转@OneToMany或@ManyToOne的用法-annotation关系映射篇（上)

原文：http://blog.sina.com.cn/s/blog_6fef491d0100obdm.html

例如我们用一个例子来开启JPA的一对多和多对一的学习。

比如你去当当网上买书籍，当当网就给你一个订单。

 
通过上面的业务活动描述你可以分析得出：一个订单可以包含一个或多个订单项.那么我们将将订单和订单项设计关系为(1:N)一对多的关系(排除0)。得出关系我们就使用JPA来实现这个关系(关于建立JPA和基本的配置我就不再说了，如果不明白请看JPA入门篇)。

首先建立订单实体类

 1 import java.util.HashSet;
 2 import java.util.Set;
 3
 4 import javax.persistence.CascadeType;
 5 import javax.persistence.Column;
 6 import javax.persistence.Entity;
 7 import javax.persistence.Id;
 8 import javax.persistence.OneToMany;
 9 import javax.persistence.Table;
10
11 @Entity
12 @Table(name="orders")
13 public class Order {
14
15     private String orderid;
16     private Float amount = 0f;
17     private Set<OrderItem> items = new HashSet<OrderItem>();
18
19     @Id
20     @Column(length = 12)
21     public String getOrderid() {
22         return orderid;
23     }
24
25     public void setOrderid(String orderid) {
26         this.orderid = orderid;
27     }
28
29     @Column(nullable = false)
30     public Float getAmount() {
31         return amount;
32     }
33
34     public void setAmount(Float amount) {
35         this.amount = amount;
36     }
37
38     @OneToMany(cascade = { CascadeType.REFRESH, CascadeType.PERSIST,CascadeType.MERGE, CascadeType.REMOVE },mappedBy ="order") //这里配置关系，并且确定关系维护端和被维护端。mappBy表示关系被维护端，只有关系端有权去更新外键。这里还有注意OneToMany默认的加载方式是赖加载。当看到设置关系中最后一个单词是Many，那么该加载默认为懒加载
39     public Set<OrderItem> getItems() {
40         return items;
41     }
42
43     public void setItems(Set<OrderItem> items) {
44         this.items = items;
45     }
46
47          /**
48            *该方法用于向order中加order项
49           /*
50     public void addOrderItem(OrderItem orderItem){
51         orderItem.setOrder(this);//用关系维护端来维护关系
52         this.items.add(orderItem);
53     }
54
55 }  

订单项的实体类

 1 import javax.persistence.CascadeType;
 2 import javax.persistence.Column;
 3 import javax.persistence.Entity;
 4 import javax.persistence.GeneratedValue;
 5 import javax.persistence.Id;
 6 import javax.persistence.JoinColumn;
 7 import javax.persistence.ManyToOne;
 8
 9 @Entity
10 public class OrderItem {
11     private Integer id;
12     private String productName;
13     private Float sellPrice = 0f;
14     private Order order;
15
16     @Id
17     @GeneratedValue
18     public Integer getId() {
19         return id;
20     }
21
22     public void setId(Integer id) {
23         this.id = id;
24     }
25
26     @Column(length = 40, nullable = false)
27     public String getProductName() {
28         return productName;
29     }
30
31     public void setProductName(String productName) {
32         this.productName = productName;
33     }
34
35     @Column(nullable = false)
36     public Float getSellPrice() {
37         return sellPrice;
38     }
39
40     public void setSellPrice(Float sellPrice) {
41         this.sellPrice = sellPrice;
42     }
43
44     @ManyToOne(cascade = {CascadeType.MERGE,CascadeType.REFRESH }, optional = true)
45     @JoinColumn(name="order_id")//这里设置JoinColum设置了外键的名字，并且orderItem是关系维护端
46     public Order getOrder() {
47         return order;
48     }
49
50     public void setOrder(Order order) {
51         this.order = order;
52     }
53
54 }  

下面看看测试类

 1 import javax.persistence.EntityManager;
 2 import javax.persistence.EntityManagerFactory;
 3 import javax.persistence.Persistence;
 4
 5 import org.junit.BeforeClass;
 6 import org.junit.Test;
 7
 8 import cn.itcast.bean.Order;
 9 import cn.itcast.bean.OrderItem;
10
11 public class OneToManyTest {
12
13     @BeforeClass
14     public static void setUpBeforeClass() throws Exception {
15     }
16     @Test public void addOrder(){
17          EntityManagerFactory factory = Persistence.createEntityManagerFactory("test");
18          EntityManager em = factory.createEntityManager();
19          em.getTransaction().begin(); // start transaction
20
21                    Order order = new Order();
22          order.setAmount(34f);
23          order.setOrderid("00001");
24
25                    //order中包含的OrderItem项OrderItem1，OrderItem2
26          OrderItem orderItem1 = new OrderItem();
27          orderItem1.setProductName("书");
28          orderItem1.setSellPrice(22f);
29          order.addOrderItem(orderItem1); //add orderitem in order
30
31          OrderItem orderItem2 = new OrderItem();
32          orderItem2.setProductName("篮球");
33          orderItem2.setSellPrice(100f);
34          order.addOrderItem(orderItem2);
35
36          em.persist(order); //persist order object
37          em.getTransaction().commit(); //commit transaction
38          em.close();
39          factory.close();
40     }
41 }