XmlHttpRequest level 2 : Response returns nothing

I have this problem, I've already asked this question yesterday but i did not have any answer... :(

I have this code on the client-side:

 var formdata = new FormData();
    //fill fields of formdata... for example:
    var file = document.getElementById("file").files[0];
    formdata.append("file", file);
    //and others....but the problem is not here
    var xhr = new XMLHttpRequest();
    xhr.open("POST","http://127.0.0.1:8080/Commerciale",true);
    xhr.send(formdata);
    xhr.onreadystatechange = function() {

        if (xhr.readyState == 4) {
    if (xhr.status == 200) {
                   var str = xhr.responseText;
                   alert(str);
              }
         }
      });

so far it seems fair. In the servlet I have this code:

 protected void doPost(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException             {
   ***other code, but i think that the problem is here:
   PrintWriter ajaxWriter = response.getWriter();
   ajaxWriter.println(p.getJSON());
   ajaxWriter.flush();          
   System.out.println(p.getJSON());
   ajaxWriter.close();
 }

the problem is that with the

 System.out.println(p.getJSON()); 

prints what I expect, but it seems that

 xhr.responseText 

does not return anything, in fact, the alert is empty.

someone can explain me why?

:) After discovering that this is the cause:

You should not close the writer after flushing.
Remove the line:

ajaxWriter.close();

An interesting related question - Should one call .close() on HttpServletResponse.getOutputStream()/.getWriter()?

Although there is no specific documentation that forbids closing the writer/stream - this is something that the container should perform and not the application.