Question

I am experimenting around with JRuby – generating java from ruby files. I have an abstract class in ruby that implement a Java interface, and child classes extending this. also in ruby.

I’m running into the problem as described on http://jira.codehaus.org/browse/JRUBY-6342 where all the generated java files only extend RubyObject.

I am wondering if anyone else has encountered this and have a workaround? Right now I have used a java_implement interface in each child class as they do not extend the abstract class.

I have included the snippet from JRUBY-6342 describing the problem:

The Java code generated by jrubyc –java does not appear to support Ruby class inheritance. Given the following simple example:

class A
def my_class; self.class.name end
end

class B < A
end

The generated class in B.java inherits from RubyObject rather than A, rendering the B class completely broken in Java.
On a somewhat related note, module inclusion doesn’t seem to work either. A class with include M doesn’t get M’s methods in the generated Java code.

Am I missing something in my understanding of Ruby or JRuby?

Answer 1

This is still an issue indeed as the jruby compiler still produces RubyObject for the classes.

The only workaround that I know to this is to use the JRuby ScriptEngine from Java to eval your JRuby code. For example, here is some JRuby code:

require 'java'

java_import 'javax.swing.JFrame'
java_import 'javax.swing.JButton'

class MyFrame < JFrame
  def initialize
    super('Test')
    content_pane.add(JButton.new("Hello"))
    pack()
  end
end

This code can then be called from a Java class like this:

import javax.swing.JFrame;

import javax.script.*;
import java.io.*;

public class Main {
    public static void main(String[] args) throws Exception {

    ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("jruby");
        Reader reader = new FileReader("myframe.rb");
        engine.eval(reader);

        // Instantiate the JRuby class, and cast the result of eval.
        JFrame frame = (JFrame) engine.eval("MyFrame.new");
        frame.setVisible(true);
    }
}

Here the object returned by eval can be casted into JFrame, just as you would expect. See also this question for that problem.