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;
}

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
关联是有方向的

---

HIbernate一对多的实现

1.在数据库中建好测试的表（这里用的MYSQL）

表的实体类：

package com.psy.three.entity;

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

public class Order {

    private Integer  orderId;
    private String orderNo;
    //建立关联关系  一个订单对应多个订单项
    private List<OrderItem> orderItems =new ArrayList<>();
    private Integer initChildren =0;//0是懒加载 1是false
    
    
    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 + "]";
    }
    
}

package com.psy.three.entity;

public class OrderItem {

    private Integer orderItemId;
    private Integer productId;
    private Integer quantity; 
    private Integer oid;
    //建立关联关系   一个订单项对应一个订单
    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配置：

<class name="com.psy.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，默认值：true
             name：类的关联属性名
             cascade：级联关系  级联新增修改
             inverse：关联关系交给对方控制  默认值：true（ 当前类不维护关联关系）
             
             子标签key：
             column：从表的外键
             子标签one-to-many：
             class：外键对应的实体类
             
         -->
        <bag lazy="false" name="orderItems" cascade="save-update" inverse="true">
        <key column="oid"></key>
        <one-to-many class="com.psy.three.entity.OrderItem" />
        </bag>
    </class>

OrderItem.hbm.xml配置：

<?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.psy.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>
        <!-- Repeated column in mapping for entity  列被重复维护了-->
        <property name="oid" type="java.lang.Integer" column="oid" insert="false" update="false">
        </property>
        <many-to-one name="order" class="com.psy.three.entity.Order" column="oid">
        
        </many-to-one>
        
        
    </class>
</hibernate-mapping>

hibernate.cfg.xml主配置文件：

<!-- 一对多-->
        <mapping resource="com/psy/three/entity/OrderItem.hbm.xml" />
        <mapping resource="com/psy/three/entity/Order.hbm.xml" />

dao方法：

package com.psy.three.dao;

import java.util.List;


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

import com.psy.three.entity.Order;
import com.psy.three.entity.OrderItem;
import com.psy.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();
    }
}

因为测试的方法较多  用到JUnit测试类

package com.psy.three.dao;

import static org.junit.Assert.*;

import java.util.List;

import org.hibernate.Session;
import org.hibernate.Transaction;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

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

/**
 * setUp每测试一个@test标记的测试方法，都会在调用之前执行一次 tearDown每测试一个@test标记的测试方法，都会在调用之后执行一次
 * 
 * @author Admin
 *
 */
public class DemoDaoTest {

    private DemoDao demodao = new DemoDao();

    // @Before
    // public void setUp() throws Exception {
    // System.out.println("加载资源");
    // Session session = SessionFactoryUtils.openSession();
    // Transaction transaction = session.beginTransaction();
    //
    // }
    //
    // @After
    // public void tearDown() throws Exception {
    // System.out.println("释放资源");
    // transaction.commit();
    // session.close();
    //
    // }

    @Test
    public void testAddOrder() {
        Order order = new Order();
        order.setOrderNo("P20");
        OrderItem orderItem = null;
        for (int i = 0; i < 6; i++) {
            orderItem = new OrderItem();
            orderItem.setProductId(10 + i);
            orderItem.setQuantity(20 + i);
            //维护关联关系
            orderItem.setOrder(order);
            order.getOrderItems().add(orderItem);
        }
        demodao.addOrder(order);//只调用了一个新增的dao方法 可以增加两个表的数据
    }

    @Test
    public void testAddOrderItem() {
        OrderItem orderItem = new OrderItem();
        for (int i = 0; i < 6; i++) {
            orderItem = new OrderItem();
            orderItem.setProductId(10 + i);
            orderItem.setQuantity(20 + i);
            //维护关联关系
            Order order=new Order();
            order.setOrderId(1);
            order.getOrderItems().add(orderItem);
            orderItem.setOrder(order);
            demodao.addOrderItem(orderItem);
        }
        
        
        
    }

    @Test
    public void testGetOrder() {
        Order order= new Order();
        order.setOrderId(1);
        Order o = this.demodao.getOrder(order);
        //操作了两次数据库  当lazy=false的时候  会让hibernate执行完两次操作，session才会关闭
        //当lazy=true的时候hibernate执行一次就会关闭
        //从上面看lazy=false更好  但是为什么默认让它等于true
        //出于性能的考虑，所有hibernate3.0出现了lazy这个属性，并让它默认等于true  也就是说不加载关联属性
        List<OrderItem> orderItems = o.getOrderItems();
        for (OrderItem orderItem : orderItems) {
            System.out.println(orderItem);
        }
        System.out.println(o);
    }

    @Test
    public void testGetOrderList() {
        List<Order> orderList = this.demodao.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(1);
        this.demodao.delOrder(order );//调用一个dao方法 可以同时删除两个表的数据(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)
    }

}

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，拿到了com.psy.three.entity.Order，以及t_hibernate_order
类属性、以及表列段
5.生成动态的SQL：select orderNo,orderId from t_hibernate_order
执行SQL最终得到meterDate源数据模型
Order o1= Class.forName("com.psy.three.entity.Order")
o2.setOrderId(1)
o2.setOrderNo(P1)

......
最终得到:
List<Order> list=new ArrayList();
list.add(01);

.....

最终list中所有的order实例都有值了（这里只是处理了表里的非外键列段，原理同baseDao）

7.处理关联关系：orderItems oid com.psy.three.entity.OrderItem
通过one-to-many这个标签以及class对应全路径名会找到class对应的全路径名的专属映射文件
也就是找到了order.item.xml这个文件，拿到了它之后就可以拿到table t_hibernate_order_item

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

9.给order的关联关系属性赋值
List(Order o:list){
o.setOrderItems(orderItems);
}