JPA（三）之实体关系一对多（多对一）



1.背景介绍：

对于购买商品时，订单信息（Order）和订单商品信息（OrderItem）的关系就是一对多的关系。

2.实体bean：

Order.java代码

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package com.sinoi.bean;   import java.util.HashSet; import java.util.Set;   import javax.persistence.CascadeType; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.FetchType; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.OneToMany; import javax.persistence.Table;   @Entity @Table(name="my_order") public class Order {     @Id @Column(length=12)     private String orderId;     @Column(nullable=false)     private Float amount = 0f;     @OneToMany(cascade={CascadeType.REMOVE, CascadeType.MERGE, CascadeType.PERSIST, CascadeType.REFRESH},             mappedBy="order")     private Set<OrderItem> orderItems = new HashSet<OrderItem>();     public String getOrderId() {         return orderId;     }     public void setOrderId(String orderId) {         this.orderId = orderId;     }     public Float getAmount() {         return amount;     }     public void setAmount(Float amount) {         this.amount = amount;     }           public Set<OrderItem> getOrderItems() {         return orderItems;     }     public void setOrderItems(Set<OrderItem> orderItems) {         this.orderItems = orderItems;     }     public void addOrderItem(OrderItem orderItem) {         orderItem.setOrder(this);//这句代码不加也可！         this.orderItems.add(orderItem);     }             }

OrderItem.java代码

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package com.sinoi.bean;   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 {     @Id @GeneratedValue     private Integer id;     @Column(length=20, nullable=false)     private String productName;     @Column(nullable=false)     private Float sellPrice;     @ManyToOne(cascade={CascadeType.REFRESH, CascadeType.MERGE}, optional=false)     @JoinColumn(name="item_order_for")     private Order order;     public Order getOrder() {         return order;     }     public void setOrder(Order order) {         this.order = order;     }     public Integer getId() {         return id;     }     public void setId(Integer id) {         this.id = id;     }     public String getProductName() {         return productName;     }     public void setProductName(String productName) {         this.productName = productName;     }     public Float getSellPrice() {         return sellPrice;     }     public void setSellPrice(Float sellPrice) {         this.sellPrice = sellPrice;     } }

3.测试代码

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package junit.test;   import javax.persistence.EntityManager; import javax.persistence.EntityManagerFactory; import javax.persistence.Persistence;   import org.junit.Test;   import com.sinoi.bean.Order; import com.sinoi.bean.OrderItem;   public class UnitTest {     @Test     public void save() {         EntityManagerFactory entityManagerFactory = Persistence                 .createEntityManagerFactory("unitname");         EntityManager entityManager = entityManagerFactory                 .createEntityManager();         entityManager.getTransaction().begin();                   Order order = new Order();         order.setOrderId("jfdkf323");         order.setAmount(232f);         OrderItem orderItem = new OrderItem();         orderItem.setProductName("basketball");         orderItem.setSellPrice(221f);         OrderItem orderItem2 = new OrderItem();         orderItem2.setProductName("football");         orderItem2.setSellPrice(11f);         orderItem.setOrder(order);         orderItem2.setOrder(order);         order.addOrderItem(orderItem);         order.addOrderItem(orderItem2);           entityManager.persist(order);         entityManager.getTransaction().commit();         entityManager.close();         entityManagerFactory.close();               } }

4.配置文件persistence.xml和前面JPA（一）、、、一样，在此省略