AJAX POST call with application/json contentType gets “no 'Access-Control-Allow-Origin' header” error

Question

I have the Javascript function below. Whenever I run it (calling it from an Android app using WebView), it is sent as an application/x-www-form-urlencoded despite h

Answer 1

This works for me (I'm using Tomcat, not Jetty though). I didn't do anything with the JSON payload, just tested if the URL on the server is hit.

AJAX (plain javascript)

function issueAjaxPost(){
  var url = "http://localhost:8085/some_url/servlet/PostJson";
  var json = '{ "action": "create", "userId": userId }';

  xmlhttp.open("POST", url, true);
  xmlhttp.setRequestHeader("Content-type", "application/json");
  xmlhttp.setRequestHeader("Access-Control-Allow-Origin", "*");
  xmlhttp.send(json);
}

Servlet

public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException  {
  String url = request.getRequestURL().toString();
  String page = url.substring(url.lastIndexOf("/"));

  ...

  if("/PostJson".equals(page)){
    response.setContentLength("success".length());
    response.setContentType("text/plain");
    response.setStatus(HttpServletResponse.SC_OK);
    response.getWriter().write("success");
  }
}

AJAX callback (the result)

xmlhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    document.getElementById("txt2").innerHTML = this.responseText;      //the result (success)
  }
};