validate input in Thymeleaf

@PostMapping("/cal-bmi")
public String calculateBmiForm(Model model, Integer masa, Integer wzrost) {

String x = String.valueOf(masa);
    String y = String.valueOf(wzrost);


    if(x==null ){
    model.addAttribute("wzrost",true);
    return"views/success";
}
    if(y==null ){
        model.addAttribute("wzrost",true);
        return"views/success";
    }
}

ANd when i get a valu form masa and wzrost i check from null, i click submit alwas nullpointerexception

 <form th:action="@{/cal-bmi}" method="post">

    <ul class="gender-options">
        <input id="man" type="radio" name="gender" value="male" required  />
        <label for="man">mężczyzna</label> &frasl;
        <input id="woman" type="radio" name="gender" value="female"/>
        <label for="woman">kobieta</label>
    </ul>


    Masa: <input type="number" class="form-control form-text" required placeholder="(kg)" name="masa"/>

    <!--<div class="text col-sm-12 error" th:if="${wzrost}">-->
        <!--<p class="text text-center">-->
            <!--To pole jest wymagane-->
        <!--</p>-->
    <!--</div>-->
    Wzrost: <input type="number" class="form-control form-text " required placeholder="(cm)" name="wzrost"/>

    <!--<div class="text col-sm-12 error" th:if="${wzrost}">-->
        <!--<p class="text text-center">-->
            <!--To pole jest wymagane-->
        <!--</p>-->
    <!--</div>-->
    <input type="submit" class="col-lg-10 btn btn-primary" value="Oblicz"/>
</form>

Now i used required but is not good solution

It seems like you want to implement server side validation. For this the best approach is to use validators and its bindingResult. Steps to implement server side validation is

1. Have for model

   public class PersonForm {
     private String name;
   
    public String getName() {
       return this.name;
   }
   
    public void setName(String name) {
       this.name = name;
   }
   }
   

2. Use form model in html

   <form action="#" th:action="@{/personForm}" th:object="${personForm}" method="post">
   <table>
       <tr>
           <td><label th:text="#{label.name}+' :'"></label></td>
           <td><input type="text" th:field="*{name}" /></td>
           <td th:if="${#fields.hasErrors('name')}" th:errors="*{name}">Generic Error</td>
       </tr>
       <tr>
           <td><button type="submit">Submit</button></td>
       </tr>
   </table>
   </form>
   

   3. Have validator class

   @Component

   public class PersonFormValidator implements Validator {
   @Override
   public boolean supports(Class<?> clazz) {
       return PersonForm.class.equals(clazz);
   }
   
   @Override
   public void validate(Object target, Errors errors) {
       ValidationUtils.rejectIfEmpty(errors, "name", "field.name.empty");
       PersonForm p = (PersonForm) target;
   
       if (p.getName().equalsIgnoreCase("XXX")) {
           errors.rejectValue("name", "Name cannot be XXX");
       }
   }}
   

   4. Bind validator to controller and let spring do the magic.

   @Controller

   public class WebController {
   @Autowired
   PersonFormValidator personFormValidator;
   
   
   @InitBinder("personForm")
   protected void initPersonFormBinder(WebDataBinder binder) {
   binder.addValidators(personFormValidator);
   }
   
   @PostMapping("/personForm")
   public String checkPersonInfo(@Validated PersonForm personForm, BindingResult bindingResult, final RedirectAttributes redirectAttributes) {
   if (bindingResult.hasErrors()) {
       return "personForm";
   }
   redirectAttributes.addFlashAttribute("personResult",  apiClientService.getPersonResult(personForm));
   return "redirect:/spouseForm";
   }
   }