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implementation 'com.fasterxml.jackson.module:jackson-module-kotlin:2.12.1'

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<dependency>
  <groupId>com.fasterxml.jackson.module</groupId>
  <artifactId>jackson-module-kotlin</artifactId>
  <version>2.12.1</version>
</dependency>

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@CustomGoogleEmailConstraint
private String email;

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public class CustomGoogleEmailValidator implements ConstraintValidator<CustomGoogleEmailConstraint, String> {
  @Override
  public void initialize(CustomGoogleEmailConstraint email) { }
  
  @Override
  public boolean isValid(String email, ConstraintValidatorContext constraintValidatorContext) { 
    return email != null && email.matches("^[a-zA-Z0–9.]+@gmail.com"); 
  }
}

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@Documented 
@Constraint(validatedBy = CustomGoogleEmailValidator.class) 
@Target({ ElementType.METHOD, ElementType.FIELD }) 
@Retention(RetentionPolicy.RUNTIME) 
public @interface CustomGoogleEmailConstraint {
  String message() default "The email must have \"@gmail.com\" extension";
  Class[] groups() default {};
  Class[] payload() default {};
}

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<dependency> 
  <groupId>org.hibernate</groupId> 
  <artifactId>hibernate-validator</artifactId> 
  <version>6.0.10.Final</version> 
</dependency>

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@NotEmpty(message = "The Full Name can't be null") 
@Size(min = 5, message = "{Size.Person.FullName}") 
private String fullName;

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<div class="alert alert-warning" th:if="${#fields.hasErrors('age')}" th:errors="*{age}"></div>

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@GetMapping("/add")
public String showAddPersonForm(Person person) { 
  return "add-person"; 
}

@PostMapping("/add") 
public String addPerson(@Valid Person person, BindingResult result, Model model) { 
  if (result.hasErrors()) { 
    return "add-person"; 
  }

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@Repository
public interface PersonRepository extends JpaRepository<Person, Long> { }