[Spring] 영속성컨텍스트, FetchType.LAZY, 프록시로 발생하는 JSON 이슈

에러가 발생하는 코드

에러메세지
Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: failed to lazily initialize a collection of role:

@RestController
@RequestMapping("/api")
@RequiredArgsConstructor
public class FolderController {

    private final FolderService folderService;

    @PostMapping("/folders")
    public List<Folder> addFolders(
            @RequestBody FolderRequestDto folderRequestDto,
            @AuthenticationPrincipal UserDetailsImpl userDetails
    ) {

        List<String> folderNames = folderRequestDto.getFolderNames();

        return folderService.addFolders(folderNames, userDetails.getUser());
    }

@Service
@RequiredArgsConstructor
public class FolderService {

    private final ProductRepository productRepository;
    private final FolderRepository folderRepository;
    private final UserRepository userRepository;


    // 로그인한 회원에 폴더들 등록
    @Transactional
    public List<Folder> addFolders(List<String> folderNames, User user) {


        // 입력으로 들어온 폴더 이름을 기준으로, 회원이 이미 생성한 폴더들을 조회합니다.
        List<Folder> existFolderList = folderRepository.findAllByUserAndNameIn(user, folderNames);

        List<Folder> folderList = new ArrayList<>();

        for (String folderName : folderNames) {
            // 이미 생성한 폴더가 아닌 경우만 폴더 생성
            if (!isExistFolderName(folderName, existFolderList)) {
                Folder folder = new Folder(folderName, user);
                folderList.add(folder);
            }
        }

        return folderRepository.saveAll(folderList);
    }

잘 동작하는 코드

@RestController
@RequestMapping("/api")
@RequiredArgsConstructor
public class FolderController {

    private final FolderService folderService;

    @PostMapping("/folders")
    public List<Folder> addFolders(
            @RequestBody FolderRequestDto folderRequestDto,
            @AuthenticationPrincipal UserDetailsImpl userDetails
    ) {

        List<String> folderNames = folderRequestDto.getFolderNames();


        return folderService.addFolders(folderNames, userDetails.getUsername());
    }

@Service
@RequiredArgsConstructor
public class FolderService {

    private final ProductRepository productRepository;
    private final FolderRepository folderRepository;
    private final UserRepository userRepository;


    // 로그인한 회원에 폴더들 등록
    @Transactional
    public List<Folder> addFolders(List<String> folderNames, String name) {

        User user = userRepository.findByUsername(name).orElseThrow(
                () -> new IllegalArgumentException("사용자가 존재하지 않습니다.")
        );

        // 입력으로 들어온 폴더 이름을 기준으로, 회원이 이미 생성한 폴더들을 조회합니다.
        List<Folder> existFolderList = folderRepository.findAllByUserAndNameIn(user, folderNames);

        List<Folder> folderList = new ArrayList<>();

        for (String folderName : folderNames) {
            // 이미 생성한 폴더가 아닌 경우만 폴더 생성
            if (!isExistFolderName(folderName, existFolderList)) {
                Folder folder = new Folder(folderName, user);
                folderList.add(folder);
            }
        }

        return folderRepository.saveAll(folderList);
    }

에러가 발생한 이유