How to handle bindingresult errors in mustache springboot

Question

I am learning mustache with springboot. I am getting error page while handling the binding result.

My controller class method to handle request:

@PostMapping("/registerUser")
    public String createNewBuyer(@ModelAttribute("user") @Valid MyUser user, final Model model, final BindingResult result) {
        if (result.hasErrors()) {
            model.addAttribute("user", user);
            return "page-login-register";
        }
        String error = userService.checkIfUserExists(user);
        if (error != null) {
            model.addAttribute("registerError", error);
        } else {
            userService.saveUser(user);
        }
        return "page-login-register";
    }

POJO class to map request variable and validate

@Data
public class MyUser implements Serializable {

    private Integer userId;
    @NotEmpty(message = "registration.validation.userName")
    @Size(min = 3, message = "minimum size of username should be 3")
    private String userName;
    @NotEmpty(message = "registration.validation.firstName")
    private String firstName;
    @NotEmpty(message = "registration.validation.lastName")
    private String lastName;
    @NotEmpty(message = "registration.validation.email")
    @Size(min = 5, max = 64, message = "Please enter a valid email")
    private String email;
    @NotEmpty(message = "registration.validation.password")
    @Size(min = 5, max = 24, message = "Please enter a valid password.")
    private String password;
    private String confirmPassword;
    @NotEmpty(message = "registration.validation.phoneNumber")
    @Size(min = 10, message = "Please enter valid phone Number.")
    private String phoneNumber;
}

Html code to submit request and show error

{{#registerError}}
                                        <p class="mb-50 font-sm text-danger">
                                            {{registerError}}
                                        </p>
                                        {{/registerError}}
                                        {{#user}}
                                        <form method="post" action="/registerUser" name="user">
                                            {{#_csrf}}
                                            <input type="hidden" name="{{parameterName}}" value="{{token}}"/>
                                            {{/_csrf}}
                                            <div class="form-group">
                                                <input type="text" required="" name="userName" value="{{#userName}}{{userName}}{{/userName}}" placeholder="Username">
                                            </div>
                                             <div class="form-group">
                                                <input type="text" required="" name="firstName" placeholder="First Name" value="{{#firstName}}{{firstName}}{{/firstName}}">
                                            </div>
                                             <div class="form-group">
                                                <input type="text" required="" name="lastName" placeholder="Last Name" value="{{#lastName}}{{lastName}}{{/lastName}}">
                                            </div>
                                            <div class="form-group">
                                                <input type="email" required="" name="email" placeholder="Email" value="{{#email}}{{email}}{{/email}}">
                                            </div>
                                            <div class="form-group">
                                                <input type="phone number" required="" name="phoneNumber" placeholder="Phone Number" value="{{#phoneNumber}}{{phoneNumber}}{{/phoneNumber}}">
                                            </div>
                                            <div class="form-group">
                                                <input required="" type="password" name="password" placeholder="Password">
                                            </div>
                                            <div class="form-group">
                                                <input required="" type="password" name="confirmPassword" placeholder="Confirm password">
                                            </div>
                                        <div class="form-group">
                                                <button type="submit" class="btn btn-fill-out btn-block hover-up" name="login">Submit &amp; Register</button>
                                            </div>
                                        </form>
                                        {{/user}}

When I am sending request with 3 digit password am not able to show same page and show the errors. I can achieve to handle this by adding validation into controller instead of pojo class but thats not good practice. I need help how to handle bindingresult exception and show in same page(using mustache).

Update:

I found some article where they said about order of parameter of method in controller. i.e. while writing parameters in a method write bindingresult class just after pojo class. like:

public String createNewBuyer(@ModelAttribute("user") @Valid MyUser user, final BindingResult result, final Model model) {

but now how to show them in mustache page just like thymeleaf handles. am working on it but if anybody have idea please help me.