hibernate关联关系（一对多）

什么是关联

关联指的是类之间的引用关系。如果类A与类B关联，那么被引用的类B将被定义为类A的属性。例如:

  class B{
	private String name;
  }
  
  public class A{
    private B b = new B;
    public A(){}
  }

案例

订单与订单项的一对多的增删改查

Order

一个订单包含多个订单项

package com.shl.three.entity;

import java.util.ArrayList;
import java.util.List;

public class Order {
	private Integer orderId;
	private String orderNo;
	private List<OrderItem> orderitems=new ArrayList<>();
	private Integer InitChildren=0;//0是懒加载     1,false
	
	public Integer getInitChildren() {
		return InitChildren;
	}
	public void setInitChildren(Integer initChildren) {
		InitChildren = initChildren;
	}
	public List<OrderItem> getOrderitems() {
		return orderitems;
	}
	public void setOrderitems(List<OrderItem> orderitems) {
		this.orderitems = orderitems;
	}
	public Integer getOrderId() {
		return orderId;
	}
	public void setOrderId(Integer orderId) {
		this.orderId = orderId;
	}
	public String getOrderNo() {
		return orderNo;
	}
	public void setOrderNo(String orderNo) {
		this.orderNo = orderNo;
	}
	@Override
	public String toString() {
		return "Order [orderId=" + orderId + ", orderNo=" + orderNo + "]";
	}
}

OrderItem

每个订单项都有对应的订单

package com.shl.three.entity;

public class OrderItem {
	private Integer orderItemId;
	private Integer productId;
	private Integer quantity;
	private Integer oid;
	
	private Order order;
	
	public Order getOrder() {
		return order;
	}
	public void setOrder(Order order) {
		this.order = order;
	}
	public Integer getOrderItemId() {
		return orderItemId;
	}
	public void setOrderItemId(Integer orderItemId) {
		this.orderItemId = orderItemId;
	}
	public Integer getProductId() {
		return productId;
	}
	public void setProductId(Integer productId) {
		this.productId = productId;
	}
	public Integer getQuantity() {
		return quantity;
	}
	public void setQuantity(Integer quantity) {
		this.quantity = quantity;
	}
	public Integer getOid() {
		return oid;
	}
	public void setOid(Integer oid) {
		this.oid = oid;
	}
	@Override
	public String toString() {
		return "OrderItem [orderItemId=" + orderItemId + ", productId=" + productId + ", quantity=" + quantity
				+ ", oid=" + oid + "]";
	}
}

Order.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.shl.three.entity.Order" table="t_hibernate_order">
		<id name="orderId" type="java.lang.Integer" column="order_id">
			<generator class="increment" />
		</id>
		<property name="orderNo" type="java.lang.String" column="order_no">
		</property>
		
		<!-- 
			bag标签：
				lazy:是否懒加载，默认是懒加载  true
				cascade:级联关系     级联新增与修改
				name:类的关联属性
				inverse: 关联关系交给对方控制  默认是true，当前类不维护关联关系
				
			子标签key:
				column:主表的主键，从表的外键
			
			子标签one-to-many:
				class: 外键对应的实体类
		 -->
		<bag name="orderitems" lazy="true" cascade="save-update" inverse="true">
			<key column="oid"></key>
			<one-to-many class="com.shl.three.entity.OrderItem" />
		</bag>
	</class>

</hibernate-mapping>

OrderItem.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.shl.three.entity.OrderItem" table="t_hibernate_order_item">
		<id name="orderItemId" type="java.lang.Integer" column="order_item_id">
			<generator class="increment" />
		</id>
		<property name="productId" type="java.lang.Integer" column="product_id">
		</property>
		<property name="quantity" type="java.lang.Integer" column="quantity">
		</property>
		<property name="oid" type="java.lang.Integer" column="oid" insert="false" update="false">
		</property>
		<many-to-one name="order" class="com.shl.three.entity.Order" column="oid"></many-to-one>
	</class>
</hibernate-mapping>

DemoDao

package com.shl.three.dao;

import java.util.List;

import org.hibernate.Hibernate;
import org.hibernate.Session;
import org.hibernate.Transaction;

import com.shl.three.entity.Order;
import com.shl.three.entity.OrderItem;
import com.shl.two.util.SessionFactoryUtils;

public class DemoDao {
	/**
	 * 为了测试关系型映射文件配置准确
	 * 	讲解insert=false，update=false的用途
	 * @param order
	 * @return
	 */
	public Integer addOrder(Order order) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Integer oid = (Integer)session.save(order);
		transaction.commit();
		session.close();
		return oid;
	}
	
	public Integer addOrderItem(OrderItem orderItem) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Integer otid = (Integer)session.save(orderItem);
		transaction.commit();
		session.close();
		return otid;
	}
	
	
	
	/**
	 * 为了讲解懒加载的问题（hibernate3.0后所有查询方式默认采用的是懒加载方式）
	 * 	1、查单个时存在问题，代理对象已经关闭
	 * 	2、查多个存在问题，有性能的问题
	 * @param order
	 * @return
	 */
	public Order getOrder(Order order) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Order o = session.get(Order.class, order.getOrderId());
		if(o != null && new Integer(1).equals(order.getInitChildren())) {
			//强制加载关联对象
			Hibernate.initialize(o.getOrderitems());
//			System.out.println(o.getOrderItems());
		}
		transaction.commit();
		session.close();
		return o;
	}
	
	public List<Order> getOrderList() {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		List<Order> list = session.createQuery("from Order").list();
		transaction.commit();
		session.close();
		return list;
	}
	
