This document provides an introduction to JRuby, which allows Ruby code to run on the Java Virtual Machine. It discusses key features of Ruby like dynamic typing and everything being an object. It then covers how to integrate Ruby and Java code through the Java integration layer, including calling Java from Ruby and converting interfaces. The document concludes by discussing using JRuby for applications, testing Java with RSpec, and build utilities.

1. A (Quick) Introduction
to JRuby
Frederic Jean
fred@fredjean.net

2. Hello JRuby

3. Develop, Deploy, Build, Test, Monitor
Configuration
DSLs
Glue, Wiring
External System Integration
Dynamic Layer
Application Code
Stable Layer
Java, Statically typed
Legacy libraries

4. Ruby

5. Ruby is...
• Dynamically Typed
• Strongly Typed
• Everything is An Object

6. (Almost) Everything is
an Object

7. Ruby Literals
# Strings
quot;This is a Stringquot;
'This is also a String'
%{So is this}
# Numbers
1, 0xa2, 0644, 3.14, 2.2e10
# Arrays and Hashes
['John', 'Henry', 'Mark']
{ :hello => quot;bonjourquot;, :bye => quot;au revoirquot;}
# Regular Expressions
quot;Mary had a little lambquot; =~ /little/
quot;The London Bridgequot; =~ %r{london}i
# Ranges
(1..100) # inclusive
(1...100) # exclusive

8. Symbols
:symbol
• String-like construct
• Only one copy in memory

9. Blocks
['John', 'Henry', 'Mark'].each { |name| puts quot;#{name}quot;}
File.open(quot;/tmp/password.txtquot;) do |input|
text = input.read
end

10. Ruby Classes
class Person
attr_accessor :name, :title
end
p = Person.new

11. Open Classes
class Fixnum
def odd?
self % 2 == 1
end
def even?
self % 2 == 0
end
end
http://localhost:4567/code/fixnum.rb

12. Mixins
module Parity
def even?
self % 2 == 0
end
def odd?
self % 2 == 1
end
end
http://localhost:4567/code/parity.rb

13. Mixins
class Person
include Enumerable
attr_accessor :first_name, :last_name
def initialize(first_name, last_name)
@first_name = first_name
@last_name = last_name
end
def <=>(other)
if self.last_name == other.last_name
self.first_name <=> other.first_name
else
self.last_name <=> other.last_name
end
end
def to_s
quot;#{first_name} #{last_name}quot;
end
end
http://localhost:4567/code/sorting.rb

14. JRuby

15. JRuby
• Ruby Implementation on the JVM
• Compatible with Ruby 1.8.6
• Native Multithreading
• Full Access to Java Libraries

16. JRuby Hello World
java.lang.System.out.println quot;Hello JRuby!quot;
http://localhost:4567/code/jruby_hello.rb

17. Java Integration
require 'java'

18. JI: Ruby-like Methods
Java Ruby
Locale.US Locale::US
System.currentTimeMillis() System.current_time_millis
locale.getLanguage() locale.language
date.getTime() date.time
date.setTime(10) date.time = 10
file.isDirectory() file.directory?

19. JI: Java Extensions
h = java.util.HashMap.new
h[quot;keyquot;] = quot;valuequot;
h[quot;keyquot;]
# => quot;valuequot;
h.get(quot;keyquot;)
# => quot;valuequot;
h.each {|k,v| puts k + ' => ' + v}
# key => value
h.to_a
# => [[quot;keyquot;, quot;valuequot;]]
http://localhost:4567/code/ji/extensions.rb

20. JI: Java Extensions
module java::util::Map
include Enumerable
def each(&block)
entrySet.each { |pair| block.call([pair.key, pair.value]) }
end
def [](key)
get(key)
end
def []=(key,val)
put(key,val)
val
end
end

21. JI: Open Java Classes
class Java::JavaLang::Integer
def even?
int_value % 2 == 0
end
def odd?
int_value % 2 == 1
end
end
http://localhost:4567/code/ji/integer.rb

22. JI: Interface Conversion
package java.util.concurrent;
public class Executors {
// ...
public static Callable callable(Runnable r) {
// ...
}
}

23. JI: Interface Conversion
class SimpleRubyObject
def run
puts quot;hiquot;
end
end
callable = Executors.callable(SimpleRubyObject.new)
callable.call
http://localhost:4567/code/ji/if_conversion.rb

24. JI: Closure Conversion
callable = Executors.callable { puts quot;hiquot; }
callable.call
http://localhost:4567/code/ji/closure_conversion.rb

25. Develop, Deploy, Build, Test, Monitor
Configuration
DSLs
Glue, Wiring
External System Integration
Dynamic Layer
Application Code
Stable Layer
Java, Statically typed
Legacy libraries

26. Usage
• Runtime
• Wrapper around Java Code
• Calling From Java
• Utilities

27. JRuby as a Runtime

28. JRuby on Rails
• Wrap Rails app into a war file
• Direct support in Glassfish v3 Prelude
• http://kenai.com
• http://mediacast.sun.com

29. webmate
require 'rubygems'
require 'sinatra'
get '/*' do
file, line = request.path_info.split(/:/)
local_file = File.join(Dir.pwd, file)
if (File.exists?(local_file))
redirect quot;txmt://open/?url=file://#{local_file}&line=#{line}quot;
else
not_found
end
end
http://localhost:4567/code/webmate.rb

30. Wrapping Java Code

31. Swing Applications
frame = JFrame.new(quot;Hello Swingquot;)
• Reduce verbosity
• Simplify event handlers
http://localhost:4567/code/swing.rb

32. RSpec Testing Java
describe quot;An emptyquot;, HashMap do
before :each do
@hash_map = HashMap.new
end
it quot;should be able to add an entry to itquot; do
@hash_map.put quot;fooquot;, quot;barquot;
@hash_map.get(quot;fooquot;).should == quot;barquot;
end
end
JTestr Provides Ant and Maven integration
http://jtestr.codehaus.org
http://localhost:4567/code/hashmap_spec.rb

33. Utilities

34. Develop, Deploy, Build, Test, Monitor
Configuration
DSLs
Glue, Wiring
External System Integration
Dynamic Layer
Application Code
Stable Layer
Java, Statically typed
Legacy libraries

35. Build Utilities
• Rake
• Raven
• Buildr

36. Conclusion

37. Resources
• http://jruby.org
• http://wiki.jruby.org
• http://jruby.kenai.com
• http://jtester.codehaus.org
• http://raven.rubyforge.org
• http://buildr.apache.org