servlet-为什么XMLHttpRequest responseText总是空白？

Question

I am losing my mind trying to solve this problem over here. I have the following servlet deployed in Tomcat running on localhost:8080-:

@WebServlet(urlPatterns = { "/createcon" }, asyncSupported = true)
public class CreateCon extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    ConcurrentHashMap<String, AsyncContext> map;
    public CreateCon() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */

    public void init() {
         map = new ConcurrentHashMap<>();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        AsyncContext context = request.startAsync(request,response);
        context.setTimeout(10000);
        if(!map.containsKey("Hello"))
        map.put("Hello", context);
        System.out.print("Inside GET");
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        AsyncContext context = map.get("Hello");
        PrintWriter writer = context.getResponse().getWriter();
        writer.write(request.getParameter("message"));
        writer.flush();
        System.out.print(request.getParameter("message"));
    }

}

As you can see I am trying to store an AsyncContext that is created in Map. I code runs fine in Eclipse with Tomcat. As you can see above that I have added System.out.print to actually check whether the code is working properly or not. And it works exactly as expected.

But the problem is with the javascript below-:

function postMessage(){
    var xmlhttp = new XMLHttpRequest();
    xmlhttp.onreadystatechange=function(){
        if (xmlhttp.readyState==4 && xmlhttp.status==200) {

        alert(xmlhttp.responseText);
        }
    }
    xmlhttp.open("POST", "/SampleTest/createcon", true);
    xmlhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    var messageText = escape(document.getElementById("i1").value);
    document.getElementById("i1").value = "";
    xmlhttp.send("message="+messageText);
}

The onreadystatechange fires exactly when expected but the xmlhttp.responseText is always blank.

I know that there is something known as a same-origin policy. But I don't understand why that's a problem here ? I am running everything on localhost:8080 .

Why is this still happening and how do I solve this ?

Answer 1

Okay solved it. Guess it was my own silly mistake. The startchat() method should be modified like-:

function startChat() {
    var xmlhttp = new XMLHttpRequest();
        xmlhttp.open("GET", "/SampleTest/createcon", true);
        xmlhttp.send();
        xmlhttp.onreadystatechange=function(){
            if (xmlhttp.readyState==4 && xmlhttp.status==200) {
            alert(xmlhttp.responseText);
            }
        }
    }

Since I am trying to find the result from the request made at startChat() .