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();   
    }   
}