java - Ajax响应返回null

我是Ajax新手，刚刚开始学习，我已经成功地将数据传递给了我的servlet，但是一旦我写回它，然后尝试从我的web页面读取它，它就会返回undefined这里是我的代码：
爪哇语：

@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代码：

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);
            }
        }

我希望有人能指引我正确的方向，谢谢

最佳答案

responseFromServer是根元素。尝试将其包装在另一个内部并访问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];

这里的想法是当您调用xmlhttp.responseXML.documentElement时，它从根开始。因此，它检查responseFromServer标记是否有responseFromServer的子元素。如果你不想要根，你可以移除documentElement部分

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

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