This document provides an introduction to jRuby, which allows Ruby code to be run on the Java Virtual Machine. It discusses the history and creator of Ruby, Yukihiro "Matz" Matsumoto, and his goals for the language. It then provides a primer on Ruby syntax including booleans, strings, numbers, arrays, hashes, symbols, and classes. It demonstrates how Ruby code can interact with Java code when using jRuby, such as calling Java classes and methods from Ruby. It also discusses metaprogramming in Ruby and how jRuby enables cross-platform GUI development using Java libraries.

1. jRuby
The best of both worlds
Christopher Spring

2. @autonomous

3. Confessions of a... Rubyist

5. Brief history of

6. Yukihiro “Matz” Matsumoto

7. Yukihiro “Matz” Matsumoto
“I wanted a scripting language that was more powerful
than Perl, and more object-oriented than Python...”

8. “[We] need to focus on humans, on how humans
care about doing programming... We are the masters.
They are the slaves.”

10. “I hope to see Ruby help every programmer in
the world be productive, and to enjoy
programming, and to be happy.
This is the primary purpose of Ruby language”

11. A primer

12. # booleans
truth = true
lies = false
# Strings
dude = 'Matz'
credo = "#{dude} is nice,
so we are nice!"
# Numbers
42 ** 13
53.8 / 9
19 % 3

13. # Arrays
backpack = [6, 'apple', [true, some_object]]
# Hashes
hash = {
true => 'He loves me!',
dude => 'Must obey!',
backpack => false
}
# symbols
cities = [:paris, :new_york, :johannesburg]

14. Wtf is a symbol?

15. coders = [
{ 'language' => 'ruby', 'location' => 'johannesburg' },
{ 'language' => 'java', 'location' => 'port elizabeth' },
# ...
{ 'language' => 'ruby', 'location' => 'pretoria' }
]
# vs.
coders = [
{ :language => 'ruby', :location => 'johannesburg' },
{ :language => 'java', :location => 'port elizabeth' },
# ...
{ :language => 'ruby', :location => 'pretoria' }
]

16. Cool, what about
classes?

17. public Class Person {
private String firstName, lastName;
public Person(String firstName, String lastName){
setFirstName(firstName);
setLastName(lastName);
}
public String getFirstName(){
return firstName;
}
public String getLastName(){
return lastName;
}
public String setFirstName(String firstName){
this.firstName = firstName;
}
public String setLastName(String lastName){
this.lastName = lastName;
}
}

18. class Person
attr_accessor :first_name, :last_name
def initialize(first_name, last_name)
@first_name = first_name
@last_name = last_name
end
end

19. class Person
attr_accessor :first_name, :last_name
def initialize(first_name, last_name)
@first_name = first_name
@last_name = last_name
end
end
person = Person.new( 'Nick', 'Cave')
person.first_name = 'Slick'
person.last_name = 'Nave'
puts "#{person.first_name} #{person.last_name}"

20. Btw, everything is an
Object!

21. true.class() # => TrueClass
false.to_s() # => "false"
5.9.floor() # => 5
3.times{ puts('Hello JUG!') }
# => Hello JUG!
# => Hello JUG!
# => Hello JUG!

22. module Earth
end

23. module Earth
class Person
attr_reader :first_name, :last_name
def initialize(first_name, last_name)
@first_name, @last_name = first_name, last_name
end
end
end

24. module Earth
class Person
attr_reader :first_name, :last_name
def initialize(first_name, last_name)
@first_name, @last_name = first_name, last_name
end
end
end
chap = Earth::Person.new('Chandler', 'Bing')
chap.first_name # => Chandler

25. module RepublicPersonnel
def storm_trooper
Earth::Person.new('Storm', 'Trooper')
end
end
class CloningVat
extend RepublicPersonnel
end

26. module RepublicPersonnel
def storm_trooper
Earth::Person.new('Storm', 'Trooper')
end
end
class CloningVat
extend RepublicPersonnel
end
#<Earth::Person @last_name="Trooper", @first_name="Storm">
CloningVat.storm_trooper

27. module TehForce
def sense
'... a disturbance in the force'
end
end
module Earth
class Person
include TehForce
end
end

28. module TehForce
def sense
'... a disturbance in the force'
end
end
module Earth
class Person
include TehForce
end
end
farm_boy = Earth::Person.new('Luke', 'Skywalker')
# => '... a disturbance in the force'
farm_boy.sense

29. def some_days( something_happens )
puts 'The sun rises'
puts something_happens
puts 'The sun sets'
end
def other_days( this_happens, and_that_happens)
puts 'The sun rises'
puts this_happens
puts and_that_happens
puts 'The sun sets'
end
some_days 'I do some work'
other_days 'I skip', 'I read a book'

30. def some_days( something_happens )
puts 'The sun rises'
puts something_happens
puts 'The sun sets'
end

31. def some_days( something_happens )
puts 'The sun rises'
puts something_happens
puts 'The sun sets'
end

32. def some_days( something_happens )
puts 'The sun rises'
puts something_happens
puts 'The sun sets'
end
def some_days
puts 'The sun rises'
yield
puts 'The sun sets'
end
some_days { puts 'I go to work' }
some_days do
puts 'I skip'
puts 'I read a book'
end

33. file = File.open('i <3 ruby.txt', 'w')
file.puts "... i really do!"
file.close

34. file = File.open('i <3 ruby.txt', 'w')
file.puts "... i really do!"
file.close
File.open('i <3 ruby.txt', 'w') do |file|
file.puts "... i really do!"
end

35. Rubyists TATFT!
describe Calculator do
before(:each) do
@calc = Calculator.new
end
context 'Addition' do
it 'returns 10 when adding 5 and 5'
@calc.add(5,5).should == 10
end
it 'returns 0 when adding 5 and -5' do
@calc.add(5,-5).should == 0
end
end
# ...
end

36. Metaprogramming

37. class Person
def initialize
@height, @weight, @hair_color = 6.2, '70kg', 'brown'
end
%w(height weight hair).each do |property|
define_method("#{property}") do
i_var = self.instance_eval("@#{property}")
i_var.to_s.upcase
end
end
def method_missing(*args, &block)
puts "I dont have #{args[0].to_s.gsub(/_/, ' ')}"
end
end
p = Person.new
p.my_height # => 6.2
p.my_weight # => 70KG
p.my_hair_color # => BROWN
p.your_car_keys # => I dont have your car keys

38. A primer

41. or....

