How to create a Ruby module in Java using JRuby?

Question

In Ruby, I can have a module like:

module Greeter
  def greet
    print "Hello"
  end
end

And my class can get the greet method like this:

class MyClass
  include Greeter
end

obj = MyClass.new
obj.greet

Now, I would like to have my module Greeter implemented in Java instead. I'm using JRuby. I'm unsure about how to create a Ruby module in Java (in such a way that I can do include normally).

For a moment I though of making a Java interface. Including it in my Ruby class doesn't throw errors, but it really isn't the same thing since modules seem to implement the methods, whereas a Java interface doesn't.

Um... why? How is this going to help? I think it'd obfuscate what your code is doing, and not in a good way. — the Tin Man
– the Tin Man, Commented Nov 20, 2013 at 1:01
In jruby I commonly find myself pushing ruby code down into java implementations for performance reasons. — phs
– phs, Commented Nov 20, 2013 at 1:02
@theTinMan: Mostly curiosity. Frankly though, hiding such module wouldn't be so bad for me. I'm writing a game development framework and honestly, the end-user definitely doesn't want to mess around with this module - they only need (and should) include it in their Ruby classes. — Saturn
– Saturn, Commented Nov 20, 2013 at 1:20

Maurício Linhares · Accepted Answer · 2013-11-23 13:48:49Z

Here's how would implement exactly the same code you have above using a Java extension.

Everything here is at the default package level (no package), if you would like to have packages just change the module name.

First, create the Greeter class:

import org.jruby.anno.JRubyMethod;
import org.jruby.runtime.ThreadContext;
import org.jruby.runtime.builtin.IRubyObject;

public class Greeter {

    @JRubyMethod
    public static void greet( ThreadContext context, IRubyObject self ) {
        System.out.printf("Hello from %s%n", self);
    }

}

Then you need the GreeterService to load it:

import org.jruby.Ruby;
import org.jruby.RubyModule;
import org.jruby.runtime.load.BasicLibraryService;

import java.io.IOException;

public class GreeterService implements BasicLibraryService {

    @Override
    public boolean basicLoad(final Ruby runtime) throws IOException {
        RubyModule greeter = runtime.defineModule(Greeter.class.getSimpleName());
        greeter.defineAnnotatedMethods(Greeter.class);

        return true;
    }

}

And with these classes defined, here's how you can use them in a JRuby script:

require 'target/jruby-example.jar'
require 'greeter'

class MyClass
  include Greeter
end

obj = MyClass.new
obj.greet

The jruby-example.jar contains both classes above compiled and packaged. There isn't much else to be done here, now you have your module that you can include anywhere. For a bigger example, just check how the Enumerable module is implemented.

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