Custom Script subclass. Example taken from http://zeroturnaround.com/labs/scripting-your-java-application-with-groovy

gistfile1.groovy

// zt.Engine.java
package zt;

public interface Engine {
   void start();
   void stop();
}

// zt.EngineScript.java
package zt;

import groovy.lang.Script;

public abstract class EngineScript extends Script implements Engine {
}

// engine.groovy
void start(){
   println "Start the engines!"
}
void stop(){
   println "Stop at once!"
}
this

// runner.groovy
import zt.*
import org.codehaus.groovy.control.CompilerConfiguration

// create a configuration
def config = new CompilerConfiguration()
// tweak the configuration
config.scriptBaseClass = 'zt.EngineScript'
// run your script
def shell = new GroovyShell(config)
engineScript = shell.evaluate(new File('engine.groovy').text)

assert engineScript instanceof Engine
engineScript.start()
engineScript.stop()

// run it with
javac -d classes -cp $GROOVY_HOME/embeddable/groovy-all-<version>.jar zt/*
groovy -cp classes runner.groovy

good