[java] Call external javascript functions from java code

By using Java Scripting API, I am able to execute JavaScript within Java. However, can someone please explain what I would need to add to this code in order to be able to call on functions that are in C:/Scripts/Jsfunctions.js

import javax.script.*;

public class InvokeScriptFunction {
public static void main(String[] args) throws Exception {
    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("JavaScript");

    // JavaScript code in a String
    String script1 = "function hello(name) {print ('Hello, ' + name);}";
    String script2 = "function getValue(a,b) { if (a===\"Number\") return 1; 
                     else return b;}";
    // evaluate script
    engine.eval(script1);
    engine.eval(script2);

    Invocable inv = (Invocable) engine;

    inv.invokeFunction("hello", "Scripting!!");  //This one works.      
 }
}

try {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("JavaScript");
        System.out.println("okay1");
        FileInputStream fileInputStream = new FileInputStream("C:/Users/Kushan/eclipse-workspace/sureson.lk/src/main/webapp/js/back_end_response.js");
        System.out.println("okay2");
        if (fileInputStream != null){
         BufferedReader reader = new BufferedReader(new InputStreamReader(fileInputStream));
         engine.eval(reader);
         System.out.println("okay3");
        // Invocable javascriptEngine = null;
         System.out.println("okay4");
        Invocable invocableEngine = (Invocable)engine;
         System.out.println("okay5");
         int x=0;
         System.out.println("invocableEngine is : "+invocableEngine);
         Object object = invocableEngine.invokeFunction("backend_message",x);

         System.out.println("okay6");
        }
        }catch(Exception e) {
            System.out.println("erroe when calling js function"+ e);
        }

Let us say your jsfunctions.js file has a function "display" and this file is stored in C:/Scripts/Jsfunctions.js

jsfunctions.js

var display = function(name) {
print("Hello, I am a Javascript display function",name);
return "display function return"
}

Now, in your java code, I would recommend you to use Java8 Nashorn. In your java class,

import java.io.FileNotFoundException;
import java.io.FileReader;

import javax.script.Invocable;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

class Test {
public void runDisplay() {
ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
try {
  engine.eval(new FileReader("C:/Scripts/Jsfunctions.js"));
  Invocable invocable = (Invocable) engine;
  Object result;
  result = invocable.invokeFunction("display", helloWorld);
  System.out.println(result);
  System.out.println(result.getClass());
  } catch (FileNotFoundException | NoSuchMethodException | ScriptException e) {
    e.printStackTrace();
    }
  }
}

Note: Get the absolute path of your javascript file and replace in FileReader() and run the java code. It should work.