PiRouter Worker
Result Result
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Red: Actors
Green: Messages (POJO)
Akka actors invoke onReceive(Object) method when a message is sent. A router routes
messages between actors.

19. Akka tutorial
Code reading and demo
Java source: http://bit.ly/lTClmr
Ruby source: https://gist.github.com/1013227 (Without comments)
(See also) https://gist.github.com/1013217
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See handout

20. Directory layout
akka-tutorial-first
├── akka-reference.conf
├── akka.conf
├── lib/
│ ├── akka/
│ │ ├── akka-actor-1.1.2.jar
│ │ ├── akka-actor-tests-1.1.2.jar
│ │ ├── akka-http-1.1.2.jar
│ │ ├── akka-remote-1.1.2.jar
│ │ ├── akka-slf4j-1.1.2.jar
│ │ ├── akka-stm-1.1.2.jar
│ │ ├── akka-testkit-1.1.2.jar
│ │ ├── akka-typed-actor-1.1.2.jar
│ │ ├── aopalliance-1.0.jar
│ │ ├── aspectwerkz-2.2.3.jar
│ │ ├── commons-codec-1.4.jar
│ │ ├── commons-io-2.0.1.jar
│ │ ├── dispatch-json_2.9.0-0.8.1.jar
│ │ ├── guice-all-2.0.jar
│ │ ├── h2-lzf-1.0.jar
│ │ ├── jackson-core-asl-1.7.1.jar
│ │ ├── jackson-mapper-asl-1.7.1.jar
│ │ ├── jsr250-api-1.0.jar
│ │ ├── jsr311-api-1.1.jar
│ │ ├── multiverse-alpha-0.6.2.jar
│ │ ├── netty-3.2.3.Final.jar
│ │ ├── objenesis-1.2.jar
│ │ ├── protobuf-java-2.3.0.jar
│ │ ├── sjson_2.9.0-0.11.jar
│ │ └── slf4j-api-1.6.0.jar
│ └── scala-library.jar
├── pi.pdf
├── pi.rb
└── pi2.rb
2 directories, 31 files
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http://akka.io/downloads/
http://akka.io/downloads/akka-actors-1.1.2.zip
Move config/* up 1 directory level

21. Maven integration
$ jruby -S gem install
mvn:org.clojure:clojure -v 1.2.1
Successfully installed
mvn:org.clojure:clojure-1.2.1-java
1 gem installed
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n.b.: artifacts with uncommon versions may not be found

22. Maven integration
require 'rubygems'
require 'java'
require 'maven/org.clojure/clojure'
java_import 'clojure.lang.Atom'
Atom.new(nil)
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https://gist.github.com/660804
"require 'maven/foo.bar/bazz.jar'" doesn't work the first time.

23. Testing frameworks in Ruby
• RSpec
• Cucumber
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24. RSpec
describe Game do
describe "#score" do
it "returns 0 for all gutter game" do
game = Game.new
20.times { game.roll(0) }
game.score.should == 0
end
end
end
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http://relishapp.com/rspec

25. Cucumber
# language: en
Feature: Addition
In order to avoid silly mistakes
As a math idiot
I want to be told the sum of two numbers
Scenario Outline: Add two numbers
Given I have entered <input_1> into the calculator
And I have entered <input_2> into the calculator
When I press <button>
Then the result should be <output> on the screen
Examples:
| input_1 | input_2 | button | output |
| 20 | 30 | add | 50 |
| 2 | 5 | add | 7 |
| 0 | 40 | add | 40 |
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http://cukes.info
https://github.com/cucumber/cucumber/blob/master/examples/i18n/en/features/
addition.feature

26. IDE
• RubyMine
• NetBeans (separate Ruby plugin for 7.0)
• Eclipse
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27. Other stuff
• Ant integration
• warbler (generate WAR files)
• Ruboto (JRuby on Android)
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28. Sponsored plug
• JRuby on Rails hosting now available on
Engine Yard's AppCloud
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29. JRubyConf 2011
Washington, DC
August 3-5
http://jrubyconf.com
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30. Thank you!
@hiro_asari
hasari@engineyard.com
asari.ruby@gmail.com
http://github.com/banzaiman
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31. Door prizes!
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32. Door prizes!
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