22nd July 2018, 3 min read

Passing HashMap from Java to Java Nashorn

Original post is here eklausmeier.goip.de/blog/2018/07-22-passing-hashmap-from-java-to-java-nashorn.

Java Nashorn is the JavaScript engine shipped since Java 8. You can therefore use JavaScript wherever you have at least Java 8. Java 8 also has a standalone interpreter, called jjs.

It is possible to create a Java HashMap and use this structure directly in JavaScript. Here is the code:

import java.util.*;
import java.io.*;
import javax.script.*;


public class HashMapDemo {

        public static void main(String[] args) {
                HashMap hm = new HashMap();

                hm.put("A", new Double(3434.34));
                hm.put("B", new Double(123.22));
                hm.put("C", new Double(1200.34));
                hm.put("D", new Double(99.34));
                hm.put("E", new Double(-19.34));

                for( String name: hm.keySet() )
                        System.out.println(name + ": "+ hm.get(name));

                // Increase A's balance by 1000
                double balance = ((Double)hm.get("A")).doubleValue();
                hm.put("A", new Double(balance + 1000));
                System.out.println("A's new account balance : " + hm.get("A"));

                // Call JavaScript from Java
                try {   
                        ScriptEngine engine = new ScriptEngineManager().getEngineByName("nashorn");
                        engine.eval("print('Hello World');");
                        engine.eval(new FileReader("example.js"));
                        Invocable invocable = (Invocable) engine;
                        Object result = invocable.invokeFunction("sayHello", "John Doe");
                        System.out.println(result);
                        System.out.println(result.getClass());

                        result = invocable.invokeFunction("prtHash", hm);
                        System.out.println(result);
                } catch (FileNotFoundException | NoSuchMethodException | ScriptException e) {
                        e.printStackTrace();
                        System.out.println(e);
                }

        }
}

And here is the corresponding JavaScript file example.js:

var sayHello = function(name) {
        print('Hello, ' + name + '!');
        return 'hello from javascript';
};

var prtHash = function(h) {
        print('h.A = ' + h.A);
        print('h.B = ' + h["B"]);
        print('h.C = ' + h.C);
        print('h.D = ' + h["D"]);
        print('h.E = ' + h.E);
};

Output is:

$ java HashMapDemo
A: 3434.34
B: 123.22
C: 1200.34
D: 99.34
E: -19.34
A's new account balance : 4434.34
Hello World
Hello, John Doe!
hello from javascript
class java.lang.String
h.A = 4434.34
h.B = 123.22
h.C = 1200.34
h.D = 99.34
h.E = -19.34
null

Above example uses sample code from

1. Riding the Nashorn: Programming JavaScript on the JVM (dead link)
2. Simple example for Java HashMap
3. Nashorn: Run JavaScript on the JVM

Decisive was the statement in https://winterbe.com/posts/2014/04/05/java8-nashorn-tutorial/:

Java objects can be passed without loosing any type information on the javascript side. Since the script runs natively on the JVM we can utilize the full power of the Java API or external libraries on nashorn.

Above program works the same if one changes HashMap to HashMap and populating accordingly, e.g.:

HashMap hm = new HashMap();

hm.put("A", new Double(3434.34));
hm.put("B", new String("Test"));
hm.put("C", new Date(5000));
hm.put("D", new Integer(99));
hm.put("E", new Boolean(Boolean.TRUE));

Output from JavaScript would be

h.A = 4434.34
h.B = Test
h.C = Thu Jan 01 01:00:05 CET 1970
h.D = 99
h.E = true

Entries changed in JavaScript can be returned back to Java. Assume JavaScript program changes values:

var prtHash = function(h,hret) {
        hret.U = 57;
        hret.V = "Some text";
        hret.W = false;
};

Then these changed arguments can be used back in Java program:

HashMap hret = new HashMap();

result = invocable.invokeFunction("prtHash", hm, hret);
System.out.println(result);
System.out.println("hret.U = " + hret.get("U"));
System.out.println("hret.V = " + hret.get("V"));
System.out.println("hret.W = " + hret.get("W"));

Output is then

hret.U = 57
hret.V = Some text
hret.W = false

Added 09-Dec-2020: Since JDK 15 Nashorn is no longer available. It is now necessary to use GraalVM. Here is a Migration Guide from Nashorn to GraalVM JavaScript. See Compiling Java source to binary (native).