Java Object as prototype in Rhino

Question

I am having difficulties using Java objects as JavaScript prototypes. This illustrates my problem:

var test = {};
test.prototype = new java.util.ArrayList();
test.prototype.add(1); // works
test.add(1);           // fails

It seems to me that this should never happen: any function property accessible on the prototype must be accessible on the object itself.

My goal is to add more function properties to the wrapped Java object. Is there at least a workaround to get the desired effect with Rhino? The constraint is that the adding of properties must be done on the JavaScript side.

My Rhino is 1.7R4.

@Andremoniy I am looking for a way to add more properties to the Java object. — Marko Topolnik
– Marko Topolnik, Commented Jul 11, 2013 at 11:38

Marko Topolnik · Accepted Answer · 2013-07-11 13:55:30Z

1

As explained here, the workaround is to set the prototype of the wrapped Java object to new NativeObject(). Note that in the linked thread there was no confirmation from any authority that this is in fact the right thing to do, so it may just happen to work for your use case and break others.

Anyway, the best place to set the prototype on the Java side is in a custom wrapFactory:

cx.setWrapFactory(new WrapFactory() {
  @Override public Object wrap(Context cx, Scriptable scope, Object obj, Class<?> staticType) {
    final Object ret = super.wrap(cx, scope, obj, staticType);
    if (ret instanceof Scriptable) {
      final Scriptable sret = (Scriptable) ret;
      if (sret.getPrototype() == null) sret.setPrototype(new NativeObject());
    }
    return ret;
  }
});

and on the JavaScript side this will now work:

var test = new java.util.ArrayList();
test.x = 'a';

So, compared to your posted attempt, you need to invert the roles of object and prototype.

A funny thing: now you can freely set the prototype on the JavaScript side as well:

test.prototype = {};

This sounds like a Rhino bug to me, or at least a point to improve.

answered Jul 11, 2013 at 13:55

Marko Topolnik

201k31 gold badges336 silver badges455 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Frederic Leitenberger Over a year ago

new NativeObject() is not exactly the same as doing {} in JS. What is missing is its default prototype Object. Consider this:

NativeObject prototype = new NativeObject(); prototype.setPrototype(ScriptableObject.getObjectPrototype(scope)); sret.setPrototype(prototype);

The difference is for example: the NativeObject without prototype does not have the default toString() implementation. So trying to print it somewhere will cause an error.

Collectives™ on Stack Overflow

Java Object as prototype in Rhino

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related