Spring Data JPA 与实体建模

第 5 章 · Spring Data JPA 与实体建模

本章目标：使用 @Entity 定义商品与分类；编写 JpaRepository 接口实现 CRUD；掌握 一对多 / 多对一 关系映射与懒加载注意点；配置 H2/MySQL 数据源；了解 Flyway 数据库版本迁移；将 ch02～ch04 的内存数据替换为持久化层；对照 shop-demo Django ORM 说明异同。

学时建议：6～7 小时（含 2.5 小时跟练）

前置：完成 spring-boot-web ch04；java-dev ch11（JDBC/SQL）；django-web ch05 Model 章节更佳。

5.1 场景说明：从内存 List 到数据库

ch02 的 InMemoryProductCatalogService 重启即丢失。运营后台需要持久化商品与分类，与 shop-demo 的 Product、Category 模型对齐（教学虚构字段）。

API 仍经虚构 https://api.example.com 对外；数据库连接串仅用本地或占位 hostname，勿写真实生产凭证。

5.2 依赖与数据源

5.2.1 pom.xml

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
</dependency>

5.2.2 application-dev.yml

spring:
  datasource:
    url: jdbc:h2:mem:shop_spring;DB_CLOSE_DELAY=-1;MODE=MySQL
    driver-class-name: org.h2.Driver
    username: sa
    password:
  h2:
    console:
      enabled: true
      path: /h2-console
  jpa:
    hibernate:
      ddl-auto: validate   # 配合 Flyway，勿用 create-drop 上生产
    show-sql: true
    properties:
      hibernate:
        format_sql: true

Django：migrate 由迁移文件驱动；Spring 推荐 validate + Flyway 而非 ddl-auto=update。

5.3 实体 Category 与 Product

5.3.1 Category

package com.example.shop.catalog.entity;

import jakarta.persistence.*;
import java.util.ArrayList;
import java.util.List;

@Entity
@Table(name = "categories")
public class Category {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false, unique = true, length = 64)
    private String name;

    @OneToMany(mappedBy = "category", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Product> products = new ArrayList<>();

    protected Category() {}

    public Category(String name) {
        this.name = name;
    }

    public Long getId() { return id; }
    public String getName() { return name; }
    public List<Product> getProducts() { return products; }
}

5.3.2 Product

@Entity
@Table(name = "products", indexes = {
        @Index(name = "idx_product_sku", columnList = "sku", unique = true)
})
public class Product {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false, unique = true, length = 32)
    private String sku;

    @Column(nullable = false, length = 128)
    private String name;

    @Column(nullable = false, precision = 12, scale = 2)
    private BigDecimal price;

    @Column(nullable = false)
    private int stock = 0;

    @Column(nullable = false)
    private boolean active = true;

    @Column(nullable = false, updatable = false)
    private Instant createdAt = Instant.now();

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "category_id")
    private Category category;

    protected Product() {}

    public Product(String sku, String name, BigDecimal price, Category category) {
        this.sku = sku;
        this.name = name;
        this.price = price;
        this.category = category;
    }
}

5.3.3 与 Django Model 对照

5.4 Repository 接口

public interface ProductRepository extends JpaRepository<Product, Long> {

    Optional<Product> findBySku(String sku);

    Page<Product> findByActiveTrueAndNameContainingIgnoreCase(String name, Pageable pageable);

    Page<Product> findByActiveTrue(Pageable pageable);
}

public interface CategoryRepository extends JpaRepository<Category, Long> {
    Optional<Category> findByName(String name);
}

无需实现类——Spring Data 在运行时生成代理。

5.4.1 自定义 @Query 示例

@Query("SELECT p FROM Product p JOIN FETCH p.category WHERE p.active = true")
List<Product> findActiveWithCategory();

JOIN FETCH 一次加载关联分类，避免列表页 N+1。