Creating a simple Swing application in JRuby

First of all, for those who may not know, JRuby is an implementation of the Ruby programming language in Java. In addition to being a Ruby interpreter, JRuby also integrates with the Java platform, so you can interact with your existing Java classes from JRuby.

Being implemented in Java, JRuby of course has full support for Unicode. It also supports the Bean Scripting Framework. Supposedly you can run Ruby on Rails with JRuby, although I have not yet tried doing so myself. For more information about JRuby, go to their web site.

For those who’d like to try it, I’ll give you a very simple Swing application. If you are a bit familiar with both Ruby and Java, then you’ll get into JRuby in no time.

 1 #! /usr/local/bin/jruby -w
 2 
 3 require 'java'
 4 
 5 include_class 'java.awt.event.ActionListener'
 6 include_class 'javax.swing.JButton'
 7 include_class 'javax.swing.JFrame'
 8 
 9 class ClickAction < ActionListener
10   def actionPerformed(event)
11     puts "Button got clicked."
12   end
13 end
14 
15 class MainWindow < JFrame
16   def initialize
17     super "JRuby/Swing Demo"
18     setDefaultCloseOperation JFrame::EXIT_ON_CLOSE
19 
20     button = JButton.new "Click me!"
21     button.addActionListener ClickAction.new
22     add button
23     pack
24   end
25 end
26 
27 MainWindow.new.setVisible true

There that should be it. Remember, this is Ruby, so there’s no need to call the file anything special, and no “only one public class per file” restriction.

So how do you run this program? First of all, you need to have JRuby installed, which I assume you have. I installed my copy into /usr/local/jruby. I then created a symlink from /usr/local/jruby/bin/jruby to /usr/local/bin/jruby. That way the JRuby interpreter behaves just like the “normal” Ruby interpreter.

Assuming you have saved your program as swingtest.rb, installed JRuby and created your symlink, you should just have to do the following:


% chmod +x swingtest.rb
% ./swingtest.rb


Simple, wasn’t it?

Update

As I was writing this post, I downloaded the latest version of JRuby and the code above no longer runs. It turns out that you can no longer extend an interface. You are affected if you get this error:


% jruby button.rb
button.rb:[13,19]:[326,432]: superclass must be a Class (#) given (TypeError)


Fortunately, it’s very simple to fix. Simply change the ClickAction class to look as follows:

class ClickAction
  include ActionListener

  def actionPerformed(event)
    puts "Button got clicked."
  end
end

Now it should run fine. Still simple.

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Update 2007.05.17: fixed the code to work with JRuby-1.0RC2.

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Possibly related posts: (automatically generated)

• Getting started with JRuby
• Getting started with JRuby-on-Rails (Part 1 of 3)
• Getting started with JRuby2