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I have been using GraalVM to execute Javascript from java. But i have a problem: I don't know how to get fields or invoke methods of a js object from java. This is my code, java:

public static class JavaMethods {
    public void printWidthOf(Object size) {
        System.out.println("printWidthOf started");
        System.out.println("size class is " + size.getClass().getName()); // prints PolyglotMap
        Map map = (Map) size; // PolyglotMap implements Map so i tried that
        System.out.println("size width is " + map.get("size")); // doesn't work, prints null instaed of 40
    }
}

public static void main(String[] args) throws Exception {
    GraalJSEngineFactory factory = new GraalJSEngineFactory();
    GraalJSScriptEngine engine = factory.getScriptEngine();

    Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
    bindings.put("polyglot.js.allowHostAccess", true);
    bindings.put("JavaMethods", new JavaMethods());

    engine.eval(new FileReader("MyScript.js"));
    engine.invokeFunction("start");
}

javascript:

class Size {
    #width;
    #height;

    constructor(w, h) {
        this.#width = w;
        this.#height = h;
    }

    getWidth() {
        return this.#width;
    }

    getHeight() {
        return this.#height;
    }
}

function start() {
    var mySize = new Size(40, 60);
    JavaMethods.printWidthOf(mySize);
}

Output:

printWidthOf started 
size class is com.oracle.truffle.polyglot.PolyglotMap
size width is null

Then how can i get a field of a PolyglotMap (js object) from java?

1 Answers1

0

I found the solution.

public static class JavaMethods {
    public void printWidthOf(Object size) throws Exception {
        Field f = size.getClass().getDeclaredField("guestObject");
        f.setAccessible(true);
        JSOrdinaryObject obj = (JSOrdinaryObject) f.get(size);
        System.out.println(obj.getValue("width"));
    }
}