public interface IFoo {

    public Object run(Object foo);

}

class Foo implements IFoo {

       public Object run(Object foo) {

              println 'Hello World!'

              x = 123

              foo * 10

       }

}

import groovy.lang.GroovyClassLoader;

import java.io.File;

public class InvokeGroovy {

    public static void main(String[] args) {

        ClassLoader cl = new InvokeGroovy().getClass().getClassLoader();

        GroovyClassLoader groovyCl = new GroovyClassLoader(cl);

        try {

            Class groovyClass = groovyCl.parseClass(new File("src/Foo.groovy"));

            IFoo foo = (IFoo) groovyClass.newInstance();

            System.out.println(foo.run(new Integer(2)));

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

}