Want to use Ruby functionality in Java? Compile it into a JAR

JavaUsingRubyLib.java

import org.jruby.*;
import org.jruby.runtime.ThreadContext;

public class JavaUsingRubyLib {
  public static void main(String[] args) {
    Service s = new Service();
    Ruby rt = Ruby.getGlobalRuntime();
    ThreadContext ctx = rt.getCurrentContext();
    
    RubyArray response1 = (RubyArray) s.send_msg("Test message 1");
    RubyArray response2 = (RubyArray) s.send_msg(2000);

    RubySymbol sym2 = RubySymbol.newSymbol(rt, ":testSymbol");
    RubyArray response3 = (RubyArray) s.send_msg(sym2);
    
    RubyString joinString = RubyString.newString(rt, "->");

    System.out.println(response1.join(ctx, joinString));
    System.out.println(response2.join(ctx, joinString));
    System.out.println(response3.join(ctx, joinString));
  }
}

Service.java

public class Service extends RubyObject  {
    private static final Ruby __ruby__ = Ruby.getGlobalRuntime();
    private static final RubyClass __metaclass__;

    static {
        String source = new StringBuilder("class Service\n" +
            "  attr_accessor :count  # This doesn't generate getters/setters in Java class\n" +
            "\n" +
            "  def count(count)\n" +
            "    @count = count\n" +
            "  end\n" +
            "\n" +
            "  def count\n" +
            "    @count ||= 0\n" +
            "  end\n" +
            "\n" +
            "  def send_msg(msg=\"this is the default message\")\n" +
            "    puts \"Sending #{msg}\"\n" +
            "    [msg, self.count += 1]\n" +
            "  end\n" +
            "end\n" +
            "").toString();
        __ruby__.executeScript(source, "service.rb");
        RubyClass metaclass = __ruby__.getClass("Service");
        metaclass.setRubyStaticAllocator(Service.class);
        if (metaclass == null) throw new NoClassDefFoundError("Could not load Ruby class: Service");
        __metaclass__ = metaclass;
    }

    /**
     * Standard Ruby object constructor, for construction-from-Ruby purposes.
     * Generally not for user consumption.
     *
     * @param ruby The JRuby instance this object will belong to
     * @param metaclass The RubyClass representing the Ruby class of this object
     */
    private Service(Ruby ruby, RubyClass metaclass) {
        super(ruby, metaclass);
    }

    /**
     * A static method used by JRuby for allocating instances of this object
     * from Ruby. Generally not for user comsumption.
     *
     * @param ruby The JRuby instance this object will belong to
     * @param metaclass The RubyClass representing the Ruby class of this object
     */
    public static IRubyObject __allocate__(Ruby ruby, RubyClass metaClass) {
        return new Service(ruby, metaClass);
    }
        
    /**
     * Default constructor. Invokes this(Ruby, RubyClass) with the classloader-static
     * Ruby and RubyClass instances assocated with this class, and then invokes the
     * no-argument 'initialize' method in Ruby.
     *
     * @param ruby The JRuby instance this object will belong to
     * @param metaclass The RubyClass representing the Ruby class of this object
     */
    public Service() {
        this(__ruby__, __metaclass__);
        RuntimeHelpers.invoke(__ruby__.getCurrentContext(), this, "initialize");
    }

    
    public Object count(Object count) {
        IRubyObject ruby_count = JavaUtil.convertJavaToRuby(__ruby__, count);
        IRubyObject ruby_result = RuntimeHelpers.invoke(__ruby__.getCurrentContext(), this, "count", ruby_count);
        return (Object)ruby_result.toJava(Object.class);

    }

    
    public Object count() {

        IRubyObject ruby_result = RuntimeHelpers.invoke(__ruby__.getCurrentContext(), this, "count");
        return (Object)ruby_result.toJava(Object.class);

    }

    
    public Object send_msg(Object msg) {
        IRubyObject ruby_msg = JavaUtil.convertJavaToRuby(__ruby__, msg);
        IRubyObject ruby_result = RuntimeHelpers.invoke(__ruby__.getCurrentContext(), this, "send_msg", ruby_msg);
        return (Object)ruby_result.toJava(Object.class);

    }

}

service.rb

class Service
  attr_accessor :count  # This doesn't generate getters/setters in Java class

  def count(count)
    @count = count
  end

  def count
    @count ||= 0
  end

  def send_msg(msg="this is the default message")
    puts "Sending #{msg}"
    [msg, self.count += 1]
  end
end

# compile with: jrubyc --javac service.rb

service.rb is compiled into Service.java/Service.class by "jrubyc --javac service.rb"
compile JavaUsingRubyLib.java with: javac -cp ~/.rvm/rubies/jruby-1.5.6/lib/jruby.jar:. JavaUsingRubyLib.java
run all with: java -cp ~/.rvm/rubies/jruby-1.5.6/lib/jruby.jar:. JavaUsingRubyLib