상품 엔티티

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "dtype")
@Data @Getter
public abstract class Item {

    @Id
    @GeneratedValue
    @Column(name = "item_id")
    private Long id;

    private String name;
    private int price;
    private int stockQuantity;

    @ManyToMany(mappedBy = "items")
    private List<Category> categories = new ArrayList<Category>();

    //==비지니스 로직==//
    public void addStock(int quantity){
        this.stockQuantity += stockQuantity;
    }

    public  void removeStock(int quantity){
        int restStock = this.stockQuantity - quantity;
        if (restStock < 0){
            throw new NotEnoughStockException("need more stock");
        }
        this.stockQuantity = restStock;
    }
}

상품 레포지토리

@Repository
@RequiredArgsConstructor
public class ItemRepository {

    private final EntityManager em;

    public void save(Item item){
        if (item.getId() == null){  //새롭게 db에 등록
            em.persist(item);
        } else {    //이미 등록된 item 업데이트.
            em.merge(item);
        }
    }

    public Item findOne(Long id){
        return em.find(Item.class, id);
    }

    public List<Item> findAll(){
        return em.createQuery("select i from Item i", Item.class)
                .getResultList();
    }
}

상품 서비스

@Service
@Transactional(readOnly = true)
@RequiredArgsConstructor
public class ItemService {

    private final ItemRepository itemRepository;

    @Transactional
    public void saveItem(Item item){
        itemRepository.save(item);
    }

    public List<Item> findItems(){
        return itemRepository.findAll();
    }

    public Item findOne(Long itemId){
        return itemRepository.findOne(itemId);
    }
}