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EJB3的一对多映射使用@OneToMany来设置，如果是双向的一对多映射，在many方需要使用@ManyToOne设置。在本书中给出两个表，其他一个表t_customers在上一篇文章中已给出了，另一个表的结构如图1所示。
t_customers和t_orders表是一对多关系，一个Customer可能有多个Order，而一个Order只能有一个Customer。
在Customer类中需要定义一个集合类型的属性，用来保存多个Order对象，Customer类的代码如下：
- package entity;
- import java.util.Collection;
- import javax.persistence.CascadeType;
- import javax.persistence.Entity;
- import javax.persistence.FetchType;
- import javax.persistence.GeneratedValue;
- import javax.persistence.GenerationType;
- import javax.persistence.Id;
- import javax.persistence.JoinColumn;
- import javax.persistence.JoinTable;
- import javax.persistence.ManyToMany;
- import javax.persistence.OneToMany;
- import javax.persistence.OneToOne;
- import javax.persistence.PrimaryKeyJoinColumn;
- import javax.persistence.Table;
- @Entity
- @Table(name = "t_customers")
- public class Customer
- {
- private int id;
- private String name;
- private Referee referee;
- private Collection<Order> orders;
- @OneToMany(mappedBy = "customer", cascade = CascadeType.ALL)
- public Collection<Order> getOrders()
- {
- return orders;
- }
- public void setOrders(Collection<Order> orders)
- {
- this.orders = orders;
- }
- @Id
- @GeneratedValue(strategy = GenerationType.IDENTITY)
- public int getId()
- {
- return id;
- }
- ... ...
- }
其中@OneToMany的mappedBy属性指定了Order类中获得Customer对象的属性名。Order类的代码如下：
- package entity;
- import javax.persistence.Column;
- import javax.persistence.Entity;
- import javax.persistence.GeneratedValue;
- import javax.persistence.GenerationType;
- import javax.persistence.Id;
- import javax.persistence.JoinColumn;
- import javax.persistence.JoinColumns;
- import javax.persistence.JoinTable;
- import javax.persistence.ManyToOne;
- import javax.persistence.Table;
- @Entity
- @Table(name = "t_orders")
- public class Order
- {
- private int id;
- private String productId;
- private int count;
- private Customer customer;
- @ManyToOne
- @JoinColumn(name = "customer_id")
- public Customer getCustomer()
- {
- return customer;
- }
- public void setCustomer(Customer customer)
- {
- this.customer = customer;
- }
- @Id
- @GeneratedValue(strategy = GenerationType.IDENTITY)
- public int getId()
- {
- return id;
- }
- ... ...
- }
其中@JoinColumn注释的name属性指定t_orders表中用于连接t_customers表的外键名。
可以使用下面的代码进行测试：
- Customer customer = new Customer();
- customer.setName("微软");
- List<Order> orders = new ArrayList<Order>();
- Order order =new Order();
- order.setProductId("1234");
- order.setCount(20);
- order.setCustomer(customer);
- orders.add(order);
- order = new Order();
- order.setProductId("4321");
- order.setCount(12);
- order.setCustomer(customer);
- orders.add(order);
- customer.setOrders(orders);
- em.persist(customer);
除此之外，还可以使用@JoinTable指定连接表来映射one-to-many关系。连接表的结构如图2所示。
首选需要将Customer类的getOrders方法修改成下的形式：
- @OneToMany
- @JoinTable(name = "t_customers_orders", joinColumns = @JoinColumn(name = "t_customers_id",
- referencedColumnName = "id"), inverseJoinColumns =
- @JoinColumn(name = "orders_id", referencedColumnName = "id"))
- public Collection<Order> getOrders()
- {
- return orders;
- }
其中name属性指定图2所示的连接表的名称。joinColumns指定了t_customers表和t_customers_orders表中相连接 的字段。inverseJoinColumns指定了t_orders表和t_customers_orders表中相连接的字段。这时t_orders 表中的customer_id字段就不再需要了。因此，需要将Order表中的customer属性（getter和setter方法）去掉。
在持久化Customer和Order对象时，需要对Order对象进行持久化后才能对Customer对象进行持久化。
