engine.groovy

def start(){
    println "Start the engines!"
}
def stop(){
    println "Stop at once!"
}
this as Engine

Engine.java

// Engine.java
public interface Engine {
    void start();
    void stop();
}

Main.java

// invoke the script using JSR
import java.io.File;
import java.io.FileReader;
import java.io.Reader;
import javax.script.ScriptContext;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.SimpleScriptContext;

public class Main {

    public static void main(String[] args) {        

        ScriptEngineManager scriptEngineManager = new ScriptEngineManager();
        ScriptEngine groovy = scriptEngineManager.getEngineByExtension("groovy");

        try (Reader reader = new FileReader(new File("engine.groovy"))) {
            ScriptContext scriptContext = new SimpleScriptContext();
            Engine result = (Engine) groovy.eval(reader, scriptContext);
            result.start();
        } catch (Exception e) {
            e.printStackTrace();
        } 
    }
}