java验证不起作用？

Question

i am trying to validate Link Identifier input field when user enter dublicate data which already exist in database so it popup a alert link already exist if user enter new data related to link so code will allow to user sava data but this code is not working i do not know where am i doing mistake please help me.

jsp code

    <input type="text" name="txtLinkIdentifier" id = "txtLinkIdentifier" class="form-control" placeholder="Link Identifier" onblur="checkLinkId();" />

js code

    function checkLinkId() {
        var idValue = $("#txtLinkIdentifier").val();

        $.ajax(
                {
                    data: {action: 'Check'},
                    method: "POST",
                    url: "LinkPiuServlet?idvalue=" + idValue,
                    dataType: 'json',
                    success: function (data) {
                        if (data.success)
                        {
                            alert(data.msg);
                        }
                    }

                });

servlet code

    else if (action.equals("Check")) {
                    String txtLinkIdentifier = request.getParameter("idvalue");

                    boolean res = LinkPiuDao.checkId(txtLinkIdentifier);
                    if (res == true) {
                        jsonObj.addProperty("success", true);
                        jsonObj.addProperty("msg", "Link Id Already Exist");
                    }

Dao code

    public static boolean checkId(String linkIdentifier) {
            boolean status = false;
            try {
                DBCollection collection = new MongoSetup().getDBCollection("linkPiu");
                BasicDBObject fields = new BasicDBObject();
                DBObject obj;
                fields.put("linkIdentifier", linkIdentifier);
                DBCursor cursor = collection.find(fields);
                while (cursor.hasNext()) {
                    obj = cursor.next();
                    String data = obj.get("linkIdentifier").toString();
                    System.out.println(data);
                    if (data.equals(linkIdentifier)) {
                        status = true;
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
            return status;
        }