	/**
	 * z主表的数据不能随便删除，得先删除从表中对应信息，才能删除主表的信息。
	 * @param order
	 */
	public void delOrder(Order order) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Order order2 = session.get(Order.class, order.getOrderId());
		for (OrderItem oi : order2.getOrderitems()) {
			session.delete(oi);
		}
		session.delete(order2);
//		session.delete(order);
		transaction.commit();
		session.close();
	}
}

DemoDaoTest

利用junit进行测试

package com.shl.three.dao;

import static org.junit.Assert.*;

import java.util.List;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import com.shl.three.entity.Order;
import com.shl.three.entity.OrderItem;

/**
 * @Before没测试一个@Test标记的方法，都会执行一次
 * @After没测试一个@Test标记的方法，都会调用之后执行一次
 * @author Administrator
 *
 */
public class DemoDaoTest {
	
	private DemoDao dd=new DemoDao();

//	@Before
//	public void setUp() throws Exception {
//		System.out.println("加载资源");
//	}
//
//	@After
//	public void tearDown() throws Exception {
//		System.out.println("释放资源");
//	}

	@Test
	public void testAddOrder() {
		Order o=new Order();
		o.setOrderNo("p30");
		OrderItem oi=null;
		for (int i = 0; i < 8; i++) {
			oi=new OrderItem();
			oi.setProductId(10+i);
			oi.setQuantity(99+i);
			//维护关联关系
			oi.setOrder(o);
			o.getOrderitems().add(oi);
		}
		dd.addOrder(o);
	}

	@Test
	public void testAddOrderItem() {
		OrderItem oi=null;
		for (int i = 0; i < 8; i++) {
			oi=new OrderItem();
			oi.setProductId(88+i);
			oi.setQuantity(22+i);
			//维护关联关系
			Order o=new Order();
			o.setOrderId(1);
			o.getOrderitems().add(oi);
			oi.setOrder(o);
			dd.addOrderItem(oi);
		}
	}

	@Test
	public void testGetOrder() {
		Order order=new Order();
		order.setOrderId(1);
		order.setInitChildren(1);
		Order o = this.dd.getOrder(order);
		List<OrderItem> ois=o.getOrderitems();
		//当lazy=false时，会让hibernate执行两次操作,session才会关闭，将关联表中的数据查出来
		//当lazy=true时，会让hibernate执行一次操作,session就会关闭
		//但是出于性能的考虑，所有lazy默认为true
		for (OrderItem orderItem : ois) {
			System.out.println(orderItem);
		}
		System.out.println(o);
	}

	@Test
	public void testGetOrderList() {
		List<Order> orderList = this.dd.getOrderList();
		for (Order order : orderList) {
			for (OrderItem orderItem : order.getOrderitems()) {
				System.out.println(orderItem);
			}
			System.out.println(order);
		}
	}

	@Test
	public void testDelOrder() {
		Order order=new Order();
		order.setOrderId(2);
		this.dd.delOrder(order);
	}

}

hibernate框架一对多的执行原理

1.对hibernate.cfg.xml进行建模，等到sessionFactory对象
	2.并且拿到了mapping resource里的内容
	3.拿到了Order.hbm.xml配置文件
	4.可以再次建模，拿到com.shl.three.entity.Order，以及t_hibernate_order
	  类属性，以及表列段
	5.生成动态sql
	  执行sql最终得到meterData（源数据模型）
	6.Order o1 = Class.forName("com.shl.three.entity.Order").newInstance(0);
	   o1.set...
	    ...
	   最终得到：
	   List<Order> list=new arr..
	   list.add(o1);
	   ..

	   最终list中所有order实例都有值了（这里只是处理表面的非外键列段，原理完全跟baseDao一样）

	7.处理关联关系   orderitems   oid   com.shl.three.entity.OrderItem
	  通过one-to-many这个标签以及class对应的全路径名会找到对class对应的全路径名对应的专属映射文件
	  也就是找到了 OrderItem.hbm.xml,拿到他之后就可以拿到t_hibernate_order_item

	8.select * from t_hibernate_order_item;
	  最终得到了一个List<OrderItem> ois

	9.给order的关联关系属性赋值
	    List<Order> list=new arr..
	    for(Order o:list){
	    	o.setOrderItem(ois);
	    }

懒加载

当我们在配置Order.hbm.xml时，有一个lazy的属性
当lazy=true时：懒加载，当lazy=false时：不是懒加载，默认为懒加载

获取订单表的内容时，如果lazy为true，就只能获取订单表的内容，获取不到对应订单项的内容。要想获取到对应订单项的内容，需要强制加载关联对象。

Hibernate.initialize(o.getOrderitems());

如果lazy为false，就会自动获取到对应订单项的内容，就很方便。

但是出于性能的考虑，所有lazy默认为true，看情况自己设置。