xmlhttp.open("POST", url, true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xmlhttp.send(passParameter);

    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
       document.getElementById("printBack").innerHTML = xmlhttp.responseText;
    }

<%@page contentType="text/html; charset=UTF-8"%>
<%@page import="org.json.simple.JSONObject"%>
<%
JSONObject obj=new JSONObject();
obj.put("name","foo");
obj.put("num",new Integer(100));
obj.put("balance",new Double(1000.21));
obj.put("is_vip",new Boolean(true));
obj.put("nickname",null);
out.print(obj);
out.flush();
%>