Save a form jsp

When you save a form, that is, it goes to the Spring control class, I need it to throw an exception, to warn the user that some field is required.

In JSP it looks like this:

         <c:if test="${not empty param.e}">
         <div class="form-group has-error">
         <label for="inputError" class="col-xs-12 col-sm-3 col-md-3 control-label no-padding-right">
         <i class="ace-icon fa fa-times-circle"></i> Campos com * são obrigatórios !</label>
         </div>
         </c:if>

Do not control it like this:

     @RequestMapping("salvarPais")
     @Transactional
     public String cadastrar(PaisEntity pais, Model model) {
        if (pais.getDescricao() == null || pais.getDescricao().isEmpty()) {
            model.addAttribute("e", "e");
            return "redirect:novoPais";
        }
        String pagina = "";
        if (pais.getId() == null) {
            pagina = "redirect:novoPais/?i=ok";
        } else {
            pagina = "redirect:editarPais?id=" + pais.getId() + "&a=ok";
        }
        repository.salvar(pais);
        return pagina;
     }

The problem is that when you save and emit the error message correctly, it clears the other fields.

What would be the best way?