轉@OneToMany或@ManyToOne的用法-annotation關系映射篇（上)

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

例如我們用一個例子來開啟JPA的一對多和多對一的學習。

 

比如你去當當網上買書籍，當當網就給你一個訂單。
  通過上面的業務活動描述你可以分析得出：一個訂單可以包含一個或多個訂單項.那么我們將將訂單和訂單項設計關系為(1:N)一對多的關系(排除0)。得出關系我們就使用JPA來實現這個關系(關於建立JPA和基本的配置我就不再說了，如果不明白請看JPA入門篇)。

首先建立訂單實體類

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") //這里配置關系，並且確定關系維護端和被維護端。mappBy表示關系被維護端，只有關系端有權去更新外鍵。這里還有注意OneToMany默認的加載方式是賴加載。當看到設置關系中最后一個單詞是Many，那么該加載默認為懶加載   
    public Set<OrderItem> getItems() {   
        return items;   
    }   
  
    public void setItems(Set<OrderItem> items) {   
        this.items = items;   
    }   
       
         /**   
           *該方法用於向order中加order項   
          /*   
    public void addOrderItem(OrderItem orderItem){   
        orderItem.setOrder(this);//用關系維護端來維護關系   
        this.items.add(orderItem);   
    }   
  
}  

訂單項的實體類

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")//這里設置JoinColum設置了外鍵的名字，並且orderItem是關系維護端   
    public Order getOrder() {   
        return order;   
    }   
  
    public void setOrder(Order order) {   
        this.order = order;   
    }   
  
}  

下面看看測試類

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項OrderItem1，OrderItem2   
         OrderItem orderItem1 = new OrderItem();   
         orderItem1.setProductName("書");   
         orderItem1.setSellPrice(22f);   
         order.addOrderItem(orderItem1); //add orderitem in order   
            
         OrderItem orderItem2 = new OrderItem();   
         orderItem2.setProductName("籃球");   
         orderItem2.setSellPrice(100f);   
         order.addOrderItem(orderItem2);   
            
         em.persist(order); //persist order object   
         em.getTransaction().commit(); //commit transaction   
         em.close();   
         factory.close();   
    }   
}