I'm trying to use JRuby (through the JSR233 interface included in JRuby 1.5) from a Java application to load a ruby implementation of a Java interface.

My sample implementation looks like this:

Interface:

package some.package;
import java.util.List;
public interface ScriptDemoIf {
    int fibonacci(int d);
    List<String> filterLength(List<String> source, int maxlen);
}

Ruby Implementation:

require 'java'
include Java

class ScriptDemo
  java_implements some.package.ScriptDemoIf
  java_signature 'int fibonacci(int d)'
  def fibonacci(d)
    d < 2 ? d : fibonacci(d-1) + fibonacci(d-2)
  end

  java_signature 'List<String> filterLength(List<String> source, int maxlen)'
  def filterLength(source, maxlen)
    source.find_all { |str| str.length <= maxlen }
  end
end

Class loader:

public ScriptDemoIf load(String filename) throws ScriptException {
    ScriptEngine engine = new ScriptEngineManager().getEngineByName("jruby");
    FileReader script = new FileReader(filename);
    try {
        engine.eval(new FileReader(script));
    } catch (FileNotFoundException e) {
        throw new ScriptException("Failed to load " + filename);
    }
    return (ScriptDemoIf) m_engine.eval("ScriptDemo.new");
}

(Obviously the loader is a bit more generic in real life - it doesn't assume that the implementation class name is "ScriptDemo" - this is just for simplicity).

Problem - I get a class cast exception in the last line of the loader - the engine.eval() return a RubyObject type which doesn't cast down nicely to my interface. From stuff I read all over the web I was under the impression that the whole point of use java_implements in the Ruby section was for the interface implementations to be compiled in properly.

What am I doing wrong?