Using Java from Ruby with JRuby IRB

Using Java from Ruby
with JRuby IRB
Hiro Asari
Engine Yard

Tuesday, June 14, 2011 1

Why JRuby?

• Interactive shell for looking around
(without writing lots of code)
• Testing frameworks
• Ruby on Rails


Assumptions

• JRuby 1.6.2 (or 1.7.0.dev)
• Java 6 (5 or later; 7 support in the works)
• Ruby 1.8


Ruby syntax will be introduced as we go along.
We do not use tricky syntax for this presentation

Starting IRB

jruby -S irb

http://jruby.org/tryjruby


'-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)

Loading Java support
require 'java'


Like Java's "import"

Finding Java libraries
# shell
export CLASSPATH=…

jruby -J-cp <classpath> …

jruby -I lib …

# in ruby
$: << File.join(File.dirname(__FILE__),
'lib')


Java way, or Ruby way

$: is the global for load paths where Ruby looks for libraries
__FILE__ is a shorthand for "this ﬁle"

Loading Java Library
require 'foo.jar'

$CLASSPATH << 'foo.jar'


Importing Java class
Bazz = Java::foo.bar.Bazz
java_import 'foo.bar.Bazz'
java_import java.lang.System


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.

Referring to Java class
bazz = Java::foo.bar.Bazz.new
bazz = Bazz.new # Bazz is already imported


No parentheses are necessary for Ruby methods, unless ambiguous otherwise

Implementing Java interface
class Foo
include java.lang.Runnable

def run
puts "foo"
end
end


JRuby works hard to make this simpler, but you get the idea

Calling static methods
Bazz.method(arg1, arg2)

java.lang.System.currentTimeMillis


This is similar to calling Ruby's class methods

Accessing static ﬁelds
java.lang.System.out
java.util.logging.Level::INFO


Here, "." and "::" are interchangeable

Type conversion
object.to_java(foo.bar.Bazz)
"Hello, Richmond!".to_java
5.to_java(java.lang.Double)


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.

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


: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"

Complex example with Akka
http://akka.io

“ Akka is the event-driven, the nextand
generation
platform for
scalable
fault-tolerant architectures on the JVM ”


Akka tutorial

Compute π using the Madhava-Leibniz series


http://akka.io/docs/akka/1.1.2/intro/getting-started-ﬁrst-java.html

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)
+…


http://en.wikipedia.org/wiki/Leibniz_formula_for_%CF%80
Does not converge very fast

How does it work?
Pi

Calculate
Work Work

Master PiRouter Worker

Result Result


Red: Actors
Green: Messages (POJO)

Akka actors invoke onReceive(Object) method when a message is sent. A router routes
messages between actors.

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


See handout

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

http://akka.io/downloads/
http://akka.io/downloads/akka-actors-1.1.2.zip
Move conﬁg/* up 1 directory level

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


n.b.: artifacts with uncommon versions may not be found

Maven integration
require 'rubygems'
require 'java'
require 'maven/org.clojure/clojure'

java_import 'clojure.lang.Atom'
Atom.new(nil)


https://gist.github.com/660804

"require 'maven/foo.bar/bazz.jar'" doesn't work the ﬁrst time.

Testing frameworks in Ruby
• RSpec
• Cucumber


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


http://relishapp.com/rspec

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 |

http://cukes.info
https://github.com/cucumber/cucumber/blob/master/examples/i18n/en/features/
addition.feature

IDE

• RubyMine
• NetBeans (separate Ruby plugin for 7.0)
• Eclipse


Other stuff

• Ant integration
• warbler (generate WAR ﬁles)
• Ruboto (JRuby on Android)


Sponsored plug
• JRuby on Rails hosting now available on
Engine Yard's AppCloud


JRubyConf 2011
Washington, DC
August 3-5

http://jrubyconf.com


Thank you!

@hiro_asari
hasari@engineyard.com
asari.ruby@gmail.com
http://github.com/banzaiman


Door prizes!


Using Java from Ruby with JRuby IRB

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