Using GWT and Ajax4jsf together

Recently I had to face a situation it's hard to speak about :) In a Seam+RichFaces based application we needed to develop a special component, which needs a large amount of client side JavaScript code. I decided to use GWT to develop this functionality. It's very simple to embed a GWT application supporting a JSF component to a page, especially if you have only one instance of the component on the page - you just include the compiled JS.

Everything seemed working fine until I tested it in Internet Explorer 6. In IE6, the GWT RPC calls just didn't succeed, the IE6 JavaScript error report said 'unhandled exception', in a meaningless line of JS code, of course. I found that it works fine if I don't have an a4j:form on my page. After a number of tries I found that the loading of the core JavaScript library of Ajax4Jsf already causes the problem, and then, looking through the code, I found a suspicious statement:

 XMLHttpRequest = function() {
  if (!_SARISSA_XMLHTTP_PROGID) {
   _SARISSA_XMLHTTP_PROGID = Sarissa.pickRecentProgID( [
     "Msxml2.XMLHTTP.6.0", "MSXML2.XMLHTTP.3.0",
     "MSXML2.XMLHTTP", "Microsoft.XMLHTTP" ]);
  }
  return new ActiveXObject(_SARISSA_XMLHTTP_PROGID);
 };

And this turned out to be the problem indeed. Normally, the XMLHttpRequest variable is simply null. I didn't dare look through the GWT RPC implementation to investigate the problem further :) But at this point, we can already work the thing around.

So I introduced a 'beforeRequest' and 'afterRequest' JSNI methods in my GWT code to ensure that '$wnd.XMLHttpRequest' is null during the RPC call. This way, GWT RPC calls work fine, and the normal a4j functionality is still available in the page.