How to make JSONP call in GWT

Example goes here:

String url3 = "https://ajax.googleapis.com/ajax/services/search/news?v=1.0&q=barack%20obama";
JsonpRequestBuilder jsonp = new JsonpRequestBuilder();

//jsonp.setCallbackParam("callback");
//gwt will generate a java script object of url "http://url_of_some_service?para=value%callback=_xxxx_onSuccess"
//the _xxxx_onSuccess points to below onSucess function.

jsonp.requestObject(url3, 
  new AsyncCallback<javascriptobject>() {
    public void onFailure(Throwable throwable) {
      GWT.log("got the JavaScriptObject");
      Window.alert("jsonp failed");
    }

    public void onSuccess(JavaScriptObject feed) {
      JSONObject obj = new JSONObject(feed);
      JSONValue val = obj.get("responseStatus");

      if(val != null)
      {
        JSONNumber num = val.isNumber();
        Window.alert("got the responseStatus " + num.doubleValue());  
      }
      else
      {
        Window.alert("not getting the responseData");
      }

    }
});