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.PERSIST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.REFRESH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();
	}
}