我最近一直在開發一個 Spring Boot 項目，該項目以 JSON 格式更新和從 MySQL 數據庫中獲取數據，但是當我運行我的應用程序時，我有一個錯誤頁面：[我瀏覽器上的錯誤截圖][1] [1]: https://i.stack.imgur.com/CkYmr.png我的實體類是：package com.project.project.entities;import javax.persistence.*;import java.io.Serializable;import java.util.List;@Entity@Table(name = "products")// why serializable ?? every entity in JPA is automatically-serializable,  connection between different networkspublic class Product implements Serializable {    @Id    @GeneratedValue(strategy = GenerationType.IDENTITY) // assign a unique value to your identity field automatically    private Long id;    private String designation;    private int price;    private int quantity;    private int category_id;    // the owning side of the relationship, side of the foreign key    @ManyToOne(fetch = FetchType.LAZY )// many products to one category    @JoinColumn(name = "category_id" , insertable = false , updatable = false)    // means that the product table will have a fk_column named...    private Category category;    // categoryId foreign key referencing to the primary key on Category    // Double and Integer in case both variables are unknown -> Category constructor    public Product(Long id, String designation, Integer price, Integer quantity, int categor_id) {        this.id = id;        this.designation = designation;        this.price = price;        this.quantity = quantity;        this.category_id = category_id;    }我的存儲庫：import com.project.project.entities.Product;import org.springframework.data.jpa.repository.JpaRepository;import org.springframework.stereotype.Repository;@Repositorypublic interface ProductRepository extends JpaRepository<Product,Long> {}