hibernate关联关系（一对多）

什么是关联(association)

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

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

代码案例

使用t_hibernate_order表和t_hibernate_order_item表

导进实体映射文件并建立实体类并且建立关联关系
Order.hbm.xml

OrderItem.hbm.xml（后需修改）

Order

package com.tzx.three.entity;

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

public class Order {
	private Integer orderId;
	private String orderNo;
	private List orderItems = new ArrayList<>();
	private Integer initChildren = 1;//1代表开启懒加载 0代表关闭懒加载
	
	
	public Integer getInitChildren() {
		return initChildren;
	}
	public void setInitChildren(Integer initChildren) {
		this.initChildren = initChildren;
	}
	public List getOrderItems() {
		return orderItems;
	}
	public void setOrderItems(List 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;
	}
	public Order() {
		super();
	}
	public Order(Integer orderId, String orderNo) {
		super();
		this.orderId = orderId;
		this.orderNo = orderNo;
	}

	

}

OrderItem

package com.tzx.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();
	}
	@Override
	public String toString() {
		return "OrderItem [orderItemId=" + orderItemId + ", productId=" + productId + ", quantity=" + quantity
				+ ", oid=" + oid + ", order=" + order + "]";
	}
	

}

hibernate.cfg.xml配置

准备测试：
DemoDao

package com.tzx.three.dao;

import java.util.List;

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

import com.tzx.three.entity.Order;
import com.tzx.three.entity.OrderItem;
import com.tzx.two.uitl.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.getInitChildren())) {
//			Hibernate.initialize(o.getOrderItems());
			System.out.println(o.getOrderItems());
//		}
		transaction.commit();
		session.close();
		return o;
	}
	
	public List getOrderList() {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		List 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();
	}
}

使用junit进行单个测试
增加一个订单与订单项

public void testAddOrder() {
		Order order = new Order();
		order.setOrderNo("tanzhixian");
		for (int i = 0; i < 3; i++) {
			OrderItem oi = new OrderItem();
			oi.setProductId(11+i);
			oi.setQuantity(22+i);
			oi.setOrder(order);
			order.getOrderItems().add(oi);
		}
		demoDao.addOrder(order);
//		demoDao.addOrderItem(orderItem)
	}

直接运行会出一个常见的错误
Repeated column in mapping for entity: com.tzx.three.entity.OrderItem column: oid (should be mapped with insert=“false” update=“false”)
原因：映射文件映射了两次column=“oid”

解决方法

insert="false" update="false"

OrderItem.hbm.xml

t_hibernate_order

t_hibernate_order_item（三条数据oid为10）

给订单id为5的数据增加订单项

public void testAddOrderItem() {
		Order order = new Order();
		order.setOrderId(5);
		for (int i = 0; i < 3; i++) {
			OrderItem oi = new OrderItem();
			oi.setProductId(11+i);
			oi.setQuantity(22+i);
			oi.setOrder(order);
			order.getOrderItems().add(oi);
			demoDao.addOrderItem(oi);
		}
	}

t_hibernate_order_item

懒加载

/*
	 * 关于懒加载
	 *  hiberbate3之后出现的
	 */
	@Test
	public void testGetOrder() {
		Order order = new Order();
		order.setOrderId(10);
		Order o = this.demoDao.getOrder(order);
		System.out.println(o.getOrderNo());
		System.out.println(o.getOrderItems());
	}

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

原因：
orderNo数据来源于t_hibernate_order
orderItems数据来源于t_hibernate_order_item
hiberbate默认的策略：是不查询关联关系对应关联表数据的

解决方法：
不推荐使用这种
在Order.hbm.xml中bag标签加入lay=“false”

弊端：如果说将懒加载直接关闭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

推荐一种方法：
例：
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

实体类Order定义一个开关，1代表开启懒加载 0代表关闭懒加载
将上面DemoDao中注释代码释放

@Test
	public void testGetOrderList() {
		List orderList = this.demoDao.getOrderList();
		for (Order order : orderList) {
			System.out.println(order.getOrderNo());
//	        System.out.println(order.getOrderItems());
		}
	}

删除

@Test
	public void testDelOrder() {
		Order order = new Order();
		order.setOrderId(10);
		this.demoDao.delOrder(order);
	}

刚新加的数据（orderId为10）被删除包括订单项