Check username availability with jquery validation plugin (servlet and jsp)

Question

I an using jquery validator plugin to validate my registration form. However, when I tried to check if an username is in the database, it does not work. Can anyone help take a look at my codes and let me know what I did wrong? It always shows user already exist even when the username is not in database.

I have tried the methods from

jQuery Validate remote method usage to check if username already exists and JQuery Remote Validation not displaying Error

but it still doesn't work.

My registration jquery function:

   $().ready(function() {

        $.validator.addMethod('uniqueUsername', function(value) {
            $.ajax({
                url: 'http://localhost:8080/TBS-war/RegisterServlet.java',
                type: 'POST',
                async: false,
                contentType: 'application/json',
                dataType: 'json'

            });
        });

        //Validate signup form on keyup and submit
        $("#registrationForm").validate({
            // Set rules
            rules: {
                username: {
                    required: true,
                    uniqueUsername: true
                    /*remote: {
                        url: "http://localhost:8080/TBS-war/RegisterServlet.java",
                        type: "post",
                        contentType: "application/json",
                        data: {
                            'username': $('#username').val()
                        }
                    }*/
                },
                password: {
                    required: true
                },
                confirmPassword: {
                    required: true,
                    equalTo: "#password"
                },
                contact: {
                    required: true
                },
                email: {
                    required: true,
                    email: true
                },
                address: {
                    required: true
            }},

            // Set messages
            messages: {
                username: {
                    required: "* This field is required",
                    uniqueUsername: "* User already exist"
                    //remote: "* User already exist"
                },
                password: {
                    required: "* This field is required"
                },
                confirmPassword: {
                    required: "* This field is required",
                    equalTo: "* Please enter the same password as above"
                },
                contact: {
                    required: "* This field is required"
                },
                email: {
                    required: "* This field is required",
                    email: "* Please enter a valid email"
                },
                address: {
                    required: "* This field is required"
            }},

My servlet file:

@EJB
private TBSManagerBeanRemote tBSManagerBean;


    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("application/json");

        PrintWriter out = response.getWriter();
        JSONObject json = new JSONObject();
        String username = request.getParameter("username");

        if (tBSManagerBean.userExist(username)) {
            out.print("{\"valid\" : false }");
            json.put("valid", false);
            System.out.println("false");
        }
        else {
            out.print("{\"valid\" : true }");
            json.put("valid", true);
            System.out.println("true");
        }
    }