1. 什么是关联(association)

** 1.1 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:**

   public   class B{
		private String name;
		private List<A> bs = new arraylist();
	  }
	  public class A{
        private B b = new B;
        public A(){}
      }

hibernate:orm框架、以面向对象的思想操作数据库
实体类–》表
属性–》字段
1.2 关联的分类：关联可以分为一对一、一对多/多对一、多对多关联
我们这里完成的是一对多

2.代码

实体类
order

package com.likang.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 ininChildren=1;//1代表开启懒加载，0代表关闭懒加载
	
	public Integer getIninChildren() {
		return ininChildren;
	}
	public void setIninChildren(Integer ininChildren) {
		this.ininChildren = ininChildren;
	}
	public Integer getOrderId() {
		return orderId;
	}
	public List<OrderItem> getOrderItems() {
		return orderItems;
	}
	public void setOrderItems(List<OrderItem> orderItems) {
		this.orderItems = orderItems;
	}
	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 + ", orderItems=" + orderItems + "]";
	}
	public Order(Integer orderId, String orderNo) {
		super();
		this.orderId = orderId;
		this.orderNo = orderNo;
	}
	public Order() {
		super();
	}

}

orderItem

package com.likang.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;
	}
	public OrderItem(Integer orderItemId, Integer productId, Integer quantity, Integer oid) {
		super();
		this.orderItemId = orderItemId;
		this.productId = productId;
		this.quantity = quantity;
		this.oid = oid;
	}
	public OrderItem() {
		super();
	}
	
}

然后是它们的配置文件
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.likang.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>
		<!-- 需要在映射文件中进行关联关系的维护，这里描述的是多对一的关系 -->
		<bag  name="orderItems" cascade="save-update" inverse="true">
		<!-- 从表的外键 -->
			<key column="oid"></key>
			<!-- 查询从表数据，然后形成list集合填充到orderItems属性中 -->
			<one-to-many class="com.likang.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.likang.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>
		<!-- 需要在映射文件中进行关联关系的维护，这里描述的是多对一的关系 -->
		<!--  org.hibernate.MappingException: Repeated column in mapping for entity: com.likang.three.entity.OrderItem column: oid (should be mapped with insert="false" update="false") -->
		<many-to-one name="order" class="com.likang.three.entity.Order" column="oid"></many-to-one>
	</class>
</hibernate-mapping>

然后添加映射

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
	"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
	"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
	<session-factory>
		<!-- 1. 数据库相关 -->
		<property name="connection.username">root</property>
		<property name="connection.password">123</property>
		<property name="connection.url">jdbc:mysql://localhost:3306/mysql?useUnicode=true&amp;characterEncoding=UTF-8
		</property>
		<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
		<property name="dialect">org.hibernate.dialect.MySQLDialect</property>

		<!-- 配置本地事务（No CurrentSessionContext configured!） -->
		<property name="hibernate.current_session_context_class">thread</property>

		<!-- 2. 调试相关 -->
		<property name="show_sql">true</property>
		<property name="format_sql">true</property>

		<!-- 3. 添加实体映射文件 -->
		<mapping resource="com/likang/one/entity/User.hbm.xml" />
		<!-- 主键生成策略 -->
		<mapping resource="com/likang/two/entity/Student.hbm.xml" />
		<mapping resource="com/likang/two/entity/Worker.hbm.xml" />
		<!-- 一对多关联关系 -->
		<mapping resource="com/likang/three/entity/Order.hbm.xml" />
		<mapping resource="com/likang/three/entity/OrderItem.hbm.xml" />
		
	</session-factory>
</hibernate-configuration>

dao方法

package com.likang.three.dao;

import java.util.List;

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

import com.likang.three.entity.Order;
import com.likang.three.entity.OrderItem;
import com.likang.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(0).equals(order.getIninChildren())) {
//		0代表了程序员想要关闭懒加载，也就是需要强制加载关联关系对象
			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();
	}
}

