Sunday, December 9, 2007

Is Volta's Javascript Interop better than GWT?

In this ZDNet Interview with Volta architect Eric Meijer, Eric says:

"The GWT uses Java native methods to interface to JavaScript where the JavaScript implementation of that native method is defined in a special pragma comment.

In many situations, the compiler can automatically infer the JavaScript implementation from the metadata for the corresponding function declaration in C# (or VB). The Volta toolkit therefore implements a sophisticated convention over configuration heuristic to simplify writing foreign function interfaces. As a result this typically enables programmers to import JavaScript functionality by just writing a single [Import] attribute on an extern method signature."

This sounds like Volta's Javascript interop is better than GWT, but is it?

Deferred Binding again?

I must sound like a broken record by now, but GWT's Deferred Binding allows a GWT developer to accomplish anything Volta's interop does, and more. It's a general purpose mechanism to achieve compile time metaprogramming, where Javascript Interop is just the tip of the iceberg. If you want Volta-style no-JSNI interop, here's how to get it:

Bob Vawter's No JSNI Interop

While it is not a part of GWT's core, Bob Vawter, a member of the GWT Team, released a package for no-JSNI interop 'convention over configuration' several months ago.

How does it work? Just make an interface where the method names correspond to actual Javascript object methods and extend JSWrapper, for example, with the GMap2 API:

interface GLatLng extends JSWrapper {
* The naming of the method is arbitrary, the only thing that's important is
* the presence of the gwt.constructor annotation.
* @gwt.constructor $wnd.GLatLng
public GLatLng construct(double lat, double lng);
public double lat();
public double lng();

The constructor annotation tells the generator how to map the construct() call to the Javascript constructor. JSIO supports tons of additional interoperability features that I'm not sure Volta is capable of.

My GWT Exporter library

In addition to being able to import Javascript classes/functions/fields, one also wants to export Java functions to Javascript in a way that makes them callable from non-Java code. Volta can do this for static functions only it appears via the [Export] declaration.

What if you need to export methods, fields, and interfaces, and have polymorphic method dispatch on exported class instances still work? GWT Exporter makes this possible. In fact, one of the coolest features of GWT Exporter that I like is automatic Javascript closure conversion.

Imagine you have the following Java code:

* @gwt.export
public static void foo(FooCallback callback) {

* @gwt.exportClosure
public interface FooCallback {
void doIt();

Then you can invoke the foo() method with an ordinary Javascript closure:

foo(function() { alert("Hello World") })

And that closure will automatically be converted to an instance of the FooCallback interface and mapped to the doIt() function. Neat huh? I'm not saying Volta can't do this, I'm just not sure because the docs don't mention it.

The sky's the limit

If there's a feature missing from Bob's library, or mine, you don't need to fret and think about hacking the compiler, because GWT's Deferred Binding Generator mechanism allows anyone to extend the system to support whatever annotation and code-injection techniques one desires.

Future versions of GWT are likely to obsolete Bob's library and mine, by simply unifying the JS and Java ASTs in a way that a few extra annotations will allow the compiler to 'understand' a Javascript library and automagically map Java's types on top of it. You'll get even more optimal code then, as GWT will even be able to optimize third party JS libraries.

The title of this article is somewhat flamebait, but I wouldn't want people reading the ZDNet interview to come away assuming that GWT can't handle the kind of Javascript interop that Volta can.



zool said...

Hey Ray, nice job with the GWT/JS generator. I really works out fine. I just have one question for you: How is it possible to do the following:

If you look in the bottom of the thread, you can see my question about calling and initiating new objects inside java..

Hope you have a idea how to do it..

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