//FollowCommandService.java
/*Follow 요청 처리*/
@Transactional
@Override
public FollowUserApiResponse followUser(FollowUserCommand command) {
//   ...
  /*자기 자신을 팔로유 하려는 경우 예외 발생*/
  if (Objects.equal(command.userId(), command.targetUserId())) {
    throw new UserException(ErrorCode.SELF_FOLLOW_REQUEST_NOT_ALLOWED);
  }
//   ...
}

//GlobalExceptionHandler.java
/*전역 애외 처리기*/
@RestControllerAdvice
public class GlobalExceptionHandler extends ResponseEntityExceptionHandler {

  //   ...
  /*User*/
  @ExceptionHandler(UserException.class)
  public ResponseEntity<ErrorResponse> handleUserException(UserException ex, WebRequest request) {

    return ResponseEntity.status(
            ex.getErrorCode().getHttpStatus()) //UserException 감지 후 ErrorResponse로 변환 후 반환
        .body(new ErrorResponse(ex.getErrorCode()));
  }
//   ...
}

//JwtAuthenticationFilter.java
@Component
public class JwtAuthenticationFilter extends OncePerRequestFilter {

  private final CustomAuthenticationEntryPoint authenticationEntryPoint;

  //   ...
  @Override
  protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response,
      FilterChain filterChain) throws ServletException, IOException {
    try {
//         ...
      /*블랙리스트에 등록되 토큰이면 예외 발생*/
      if (authManager.isBlacklisted(token)) {
        throw new TokenException(ErrorCode.TOKEN_INVALID); //도메인 CustomException 발생
      }

//         ...
    } catch (TokenException e) {
      TokenAuthenticationException exception = new TokenAuthenticationException(
          e.getErrorCode()); //도메인 예외 -> AuthenticationException 하위 예외로 변환
      authenticationEntryPoint.commence(request, response,
          exception); //AuthenticationEntryPoint 호출해서 변환된 예외를 직접 전달
      return;
    }

    filterChain.doFilter(request, response);
  }
}

//CustomAuthenticationEntryPoint.java
@Component
public class CustomAuthenticationEntryPoint implements AuthenticationEntryPoint {

  private final ObjectMapper objectMapper;

  @Override
  public void commence(HttpServletRequest request, HttpServletResponse response,
      AuthenticationException authException) throws IOException, ServletException {

    //응답 기본 설정
    response.setCharacterEncoding("UTF-8");
    response.setContentType("application/json; charset=utf-8");

    //커스텀 Authentication 예외 처리
    /*TokenAuthenticationException일 경우*/
    if (authException instanceof TokenAuthenticationException) {

      ErrorCode errorCode = ((TokenAuthenticationException) authException).getErrorCode();
      response.setStatus(errorCode.getHttpStatus().value()); //ErrorCode에 매핑된 HttpStatus 추출
      objectMapper.writeValue(response.getWriter(), //MVC 내부 예외 응답 포맷과 동일하게 실패 바디 작성
          ApiResponse.fail(errorCode));

      return;
    }
    if (authException instanceof UserAuthenticationException) { //다른 커스텀 Authentication 예외 처리
//      ...
      return;
    }
    // 기본 인증 예외는 401로 통일
    ErrorCode errorCode = ErrorCode.AUTHENTICATION_ERROR;
    response.setStatus(errorCode.getHttpStatus().value());
    objectMapper.writeValue(response.getWriter(),
        ApiResponse.fail(errorCode));

    return;
  }
}