JPA 一对多实例配置

<persistence xmlns="http://java.sun.com/xml/ns/persistence"  
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
   xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"  
   version="1.0">  
   <persistence-unit name="mengya" transaction-type="RESOURCE_LOCAL">  
      <properties>  
         <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5Dialect"/>  
         <property name="hibernate.hbm2ddl.auto" value="update"/>  
         <property name="hibernate.connection.driver_class" value="org.gjt.mm.mysql.Driver"/>  
         <property name="hibernate.connection.username" value="root"/>  
         <property name="hibernate.connection.password" value="123"/>  
         <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/mp?useUnicode=true&amp;characterEncoding=UTF-8"/>  
      </properties>  
   </persistence-unit>  
</persistence>  

 在JPA中双向一对多关系则多方为关系护端即负责外键记录的更新，一方为被维护端即没有权力更新外键记录

src下在面的META-INF/persistence.xml配置如下：

 

 

订单和订单项两个实例实体Bean如下：

订单表：

package com.mengya.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.Id;   
import javax.persistence.OneToMany;   
import javax.persistence.Table;   
  
@Entity  
@Table(name = "orders")   
public class Order {   
    private String orderId;   
  
    private Float amount = 0f;// 设置默认值   
  
    private Set<OrderItem> items = new HashSet<OrderItem>();   
  
    @Id  
    @Column(length = 32)   
    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;   
    }   
  
    // cascade表示级联操作   
    // CascadeType.MERGE级联更新：若items属性修改了那么order对象保存时同时修改items里的对象。对应EntityManager的merge方法   
    // CascadeType.REFRESH级联刷新：获取order对象里也同时也重新获取最新的items时的对象。对应EntityManager的refresh(object)方法有效。即会重新查询数据库里的最新数据   
    // CascadeType.PERSIST级联保存：对order对象保存时也对items里的对象也会保存。对应EntityManager的presist方法   
    // CascadeType.REMOVE级联删除：对order对象删除也对items里的对象也会删除。对应EntityManager的remove方法   
    // FetchType.LAZY表示懒加载。对于xxxtoMany时即获得多的一方fetch的默认值是FetchType.LAZY懒加载。对于xxxtoOne时即获得一的一方fetch的默认值是FetchType.EAGER立即加载   
    // mappedBy表示关系统被维护端，它的值是关系维护端维护关系的属性   
    @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH,   
            CascadeType.REMOVE }, fetch = FetchType.LAZY, mappedBy = "order")   
    public Set<OrderItem> getItems() {   
        return items;   
    }   
  
    public void setItems(Set<OrderItem> items) {   
        this.items = items;   
    }   
  
    /**  
     * 自己的加的一个方法，为了方便添加OrderItem  
     *   
     */  
    public void addOrderItem(OrderItem item) {   
        item.setOrder(this);   
        this.items.add(item);   
    }   
  
}  

 

订单项表：

package com.mengya.bean;   
  
import javax.persistence.CascadeType;   
import javax.persistence.Column;   
import javax.persistence.Entity;   
import javax.persistence.FetchType;   
import javax.persistence.GeneratedValue;   
import javax.persistence.GenerationType;   
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;// 设置默认值   
  
    private Order order;   
  
    @Id  
    @GeneratedValue(strategy = GenerationType.AUTO)   
    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;   
    }   
  
    // cascade表示级联。CascadeType.REFRESH级联刷新   
    // optional表示该对象可有可无，它的值为true表示该外键可以为null，它的值为false表示该外键为not null   
    @ManyToOne(cascade = CascadeType.REFRESH, fetch = FetchType.EAGER, optional = false)   
    // JoinColumn表示外键的列   
    @JoinColumn(name="orderId")   
   public Order getOrder() {   
        return order;   
    }   
  
    public void setOrder(Order order) {   
        this.order = order;   
    }   
  
}  

 

测试：

package com.mengya.junit.test;   
  
import javax.persistence.EntityManager;   
import javax.persistence.EntityManagerFactory;   
import javax.persistence.Persistence;   
  
import org.junit.Test;   
  
import com.mengya.bean.Order;   
import com.mengya.bean.OrderItem;   
  
  
public class JPA_OnetoManyTest {   
    @Test public void save(){   
        EntityManagerFactory factory=Persistence.createEntityManagerFactory("mengya");   
        EntityManager em=factory.createEntityManager();   
        em.getTransaction().begin();   
           
        Order order=new Order();   
        order.setOrderId("200910120001");   
        order.setAmount(450f);   
           
        OrderItem item1=new OrderItem();   
        item1.setProductName("篮球");   
        item1.setSellPrice(220f);   
        OrderItem item2=new OrderItem();   
        item2.setProductName("排球");   
        item2.setSellPrice(230f);   
        order.addOrderItem(item1);   
        order.addOrderItem(item2);   
           
        em.persist(order);   
        em.getTransaction().commit();   
        em.close();   
        factory.close();   
    }   
}