Can a Java Object be a Prototype of an JavaScript Object in Nashorn?

Question

Is it possible to make an Java object the prototype of a JavaScript object? Like the following:

var Person = Java.type("Person");
var hans = new Person();
hans.name = "Hans";

var employeeFactory = function() {
    var F = function(){};
    F.prototype = hans;
    return new F();
};

var fritz = employeeFactory();
print(fritz.name);

Here Person is a Java Bean. The variable hans is set as the instance of this Java class. The line hans.name = "Hans" sets the name field in the Java object as expected. But when the object fritz is created in the factory function, it doesn't get linked to the expected prototype. Is there any reason why the Java instance isn't accepted as prototype?

Only tangentially related, but does Nashorn not support Object.create? — T.J. Crowder
– T.J. Crowder, Commented Dec 28, 2014 at 10:15
Interesting point. Not only does Nashorn know the Object.create function. If I use it instead of the factory, and try to create a JS-object with the Java instance as prototype, there is an error: TypeError: Person@73d4cc9e is not an Object. That's a real pitty. Would have been nice to inherit from Java beans. — Gregor
– Gregor, Commented Dec 28, 2014 at 19:22
That is a pity. I haven't read up on Nashorn much, unfortunately. I wonder what happens if you try it with Rhino. — T.J. Crowder
– T.J. Crowder, Commented Dec 29, 2014 at 8:02

Attila Szegedi · Accepted Answer · 2015-02-01 09:18:52Z

4

It might work in Rhino, because in Rhino all beans are wrapped into JS native objects before being exposed to the JS program. Nashorn OTOH doesn't create wrappers. You can, however, use Nashorn's non-standard Object.bindProperties that adds properties from one object to another, bound to the original object's instance as its this. That's essentially as close as you can get (well, it's pretty darn close) to a wrapper. The exact specification is:

Object.bindProperties(dst, src)

creates bound properties from all properties in src object, puts them into the dst object, and returns the dst object. Since the properties are bound to src, dst.foo will delegate to src.foo for its value. bindProperties has been coded specifically so that it can handle ordinary Java objects as src.

With that in mind, I believe that if you change the line

F.prototype = hans;

to

F.prototype = Object.bindProperties({}, hans);

you'll get what you wanted. That would work with Object.create too, e.g. Object.create(Object.bindProperties({}, somePojo)).

edited Feb 1, 2015 at 9:18

answered Feb 1, 2015 at 9:12

Attila Szegedi

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1 Comment

Gregor Over a year ago

Thanks for your response. I'm only interested in Nashorn though. But at least this gives me a hint of what goes "wrong" in Nashorn.

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