1 概述 在本节课程中，我们继续学习多表关系，学习一对多关系。
在本例中，我们使用商品分类和商品两个实体，一个商品分类对应多个商品。
同上一节一样，继续采用Spring Boot+Spring Data JPA。工程结构如下：
重点对象，两个实体。
2 实体 分类实体Classification.java
package com.wangshenghua.entity;
import java.io.Serializable; import java.util.Set;
import javax.persistence.CascadeType; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.GenerationType; import javax.persistence.Id; import javax.persistence.OneToMany;
@Entity public class Classification implements Serializable {
private static final long serialVersionUID = -7378975775604019219L; @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private int id; private String name; private int pid; @OneToMany(mappedBy = "classification", cascade = CascadeType.ALL) private Set<Goods> goodes; public Classification() { } public Classification(String name, int pid) { super(); this.name = name; this.pid = pid; } // Getter and Setter} 一对多关系中，在一的一方，使用@OneToMany注释，多的一方作为一的一方属性，采用Set容器装。
1 2 @OneToMany(mappedBy = “classification”, cascade = CascadeType.ALL) private Set goodes; 级联方式是所有，CascadeType.ALL，这意味着删除当前分类，当前分类相关的商品都将被删除。
级联关系使用时要多加小时，特别是在修改和删除时，影响大。
商品实体Goods.java
package com.wangshenghua.entity;
import java.io.Serializable; import java.math.BigDecimal;
import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.GenerationType; import javax.persistence.Id; import javax.persistence.JoinColumn; import javax.persistence.ManyToOne;
@Entity public class Goods implements Serializable {
private static final long serialVersionUID = 5060150518287868308L; @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private int id; private String name; private String specifications; private BigDecimal price; private int quantity; @ManyToOne @JoinColumn(name = "classification_id") private Classification classification; public Goods() { } public Goods(String name, String specifications, BigDecimal price, int quantity) { super(); this.name = name; this.specifications = specifications; this.price = price; this.quantity = quantity; }// Getter and Setter
} 在分类往商品看，关系是一对多；从商品往分类看，关系是多对一。在商品实体中有分类属性：
@ManyToOne @JoinColumn(name = “classification_id”) private Classification classification; @ManyToOne注释表示多对一，即多个当前实体对应一个Classification实体。
@JoinColumn表示指向一个外键， 这里指向一的一方分类实体，在当前实体的外键。
3 数据访问对象 Spring Data JPA对Hibernate抽象后，使用更简单，实现一个接口便具有很多功能。
package com.wangshenghua.dao; import org.springframework.data.jpa.repository.JpaRepository; import com.wangshenghua.entity.Classification; public interface ClassificationRepository extends JpaRepository<Classification, Integer> { }
package com.wangshenghua.dao;
import org.springframework.data.jpa.repository.JpaRepository;
import com.wangshenghua.entity.Goods;
public interface GoodsRepository extends JpaRepository<Goods, Integer> {
} 4 驱动类
package com.wangshenghua.config;
import java.math.BigDecimal;
import org.springframework.beans.factory.annotation.Autowired; import org.springframework.boot.CommandLineRunner; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; import org.springframework.boot.autoconfigure.domain.EntityScan; import org.springframework.data.jpa.repository.config.EnableJpaRepositories;
import com.wangshenghua.dao.ClassificationRepository; import com.wangshenghua.dao.GoodsRepository; import com.wangshenghua.entity.Classification; import com.wangshenghua.entity.Goods;
@SpringBootApplication @EnableJpaRepositories(basePackages = “com.wangshenghua.dao”) @EntityScan(basePackages = “com.wangshenghua.entity”) public class HibernateOneToManyMappingApplication implements CommandLineRunner {
@Autowired private ClassificationRepository classificationRepository; @Autowired private GoodsRepository goodsRepository; public static void main(String[] args) { SpringApplication.run(HibernateOneToManyMappingApplication.class, args); } @Override public void run(String... args) throws Exception { // 清除表中已有数据 //classificationRepository.deleteAllInBatch(); //goodsRepository.deleteAllInBatch(); //新建分类 //Classification classify = new Classification("总分类",0); //classificationRepository.save(classify); //Classification subClassify = new Classification("服装",1); //classificationRepository.save(subClassify); // 新建商品 //Goods goods = new Goods("围巾","红色，1米5",new BigDecimal(200),300); //goods.setClassification(subClassify); Goods goods = new Goods("鞋子","43码黑色",new BigDecimal(388),100); goods.setClassification(classificationRepository.getOne(2)); goodsRepository.save(goods); }} 5 配置文件application.properties
spring.datasource.driver-class-name=com.mysql.jdbc.Driver spring.datasource.url=jdbc:mysql://localhost:3306/testdb?useUnicode=true&characterEncoding=UTF-8 spring.datasource.username=testdb spring.datasource.password=testdb
spring.jpa.properties.hibernate.dialect = org.hibernate.dialect.MySQL5InnoDBDialect spring.jpa.hibernate.ddl-auto = update
logging.level.org.hibernate.SQL=DEBUG logging.level.org.hibernate.type=TRACE 6 pom.xml
<?xml version="1.0" encoding="UTF-8"?>4.0.0 org.springframework.boot spring-boot-starter-parent 2.1.6.RELEASE com.wangshenghua hibernate-one-to-many-mapping 0.0.1-SNAPSHOT hibernate-one-to-many-mapping Demo project for Spring Boot
<properties> <java.version>1.8</java.version> <maven-jar-plugin.version>3.1.1</maven-jar-plugin.version> </properties> <dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-data-jpa</artifactId> </dependency> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>5.1.38</version> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-devtools</artifactId> <scope>runtime</scope> <optional>true</optional> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-test</artifactId> <scope>test</scope> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-maven-plugin</artifactId> </plugin> </plugins> </build> 7 总结 多表关系，特别要注意理清表之间的关系。共用图形工具梳理表关系，如MySQL Workbench，梳理后可以直观地认识实体之间关系。在Spring Data JPA创建表后，将表结构与 MySQL Workbench 建模图对比，看是否一致。
本节课程源码已经上传到github，可以前往下载。
