假设有两个表，订单表和产品表，订单跟产品的关系是一对多的关系，那么在JPA中怎样表示一对多的关系呢？实体关系一对多映射有两种方式：

1 外键关联

//订单表
@Entity
@Table(name = "orders")
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @OneToMany(cascade = {CascadeType.ALL})
    @JoinColumn(name = "order_id")
    private List<Product> productList;

    ...
}

//产品表
@Entity
@Table(name = "product")
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    ...

这样在表product中会增加一列order_id

2 表关联

//订单表
@Entity
@Table(name = "orders")
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @OneToMany(cascade = {CascadeType.ALL})
    @JoinTable(name = "order_has_product", joinColumns = {@JoinColumn(name = "order_id", referencedColumnName = "id")},inverseJoinColumns = {@JoinColumn(name = "product_id", referencedColumnName = "id")})
    private List<Product> productList;

    ...
}

这样在product表中不会增加任何外键，而是新建了一张order_has_product表