@RestController
public class UserController {

    @PostMapping("/register")
    public ResponseEntity<?> registerUser(@RequestBody @Valid UserDto userDto) {
        // 處理用戶注冊請求
        return ResponseEntity.ok().build();
    }

    public static class UserDto {
        @NotNull
        private String username;

        @NotNull
        @Size(min = 6, max = 20)
        private String password;

        public String getUsername() {
            return username;
        }

        public void setUsername(String username) {
            this.username = username;
        }

        public String getPassword() {
            return password;
        }

        public void setPassword(String password) {
            this.password = password;
        }
    }
}

@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<?> handleMethodArgumentNotValidException(MethodArgumentNotValidException ex) {
        List<String> errors = ex.getBindingResult()
                .getAllErrors()
                .stream()
                .map(DefaultMessageSourceResolvable::getDefaultMessage)
                .collect(Collectors.toList());
        return ResponseEntity.badRequest().body(errors);
    }
}