Ajax响应返回null

Question

I'm new to Ajax and am just starting to learn, I have managed to get data passed to my servlet but once i write back to it and then try to read from it from my web page it returns undefined Here is my code:

Java:

@WebServlet("/PeriodHandler")
public class PeriodHandler extends HttpServlet {

private static Connection conn = UserHandler.conn;
private String periodList;
public String date;

public PeriodHandler(){
    super();
}   

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{
    response.setContentType("text/xml");
    System.out.println(request.getParameter("date"));
    response.getWriter().println("<responseFromServer>The date is" + request.getParameter("date")+"</responseFromServer>");        
}
}

Javascript:

function sendData() {
            xmlhttp.open("POST", "PeriodHandler?date=" +document.getElementById('dates').value, true);
            xmlhttp.onreadystatechange=recievedMessageFromServer;  
            xmlhttp.send();                
        }
function recievedMessageFromServer(){
            if(xmlhttp.readyState===4 && xmlhttp.status===200){
                var date = xmlhttp.responseXML.documentElement.getElementsByTagName("responseFromServer")[0];
                alert(date);
            }
        }

I hope someone could point me in the right direction, thanks

Answer 1

responseFromServer is the root element. Try wrapping it inside another and access responseFromServer

//Java
response.getWriter().println("<root><responseFromServer>The date is" + request.getParameter("date")+"</responseFromServer></root>");

//Ajax - No change
var date = xmlhttp.responseXML.documentElement.getElementsByTagName("responseFromServer")[0];

The idea here is when you call xmlhttp.responseXML.documentElement it starts from the root. So its checks responseFromServer tag for a child element of responseFromServer. If you dont want a root you could should remove the documentElement part like

//Java - No change
response.getWriter().println("<responseFromServer>The date is" + request.getParameter("date")+"</responseFromServer>");

//Ajax
var date = xmlhttp.responseXML.getElementsByTagName("responseFromServer")[0];