然后junit进行测试

package com.likang.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.likang.three.entity.Order;
import com.likang.three.entity.OrderItem;

public class DemoDaoTest {
private DemoDao demoDao=new DemoDao();

	@Test
	public void testAddOrder() {
		Order order=new Order();
		order.setOrderNo("t226");
		OrderItem oi=null;
		for(int i=0;i<3;i++) {
			oi=new OrderItem();
			oi.setProductId(10+i);
			oi.setQuantity(20+i);
			oi.setOrder(order);
			order.getOrderItems().add(oi);
		}
		demoDao.addOrder(order);
	}

	@Test
	public void testAddOrderItem() {
		Order order=new Order();
		order.setOrderId(5);
		OrderItem oi=null;
		for(int i=0;i<3;i++) {
			oi=new OrderItem();
			oi.setProductId(10+i);
			oi.setQuantity(20+i);
			oi.setOrder(order);
			order.getOrderItems().add(oi);
			demoDao.addOrderItem(oi);
		}
	}

	/**
	 * 关于懒加载的问题
	 * hibernate3之后出现的
	 * 
	 * failed to lazily initialize a collection of role: com.likang.three.entity.Order.orderItems,
	 *  could not initialize proxy - no Session
	 *  
	 *  orderNo数据来源于t_hibernate_order
	 *  orderItems数据来源于t_hibernate_order_item
	 *  
	 *  
	 *  思考：
	 *  	查一张表的效率高还是两张？
	 *  答案：肯定一张。
	 *  
	 *  hibernate默认的策略，是不查询关联关系对应表数据的
	 */
	@Test
	public void testGetOrder() {
		Order order=new 	Order();
		order.setOrderId(10);
		order.setIninChildren(0);
		Order o = this.demoDao.getOrder(order);
		System.out.println(o.getOrderNo());
		System.out.println(o.getOrderItems());
	}

	/**
	 * 如果说将懒加载的功能关闭，也就是lazy=false;那么在查询多条订单数据的情况下是非常影响性能的
	 * 
	 * 思考：
	 * 		查单个需要关闭懒加载     目的是需要加载出关联表的数据
	 * 		查多个不能关闭懒加载	 加载关联表的数据的查询次数过多
	 */
	@Test
	public void testGetOrderList() {
		List<Order> orderList = this.demoDao.getOrderList();
		for (Order order : orderList) {
			System.out.println(order.getOrderNo());
//			System.out.println(order.getOrderItems());
		}
	}

	@Test
	public void testDelOrder() {
		Order order=new Order();
		order.setOrderId(5);
		this.demoDao.delOrder(order);
	}

}

关于其中的一些常见报错：

org.hibernate.MappingException: Repeated column in mapping for entity: com.likang.three.entity.OrderItem column: oid (should be mapped with insert="false" update="false")

原因是其中的column="oid"在两个配置中都有
只要在其中一个中加入

insert="false" update="false"

关于懒加载

报错

failed to lazily initialize a collection of role:   com.likang.three.entity.Order.orderItems,  could not initialize proxy - no Session  

这有两种方法解决
方法一
在order.hbm.xml的bag中加入lazy=“false"
但因为太过死板，而且在查询的数据多的话，它是非常影响性能的，所以我一般是用方法二
方法二
在order实体类中加入

	private Integer ininChildren=1;//1代表开启懒加载，0代表关闭懒加载
	
	public Integer getIninChildren() {
		return ininChildren;
	}
	public void setIninChildren(Integer ininChildren) {
		this.ininChildren = ininChildren;
	}

然后在demodao中加入

	/**
	 * 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(0).equals(order.getIninChildren())) {
//		0代表了程序员想要关闭懒加载，也就是需要强制加载关联关系对象
			Hibernate.initialize(o.getOrderItems());
			System.out.println(o.getOrderItems());
		}
		transaction.commit();
		session.close();
		return o;
	}

我上面一开始的就是这个方法

最终完成就是这个效果