public class User {
    @NotNull
    private Long id;
    @NotBlank
    @Size(min = 2, max = 50)
    private String firstName;
    @NotBlank
    @Size(min = 2, max = 50)
    private String lastName;
    @Email
    private String email;
    @NotNull
    @Min(18)
    @Max(99)
    private Integer age;
    @NotEmpty
    private List<String> hobbies;
    @Pattern(regexp = "[A-Z]{2}\d{4}")
    private String employeeId;

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = UniqueTitleValidator.class)
public @interface UniqueTitle {
    String message() default "Title must be unique";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

public interface PostRepository extends JpaRepository<Post, Long> {
    Post findByTitle(String title);
}

@Component
public class UniqueTitleValidator implements ConstraintValidator<UniqueTitle, String> {
    @Autowired
    private PostRepository postRepository;
    @Override
    public boolean isValid(String title, ConstraintValidatorContext context) {
        if (title == null) {
            return true;
        }
        return Objects.isNull(postRepository.findByTitle(title));
    }
}

public class Post {
    @UniqueTitle
    private String title;
    @NotNull
    private String body;
}

public class UserDTO {
    @NotBlank
    private String username;
    @NotBlank
    private String password;
}

@RestController
@RequestMapping("/users")
@Validated
public class UserController {
    @Autowired
    private UserService userService;
    @PostMapping
    public ResponseEntity<String> createUser(@Valid @RequestBody UserDTO userDto) {
        userService.createUser(userDto);
        return ResponseEntity.status(HttpStatus.CREATED).body("User created successfully");
    }
}

public class User {
    @NotBlank(message = "用戶名不能為空")
    private String name;
    @NotBlank(message = "Email不能為空")
    @Email(message = "無(wú)效的Emaild地址")
    private String email;
    @NotNull(message = "年齡不能為空")
    @Min(value = 18, message = "年齡必須大于18")
    @Max(value = 99, message = "年齡必須小于99")
    private Integer age;
}

@RestController
@RequestMapping("/users")
public class UserController {
    @Autowired
    private UserService userService;
    @PostMapping
    public ResponseEntity<String> createUser(@Valid @RequestBody User user, BindingResult result) {
        if (result.hasErrors()) {
            List<String> errorMessages = result.getAllErrors().stream()
                    .map(DefaultMessageSourceResolvable::getDefaultMessage)
                    .collect(Collectors.toList());
            return ResponseEntity.badRequest().body(errorMessages.toString());
        }
        // save the user to the database using UserService
        userService.saveUser(user);
        return ResponseEntity.status(HttpStatus.CREATED).body("User created successfully");
    }
}

# messages.properties
user.name.required=Name is required.
user.email.invalid=Invalid email format.
user.age.invalid=Age must be a number between 18 and 99.

user.name.required=名稱不能為空.
user.email.invalid=無(wú)效的email格式.
user.age.invalid=年齡必須在18到99歲之間.

public class User {
    @NotNull(message = "{user.id.required}")
    private Long id;
    @NotBlank(message = "{user.name.required}")
    private String name;
    @Email(message = "{user.email.invalid}")
    private String email;
    @NotNull(message = "{user.age.required}")
    @Min(value = 18, message = "{user.age.invalid}")
    @Max(value = 99, message = "{user.age.invalid}")
    private Integer age;
}

@Configuration
public class AppConfig {
    @Bean
    public MessageSource messageSource() {
        ResourceBundleMessageSource messageSource = new ResourceBundleMessageSource();
        messageSource.setBasename("messages");
        messageSource.setDefaultEncoding("UTF-8");
        return messageSource;
    }
    @Bean
    public LocalValidatorFactoryBean validator() {
        LocalValidatorFactoryBean validatorFactoryBean = new LocalValidatorFactoryBean();
        validatorFactoryBean.setValidationMessageSource(messageSource());
        return validatorFactoryBean;
    }
}

public class User {
    @NotBlank(groups = Default.class)
    private String firstName;
    @NotBlank(groups = Default.class)
    private String lastName;
    @Email(groups = EmailNotEmpty.class)
    private String email;
    // getters and setters omitted for brevity
    public interface EmailNotEmpty {}
    public interface Default {}
}

@RestController
@RequestMapping("/users")
@Validated
public class UserController {
    public ResponseEntity<String> createUser(
            @Validated({org.example.model.ex6.User.EmailNotEmpty.class}) @RequestBody User userWithEmail,
            @Validated({User.Default.class}) @RequestBody User userWithoutEmail)
    {
        // Create the user and return a success response
    }
}

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = EndDateAfterStartDateValidator.class)
public @interface EndDateAfterStartDate {
    String message() default "End date must be after start date";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

public class EndDateAfterStartDateValidator implements ConstraintValidator<EndDateAfterStartDate, TaskForm> {
    @Override
    public boolean isValid(TaskForm taskForm, ConstraintValidatorContext context) {
        if (taskForm.getStartDate() == null || taskForm.getEndDate() == null) {
            return true;
        }
        return taskForm.getEndDate().isAfter(taskForm.getStartDate());
    }
}

@EndDateAfterStartDate
public class TaskForm {
    @NotNull
    @DateTimeFormat(pattern = "yyyy-MM-dd")
    private LocalDate startDate;
    @NotNull
    @DateTimeFormat(pattern = "yyyy-MM-dd")
    private LocalDate endDate;
}

@RestControllerAdvice
public class RestExceptionHandler extends ResponseEntityExceptionHandler {
    @ExceptionHandler(MethodArgumentNotValidException.class)
    protected ResponseEntity<Object> handleMethodArgumentNotValid(MethodArgumentNotValidException ex,
                                                                  HttpHeaders headers, HttpStatus status,
                                                                  WebRequest request) {
        Map<String, Object> body = new LinkedHashMap<>();
        body.put("timestamp", LocalDateTime.now());
        body.put("status", status.value());
        // Get all errors
        List<String> errors = ex.getBindingResult()
                .getFieldErrors()
                .stream()
                .map(x -> x.getDefaultMessage())
                .collect(Collectors.toList());
        body.put("errors", errors);
        return new ResponseEntity<>(body, headers, status);
    }
}

@DataJpaTest
public class UserValidationTest {
    @Autowired
    private TestEntityManager entityManager;
    @Autowired
    private Validator validator;
    @Test
    public void testValidation() {
        User user = new User();
        user.setFirstName("John");
        user.setLastName("Doe");
        user.setEmail("invalid email");
        Set<ConstraintViolation<User>> violations = validator.validate(user);
        assertEquals(1, violations.size());
        assertEquals("must be a well-formed email address", violations.iterator().next().getMessage());
    }
}