42. Demo time!!

43. Interactive Ruby
$ jirb
jruby-1.6.7 :001 > require 'java'
=> true
jruby-1.6.7 :001 > #...

44. Ackermann

45. class Ackermann
def self.ack(m, n)
return n + 1 if m.zero?
return ack(m - 1, 1) if n.zero?
ack( m - 1, ack(m, n - 1) )
end
end

46. include Rubeus::Swing
JFrame.new('Ackerizer') do |frame|
frame.layout = java.awt.FlowLayout.new
@m, @n = JTextField.new('3'),JTextField.new('10')
JButton.new('->') do
start = Time.now
@result.text = Ackermann.ack(
@m.text.to_i,
@n.text.to_i
).to_s
puts "#{Time.now - start}"
end
@result = JTextField.new 10
frame.pack
frame.show
end

47. class Ackermann
def self.ack(m, n)
return n + 1 if m.zero?
return ack(m - 1, 1) if n.zero?
ack( m - 1, ack(m, n - 1) )
end
end

48. class Ackermann
def self.ack(m, n)
return n + 1 if m.zero?
return ack(m - 1, 1) if n.zero?
ack( m - 1, ack(m, n - 1) )
end
end
public class Ackermann {
public static int ack(int m, int n) {
if (m == 0)
return n + 1;
if (n == 0)
return ack(m - 1, 1);
return ack(m - 1, ack(m, n - 1));
}
}

49. include Rubeus::Swing
JFrame.new('Ackerizer') do |frame|
frame.layout = java.awt.FlowLayout.new
@m, @n = JTextField.new('3'),JTextField.new('10')
JButton.new('->') do
start = Time.now
@result.text = Java::Ackermann.ack(
@m.text.to_i,
@n.text.to_i
).to_s
puts "#{Time.now - start}"
end
@result = JTextField.new 10
frame.pack
frame.show
end

50. Method overloading
public class OverloadTypeChecker {
public static String inspect(long value) {
return "long";
}
public static String inspect(String value) {
return "string";
}
public static String inspect(Object value) {
return "object";
}
}

51. require 'java'
java_import 'OverloadTypeChecker' do
'OTC'
end
# jRuby happily handles the simple case
OTC.inspect(5) # long
OTC.inspect('Helloooo!') # string
OTC.inspect([]) # object

52. public class BitLength {
public int neededFor(int i) {
return 32;
}
public int neededFor(long l) {
return 64;
}
}

53. # jRuby can be forced to use a specific overload
require 'java'
java_import 'BitLength'
bits = BitLength.new
bits.needed_for(1_000_000) # 64

54. # jRuby can be forced to use a specific overload
require 'java'
java_import 'BitLength'
bits = BitLength.new
bits.needed_for(1_000_000) # 64
# java_send
bits.java_send :neededFor, [Java::int], 1_000_000 # 32

55. # jRuby can be forced to use a specific overload
require 'java'
java_import 'BitLength'
bits = BitLength.new
bits.needed_for(1_000_000) # 64
# java_send
bits.java_send :neededFor, [Java::int], 1_000_000 # 32
# java_alias
class BitLength
java_alias :needed_for_int, :neededFor, [Java::int]
end
bits.needed_for_int 1_000_000 # 32

56. # jRuby can be forced to use a specific overload
require 'java'
java_import 'BitLength'
bits = BitLength.new
bits.needed_for(1_000_000) # 64
# java_send
bits.java_send :neededFor, [Java::int], 1_000_000 # 32
# java_alias
class BitLength
java_alias :needed_for_int, :neededFor, [Java::int]
end
bits.needed_for_int 1_000_000 # 32
# java_method
needed = bits.java_method :neededFor, [Java::int]
needed.call 1_000_000 # 32

57. ... and more about
types

58. public class TypeChecker{
public static String check(Object o){
return o.getClass().getName();
}
}
require 'java'
ruby_array = [1, 2, 'Mushrooms', :sample]
java_array = ruby_array.to_java
# org.jruby.RubyArray
Java::TypeChecker.check( ruby_array )
# [Ljava.lang.Object;
Java::TypeChecker.check( java_array )
# org.jruby.RubyArray
Java::TypeChecker.check( java_array.to_a )

59. Sooo, what about those
C extensions... ?

60. Awesome, but why?
• Cross platform GUI dev
• Performance
• Ruby interface to legacy code
• Use a ruby test framework
(Seriously, they’re amazing!)
• Utilize java libraries
• Sneak ruby into a java shop
• Threading

62. import org.jruby.embed.InvokeFailedException;
import org.jruby.embed.ScriptingContainer;
public class RubyFromJava {
public static void main(String[] args) {
ScriptingContainer container = new ScriptingContainer();
container.runScriptlet("puts 'Java touched my Ruby'");
}
}
$ java -cp jruby-complete.jar:. RubyFromJava
Java touched my Ruby

63. More Demos!!

64. http://pragprog.com/book/jruby/using-jruby

65. http://www.meetup.com/Code-Coffee-JHB/

66. http://www.meetup.com/RubyOnBeer/

67. Questions?