problem z z walidacją

@Documented
@Constraint(validatedBy = UniqueLoginValidator.class)
@Target({ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
public @interface UniqueLogin {

    String message() default "This login already exists, please choose another one.";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};

}

public class UniqueEmailValidator implements ConstraintValidator<UniqueEmail, User> {

    @Autowired
    private UserService userService;

    @Override
    public void initialize(UniqueEmail uniqueEmail) {
    }

    public boolean isValid(User user, ConstraintValidatorContext context) {

        return !userService.findByEmail(user.getEmail()).isPresent();
    }
}

!userService.findByEmail(user.getEmail()).isPresent();

@PostMapping("/register")
    public String registerNewUser(@Valid User user, BindingResult bindingResult, Model model, RedirectAttributes redirectAttributes) {

if (!userService.uniqueEmail(user.getEmail())) bindingResult.addError(new ObjectError("inValid e-mail", "This e-mail already exists, please choose another one."));

...

public class MyPojo {

    @MyConstraint
    private String field;

    public String getField() {
        return field;
    }

    public void setField(String field) {
        this.field = field;
    }
}

@Documented
@Constraint(validatedBy = MyValidator.class)
@Target( { ElementType.METHOD, ElementType.FIELD })
@Retention(RetentionPolicy.RUNTIME)
public @interface MyConstraint {
    String message() default "Invalid String";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

public class MyValidator implements ConstraintValidator<MyConstraint, String> {

    @Autowired
    private MyService service;

    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        if (value == null) {
            return true;
        }

        return service.m(value);
    }

    @Override
    public void initialize(MyConstraint constraintAnnotation) {

    }
}

@Service
public class MyService {
    public boolean m(String s) {
        return "a".equals(s);
    }
}

@RestController
public class MyController {

    @PostMapping("/")
    public String m(@Validated @RequestBody MyPojo body) {
        return "ok";
    }
}

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

}

POST http://localhost:8080
Accept: application/json
Content-Type: application/json

{
    "field": "a"
}

POST http://localhost:8080

HTTP/1.1 200 
Content-Type: application/json;charset=UTF-8
Content-Length: 2
Date: Wed, 19 Jun 2019 20:41:45 GMT

ok

POST http://localhost:8080
Accept: application/json
Content-Type: application/json

{
    "field": "b"
}

POST http://localhost:8080

HTTP/1.1 400 
Content-Type: application/json;charset=UTF-8
Transfer-Encoding: chunked
Date: Wed, 19 Jun 2019 20:46:14 GMT
Connection: close

{
    (...)
    "defaultMessage": "Invalid String",
    (...)
}

public class EmailAvailableValidator implements ConstraintValidator<EmailAvailable, String> {

	@Resource
	private UserService userService;
	
    public void initialize(EmailAvailable constraintAnnotation) {}

    public boolean isValid(String object, ConstraintValidatorContext constraintContext) {
    	
        if (object == null)
            return true;

        return userService.isEmailAvailable(object);
    }
}

        @Bean
        public Validator validatorFactory () {
            return new LocalValidatorFactoryBean();
        }