how can i modify a freemarker variable value inside a success block in jQuery AJAX

how can i change a value of a freemarker variable inside a success block in jQuery AJAX, i have two controllers for my page the first one returns me a simple string with the name of the view with a GET method, the second one is the one that process the data using a json with a POST method

here they are

@RequestMapping(value = "myform", method = RequestMethod.GET)
    public String formmethod(Model model) {



        model.addAttribute("successMessage", "i'm in the firts controller");

        return "forms/myform";
    }

my second controller

@RequestMapping(value = "myform", method = RequestMethod.POST)
    public @ResponseBody String getTags(@RequestBody final String json, Model model) 
    throws IOException 
    {
         ObjectMapper mapper  =  new ObjectMapper();

         User userMapped=  mapper.readValue(json, User.class);

         User person  =  new Usuario();
         person.setName("new name");
         person.setLastName("new lastname");


         model.addAttribute("successMessage", person.getName());

         return toJson(userMapped);
    }

my to Json method

private String toJson(User person) 
    {
        ObjectMapper mapper = new ObjectMapper();
        try 
        {
            String value = mapper.writeValueAsString(person);
            // return "["+value+"]";
            return value;
        } 
        catch (JsonProcessingException e) 
        {
            e.printStackTrace();
            return null;
        }
    }

and my page myform.html

<script src="http://ift.tt/KA8c68"></script>

<script type="text/javascript">

        function doAjaxPost() 
        {
            // get the form values
            var name= $('#name').val();
            var lastName = $('#lastName ').val();

            var json = {"name" : name, "lastName " : lastName };
            console.log(json);
            var FreeMarkervariable = "${successMessage}";

            //this brings me the value that i put in the firts controller


            $.ajax(
            {
                type: "POST",
                url: "myform",
                data: JSON.stringify(json), 

                contentType: "application/json; charset=utf-8",
                dataType: "json",
                cache: false,

                beforeSend: function(xhr) 
                            {
                                xhr.setRequestHeader("Accept", "application/json");  
                                xhr.setRequestHeader("Content-Type", "application/json");  
                            },
                success: function(data) 
                        {


//HERE I WANT TO CHANGE THE VALUE OF MY FREEMARKER VARIABLE SO I CAN 
//PRINT A SUCCESS MESSAGE IN A DIV


                            <#assign successMessage = "success">



                        },
                error:function(data,status,er) { 
                    alert("error: "+data+" status: "+status+" er:"+er);
                }

            });
        }
</script>
    <!-- NEW WIDGET START -->
    <article class="col-sm-12">
        <div class="alert alert-warning fade in">
            <button class="close" data-dismiss="alert">
                ×
            </button>
            <i class="fa-fw fa fa-warning"></i>
            <strong>${successMessage} I WANT TO PRINT A SUCCESS MESSAGE HERE </strong> 
        </div>
    </article>
    <!-- WIDGET END -->
<fieldset>
 <legend>Name in view</legend>
    <form name="myform">
        Name in view:   <input type="text"   name="name">
        <br>
        Last Name in view:   <input type="text" id="lastName" name="lastName">
        <br>
        <input type="button" value="Add Users" onclick="doAjaxPost()">

    </form>
</fieldset>
 <br>

so far my freemarker variable gets the value that i put inside the success block but it appears "success" before i press my submit button, i believed that the succes block was executed after i hit the submit button so i dont know why it have the value even before i press the button it should have "i'm in the firts controller" before i press the submit button