Question

I’ve been implementing a GWT application that calls a REST-service (which we’re also developing). When the REST-service returns anything with a HTTP-status other than 200 I would expect the onFailure method of AsyncCallback to be called. However I can’t get this to happen.

To test it further I created a test GWT app and a test servlet. The part of the GWT app that calls the service looks like this:

JsonpRequestBuilder jsonp = new JsonpRequestBuilder();
jsonp.setCallbackParam("_jsonp");
jsonp.setFailureCallbackParam("_jsonp_failure");

jsonp.requestObject(url, new AsyncCallback<JavaScriptObject>()
{

    @Override
    public void onFailure(Throwable caught)
    {
        Window.alert("Failure: " + caught.getMessage());
    }

    @Override
    public void onSuccess(JavaScriptObject result)
    {
        Window.alert("Success");
    }
});

The servlet-code looks like this:

public class MyRestServlet extends HttpServlet
{
    protected void doGet(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse) throws ServletException, IOException
    {
        String padding = httpServletRequest.getParameter("_jsonp_failure");
        httpServletResponse.setContentType("application/x+javascript");
        httpServletResponse.setStatus(500);


        PrintWriter out = httpServletResponse.getWriter();
        out.println(padding + "({\"some\":\"json\"});");
        out.close();
    }
}

OnFailure eventually gets called when the request times out, but I would expect it to be called as soon as the http response arrives(if it’s a failure). I guess there is something I haven’t understood and I would really appreciate to get some help with this.

Thanks

Answer 1

According to HTML5, if there’s an error loading the script, an error event should be dispatched, and GWT doesn’t listen for it (because almost no browser actually fires it AFAICT).
For best browser compatibility, you’d better always send a 200 status, but then call the failure callback (or in other words, return an error state/condition, rather than throw an exception).

Also, the argument to the failure callback is expected to be a string (will be the message of the exception).