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

 import java.util.HashSet;
 import java.util.Set;   

 import javax.persistence.CascadeType;
 import javax.persistence.Column;
 import javax.persistence.Entity;
 import javax.persistence.Id;
 import javax.persistence.OneToMany;
 import javax.persistence.Table;   

 @Entity
 @Table(name="orders")
 public class Order {   

     private String orderid;
     private Float amount = 0f;
     private Set<OrderItem> items = new HashSet<OrderItem>();   

     @Id
     @Column(length = 12)
     public String getOrderid() {
         return orderid;
     }   

     public void setOrderid(String orderid) {
         this.orderid = orderid;
     }   

     @Column(nullable = false)
     public Float getAmount() {
         return amount;
     }   

     public void setAmount(Float amount) {
         this.amount = amount;
     }   

     @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
     public Set<OrderItem> getItems() {
         return items;
     }   

     public void setItems(Set<OrderItem> items) {
         this.items = items;
     }   

          /**
            *该方法用于向order中加order项
           /*
     public void addOrderItem(OrderItem orderItem){
         orderItem.setOrder(this);//用关系维护端来维护关系
         this.items.add(orderItem);
     }   

 }  

订单项的实体类

 import javax.persistence.CascadeType;
 import javax.persistence.Column;
 import javax.persistence.Entity;
 import javax.persistence.GeneratedValue;
 import javax.persistence.Id;
 import javax.persistence.JoinColumn;
 import javax.persistence.ManyToOne;   

 @Entity
 public class OrderItem {
     private Integer id;
     private String productName;
     private Float sellPrice = 0f;
     private Order order;   

     @Id
     @GeneratedValue
     public Integer getId() {
         return id;
     }   

     public void setId(Integer id) {
         this.id = id;
     }   

     @Column(length = 40, nullable = false)
     public String getProductName() {
         return productName;
     }   

     public void setProductName(String productName) {
         this.productName = productName;
     }   

     @Column(nullable = false)
     public Float getSellPrice() {
         return sellPrice;
     }   

     public void setSellPrice(Float sellPrice) {
         this.sellPrice = sellPrice;
     }   

     @ManyToOne(cascade = {CascadeType.MERGE,CascadeType.REFRESH }, optional = true)
     @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
     public Order getOrder() {
         return order;
     }   

     public void setOrder(Order order) {
         this.order = order;
     }   

 }  

下面看看测试类

 import javax.persistence.EntityManager;
 import javax.persistence.EntityManagerFactory;
 import javax.persistence.Persistence;   

 import org.junit.BeforeClass;
 import org.junit.Test;   

 import cn.itcast.bean.Order;
 import cn.itcast.bean.OrderItem;   

 public class OneToManyTest {   

     @BeforeClass
     public static void setUpBeforeClass() throws Exception {
     }
     @Test public void addOrder(){
          EntityManagerFactory factory = Persistence.createEntityManagerFactory("test");
          EntityManager em = factory.createEntityManager();
          em.getTransaction().begin(); // start transaction   

                    Order order = new Order();
          order.setAmount(34f);
          order.setOrderid("00001");   

                    //order中包含的OrderItem项OrderItem1，OrderItem2
          OrderItem orderItem1 = new OrderItem();
          orderItem1.setProductName("书");
          orderItem1.setSellPrice(22f);
          order.addOrderItem(orderItem1); //add orderitem in order   

          OrderItem orderItem2 = new OrderItem();
          orderItem2.setProductName("篮球");
          orderItem2.setSellPrice(100f);
          order.addOrderItem(orderItem2);   

          em.persist(order); //persist order object
          em.getTransaction().commit(); //commit transaction
          em.close();
          factory.close();
     }
 }