Return JSON from servlet

I was having the same problem. It was working well on Firefox but not on IE... I found out reading this post that my problem was related to the 'Content-Type'. The problem seems that that IE has problem with 'charset=UTF8'. However, if you use 'charset=UTF-8' (with a dash) it is then working! Your Content-Type should then be: application/json;charset=UTF-8

IE caches AJAX requests aggressively (more than Firefox, Chrome, and Safari, anyway). Sometimes you need to set cache header controller when request. Like cache:false. I tried to fix your code like this

request.setCharacterEncoding("utf8");
        //response.setCharacterEncoding("utf8");
        response.setContentType("application/json");
        PrintWriter out = response.getWriter();
        JSONObject jsonObj = (JSONObject) JSONValue.parse(request.getParameter("para"));
        System.out.println(jsonObj.get("message"));
        JSONObject obj = new JSONObject();
        obj.put("message", "hello from server");
        out.print(obj.toString());

I changed your response content-type from application/json; charset=utf8 to just application/json and that worked.