Next generation javascript Interoperability with GWT 3.0 02 Feb 2014

Hello everyone,

Today I am going to talk about javascript interoperability with GWT and more precisely about the new features that will be introduced in futur version of GWT. I believe this is a major topic, one that could lead to an even greater success or that could lead to the end of GWT. I have recently read the spec for GWT 3.0 and I want to share some of its content with you. You can find all the source links at the end of this article.

A little (personal) history

When I started my new job 1 year ago, as a tech lead, I was responsible for choosing the best technology in order to develop our new product. I am not going to detail the reasons that made me choose GWT here (it could be part of another article) but what I can say is that I really hesitated and one of my concerns was the actual (GWT 2.5) javascript interoperability.

I really doubted our capacity to integrate javascript components/libraries easily. The javascript community is an incredible community and I discover incredible frameworks, libraries, widgets everyday. I really wanted to be able to use those great components in our product. I had the feeling that the trend was to go 100% javascript. After using Sencha Ext Js and Sench Touch in my previous company and witnessing the incredible rise of Angular.js I was getting scared to choose GWT. So if we were going to go for GWT, at least, we needed to be able to integrate javascript lib easily !

During my previous experiences, I used JSNI a lot with GWT, in both directions, if I can say that, from GWT to javascript (which is the most common use case) and from javascript to GWT.

####GWT -> Javascript When I was working for one2team, we integrated the Highcharts library inside our GWT application and we built our own wrapper (some of this work can be found here). It was before moxie commited their own wrapper. We also coded the first backend rendering engine for highcharts. Now, for server export, you can probably do better by using Phantom.js.

####Javascript -> GWT I also wrapped a GXT customized live combo into a javascript component that was populated from jsp pages.


From what I have experienced JSNI is very powerful. I did not encounter any major blocking point using it (I coded some callbacks and some other tricky stuff). I may not have faced all the possible problems but I strongly beleive that with some efforts, you can achieve almost whatever you want, by using JSNI. The only real question is “how much time” is it going to take you. In the end, time is money, so if you are not productive, it will cost you a lot of money. Is it worth the effort ?

The example of one2team highcharts integration is a very good example. In order to be able to use it, we had to redefine all the native javascript chart options into interfaces. Interfaces are mandatory if you want to be able to test easily your client code using standard junit tests. It will make it possible to mock the highcharts options objects implementations using mockito for example.

Then we had to implement those interfaces using JavaScriptObject (JSO).

This was a tedious work and hard to maintain as you need to constantly keep up to date with the javascript library. So everything that can help here is welcome !

What is coming ? Winter ?

@JsInterface and @JsProperty

No need to implement JSO anymore !

 interface MyJsInterface {
   void action1(String x);
   AnotherObject action2(int x, int y);
   @JsProperty void setX(int x);
   @JsProperty int getX();
   @JsProperty boolean hasX();
   default void someOtherMethod() { /* some java code here */ }

This will behave like following JSO in current GWT compiler (just for illustration):

 class MyJsObjectImpl extends JavaScriptObject implements MyJsInterface {
   native void action1(String x) /*-{ this.action1(x); }-*/;
   native AnotherObject action2(int x, int y) /*-{ return this.action2(x, y); }-*/;
   native void setX(int x) /*-{ this["x"] = x; }-*/;
   native int getX() /*-{ return this["x"]; }-/;
   native boolean hasX() /*-{ return "x" in this; }-/;
   native void someOtherMethod() /*-{ 
     if ('someOtherMethod' in this) {
     } else {
   private void super$someOtherMethod() { 

It works both ways. Meaning that when @JsInterface is implemented by a regular java object, the methods will be exported to object’s prototype and names will be preserved hence make them easily callable from javascript (method names are visible in javascript)

class MyJavaObject implements MyJsInterface {
   void action1(String x) { /* some java code here */ }
   void setX(int x) { /* some java code here (called via Object.defineProperty) */ }
   void action3(int y) { /* some java code here (enabled for GWT optimizations) */ }
 // in javascript assuming obj is an instance of MyJavaObject
 obj.action1("action!!"); // works!
 obj.x = "blah"; // works!
 obj.action3(42); // will not work as action3 is not declared on a @JsInterface

Goodies : Ray Cromwell implemented the use of fluent interfaces and other nice syntax features that will make our life really easier.

@JsProperty T setX(int x)

alternate getter/setter syntax, so JavaBean as well as non-Javabean syntax is supported

// JavaBean 
getX() /* getter */
isX() /* getter */
setX() /* setter */
// non-JavaBean
x() /* getter */
x(arg) /* setter */
interface Blah {
  @JsProperty int length();
  @JsProperty void length(int x); // sets length


To export a symbol to global scope so that it can be called from javascript. Note that @JsExport can only be applied to constructors, static fields and static methods.

class MyJavaObject implements MyJsInterface {
  MyJavaObject() { ... }
  MyJavaObject(String name) { ... }
  void action1(String x) { /* some java code here */ }
  void action3(int y) { /* some java code here (enabled for GWT optimizations) */ }
  // in javascript:
  var obj = new my.java.Object();
  obj.action1("action!!"); /* works! */

  var obj2 = new my.java.ObjectPrime("Coool!!!");
  obj2.action1("action!!"); /* works! */
  obj2.action3(42); /* will not work as action3 is not declared on a @JsInterface */

How does it work ?

As Arnaud Tournier mentionned in the comments below, this javascript code generation process is integrated to the GWT compiler. If you want to have a look at the code check this code review https://gwt-review.googlesource.com/#/c/5567/

I think part of this work was inspired by the GWT generator GWT-Exporter written by Ray Cromwell and others. I think it is a good entry point if you want to understand better how this javscript code generation can work. If you want another example of generator you can have a look to restyGWT,

I wonder if there will be an impact on compilation time if there are lots of JSO involved ?

(My) Conclusion

I am really excited to try those new features and I can’t wait GWT 3.0 to do so. I think this is going in the right direction. I would like to try those new features in conjonction with GIN and see how things are testable or not. Testability is a very important topic to me (and all TDD fans !). It ensures good quality + non-regression. I will try to find out how being able to use the source code from GWT 3.0 to try those new annotation stuff.

To do so, I intend to code a new Highcharts wrapper with those new features and publish it to Github. I will give some more details about that in futur articles.


Goktug Gokdogan A glimpse into the future of GWT (js slideshow)

Ray Cromwell Gwt.Create Keynote San Francisco (slideshare)

Goktug Gokdogan Nextgen GWT/javascript Interop (google doc)

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