我在學習 spring 時做了這個例子，一切正常，現在我正在編寫自己的項目，我不明白為什么當我嘗試從EntityManager 類從 MySQL 數據庫中刪除對象產品類（我不會在此處粘貼所有方法，例如 getter 和 setter 等）import javax.persistence.*;@Entitypublic class Product {    @Id    @GeneratedValue(strategy = GenerationType.AUTO)    private Integer id;    private String name;    @OneToOne    private ProductType type;    private float price;    private String description;    //more code...}ProductRepository（我只顯示刪除方法）import javax.persistence.EntityManager;import javax.persistence.PersistenceContext;import javax.transaction.Transactional;public class DBProductRepository implements ProductRepository{    @PersistenceContext    private EntityManager entityManager;    @Override    @Transactional    public void deleteProduct(Integer id) {        entityManager.remove(id);    }    //more code...}ProductService（我只顯示刪除方法）@Servicepublic class ProductService {    public void deleteProduct(Integer id) {        productRepository.deleteProduct(id);    }    //more code...}產品控制器@Controllerpublic class ProductController {    @RequestMapping("/products")    public String getProducts(Model model){        List<Product> products = productService.getAllProducts();        model.addAttribute("products",products);        return "products";    }    @RequestMapping(value="/product/delete/{id}")    public String deleteProduct(@PathVariable("id") Integer id){        productService.deleteProduct(id);        return "redirect:/products";    }    //more code...}