How to use BindingResult Thymeleaf validation? - thymeleaf

Question

I have a bit issue related to binding result thymeleaf validation.
on form submit with empty string binding result is not validate.
please you help me to check and advise me what should I fix this issue.
Thank!!
UserDTO
#Entity
public class UserDTO {
#Id
#NotEmpty(message = "First Name is required!!")
private String firstName;
#NotEmpty(message = "Last Name is required!!")
private String lastName;
#NotEmpty(message = "Username is required!!")
private String username;
#NotEmpty(message = "Password is required!!")
private String password;
#NotEmpty(message = "Confirm password is required!!")
private String confirmPassword;
#NotEmpty(message = "Role is required!!")
private String authentication;
//setter getter
Controller
#PostMapping(value = "/addUser")
public String saveUser(#Valid #ModelAttribute("userDTO") UserDTO userDTO, BindingResult
bindingResult, ModelMap model) {
if (bindingResult.hasErrors()) {
return "createUser";
}
User userDb = userRepo.findUserByUsername(userDTO.getUsername());
if (userDb != null) {
throw new RuntimeException("User already exist.");
CreateUser.html
<form th:action="#{/addUser}" class="form-horizontal"
novalidate="novalidate" th:object="${userDTO}" method="post">
<div class="form-body">
<div class="form-group">
<label class="control-label col-md-3">Fist Name
<span class="required" aria-required="true"> * </span></label>
<div class="col-md-4">
<input type="text" th:field="*{firstName}" th:value="null"
data-required="1" class="form-control">
<span th:if="${#fields.hasErrors('firstName')}" th:errors="*{firstName}">First Name is required!</span>
</div>
</div>
<div class="form-group">
<label class="control-label col-md-3">Last Name
<span class="required" aria-required="true"> * </span>
</label>
<div class="col-md-4">
<input type="text" th:field="*{lastName}"
data-required="1" class="form-control">
<span th:if="${#fields.hasErrors('lastName')}" th:errors="*{lastName}">Last Name is required!</span>
</div>
</div>