JPA之OneToMany和ManyToOne处理

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我是靠谱客的博主飞快雨，最近开发中收集的这篇文章主要介绍JPA之OneToMany和ManyToOne处理，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

前面介绍和总结了JPA的入门知识，也就是搭建环境和简单的使用了JPA的一个实例。现在再来总结下一些常见的关系(一对多和多对一的关系)。
在学习这些的时候，我们不得单单就学习他本身来学习，我们应该联系相关的一些知识来进行学习。比如Hibernate等
-------------------------------
例如我们用一个例子来开启JPA的一对多和多对一的学习。

[color=red]比如你去当当网上买书籍，当当网就给你一个订单。[/color]
[b][color=red] 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)。[/color][/b]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[url=http://jiangduxi.iteye.com/blog/645286]JPA入门篇[/url])。

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();
	}
}