제가 JPA1강의를 듣고 SPRING DATA JPA도 듣고 난 후 JPA1에서 JPA로 만든 예제를 SpringDataJPA로 바꾸고 싶어서 시도해봤는데요.ItemRepository, MemberRepository, OrderRepository는 아래와 같이 만들고 public interface ItemRepository extends JpaRepository<Item,Long> { Item findItemById(Long id); }  public interface MemberRepository extends JpaRepository<Member, Long> { Member findMemberById(Long id); @Query("select m from Member m where m.name =:name") List<Member> findByName(@Param("name") String name); } public interface OrderRepository extends JpaRepository<Order,Long> { Order findOrderById(Long id);}동적쿼리부분은 OrderRepositoryImpl을 만들어 OrderRepository를 implements 하는 식으로 해봤는데요 @Repository@RequiredArgsConstructorpublic class OrderRepositoryImpl implements OrderRepository { private final EntityManager em; public List<Order> findAllByString(OrderSearch orderSearch) { //language=JPAQL String jpql = "select o From Order o join o.member m"; boolean isFirstCondition = true; if (orderSearch.getOrderStatus() != null) { if (isFirstCondition) { jpql += " where"; isFirstCondition = false; } else { jpql += " and"; } jpql += " o.status = :status"; } //회원 이름 검색 if (StringUtils.hasText(orderSearch.getMemberName())) { if (isFirstCondition) { jpql += " where"; isFirstCondition = false; } else { jpql += " and"; } jpql += " m.name like :name"; } TypedQuery<Order> query = em.createQuery(jpql, Order.class) .setMaxResults(1000); //최대 1000건 if(orderSearch.getOrderStatus()!=null) { query = query.setParameter("status", orderSearch.getOrderStatus()); } if(StringUtils.hasText(orderSearch.getMemberName())) { query = query.setParameter("name", orderSearch.getMemberName()); } return query.getResultList(); }@Override......@Override......}그리고 나서 ItemService, MemberService, OrderService를 밑에 처럼 작성하였고 @Service@Transactional(readOnly=true)@RequiredArgsConstructorpublic class ItemService { private final ItemRepository itemRepository; @Transactional public void saveItem(Item item){ itemRepository.save(item); } @Transactional public void updateItem(Long itemId, String name, int price, int stockQuantity){ Item findItem = itemRepository.findItemById(itemId); findItem.setName(name); findItem.setPrice(price); findItem.setStockQuantity(stockQuantity); } public List<Item> findItems(){ return itemRepository.findAll(); } public Item findOne(Long itemId){ return itemRepository.findItemById(itemId); }} @Service@Transactional(readOnly = true)@RequiredArgsConstructorpublic class MemberService { private final MemberRepository memberRepository; //회원가입 @Transactional public Long join(Member member){ validateDuplicateMember(member); Member savedMember = memberRepository.save(member); return savedMember.getId(); } // 중복회원 검증 private void validateDuplicateMember(Member member) { List<Member> findMembers = memberRepository.findByName(member.getName()); if(!findMembers.isEmpty()){ throw new IllegalStateException("이미 존재하는 회원입니다!"); } } // 회원 전체 조회 public List<Member> findAllMember(){ return memberRepository.findAll(); } // 회원 조회 public Member findMember(Long id){ return memberRepository.findMemberById(id); }}@Service@Transactional(readOnly = true)@RequiredArgsConstructorpublic class OrderService { private final OrderRepositoryImpl orderRepositoryImpl; private final MemberRepository memberRepository; private final ItemRepository itemRepository; //상품 주문 @Transactional public Long order(Long memberId, Long itemId, int count){ //엔티티 조회 Member member = memberRepository.findMemberById(memberId); Item item = itemRepository.findItemById(itemId); //배송정보 생성 Delivery delivery = new Delivery(); delivery.setAddress(member.getAddress()); //주문 상품 생성 OrderItem orderItem = OrderItem.createOrderItem(item, item.getPrice(), count); //주문 생성 Order order = Order.createOrder(member, delivery, orderItem); //주문 저장 orderRepositoryImpl.save(order); return order.getId(); } //상품 취소 @Transactional public void cancelOrder(Long orderId){ Order order = orderRepositoryImpl.findOrderById(orderId); order.cancel(); } // 상품 조회 public Order findOne(Long orderId){ return orderRepositoryImpl.findOrderById(orderId); } // public List<Order> findOrders(OrderSearch orderSearch) { return orderRepositoryImpl.findAllByString(orderSearch); }}실행을 하였을 때 회원가입과 상품등록을 하면 DB에 Member와 Item은 저장이 되지만 상품 주문을 하면 Order와 OrderItem에는 아래와 같이 아무런 정보도 저장되지 않습니다. 뭐가 틀린걸까 하고 엔티티들을 다시 작성해보고 html문서들도 다시 작성해보았는데도 아무런 이상이 없었고Spring Data jpa를 jpa로 다시 바꾸고 나니까 DB에 Order와 OrderItem들이 저장되었습니다. 제가 Spring Data Jpa를 어떤식으로 잘못 쓴건가요? 그리고 또 하나 질문이 있는데요 위에 MemberRepository를 보시면 Member findMemberById(Long id); 이런식으로 스프링데이터jpa안에 메소드를 만들고MemberService의 회원조회를public Member findMember(Long id){ return memberRepository.findMemberById(id);}이렇게 적었는데 MemberRepsitory에서 Member findMemberById(Long id) 메소드를 지우고 MemberService의 회원조회에서 public Member findMember(Long id){ return memberRepository.findById(id).orElse(null);}이런식으로 orElse(null)을 써도 되는건가요??