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

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

关键点都在数据库中的外键上面,请好好理解下面这二句SQL和一对多及多对一的关系

select * from Orders where cid=? //这条SQL返回客户对应的0-n个订单

select * from Customers where customerId=? //这条SQL返回订单对应的1个客户

通过这二条SQL不难看出,外键在这里有二个用途:查找客户的订单,查找订单对应的客户

案例:如何建立客户和订单一对多双向关联

创建与数据库表相对应的实体类，与对应的关联关系：

Order
package com.hxc.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<>();

    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 + ", orderItems=" + orderItems + "]";
    }

    public Order(Integer orderId, String orderNo) {
        super();
        this.orderId = orderId;
        this.orderNo = orderNo;
    }

    public Order() {
        super();
        // TODO Auto-generated constructor stub
    }

}
OrderItem 
package com.hxc.three.entity;

public class OrderItem {

    private Integer orderItemId;
    private Integer productId;
    private Integer quantity;
    private Integer oid;
    private 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 Order getOrder() {
        return order;
    }
    public void setOrder(Order order) {
        this.order = order;
    }
    @Override
    public String toString() {
        return "OrderItem [orderItemId=" + orderItemId + ", productId=" + productId + ", quantity=" + quantity
                + ", oid=" + oid + ", order=" + order + "]";
    }
    public OrderItem() {
        super();
        // TODO Auto-generated constructor stub
    }
    public OrderItem(Integer orderItemId, Integer productId, Integer quantity, Integer oid) {
        super();
        this.orderItemId = orderItemId;
        this.productId = productId;
        this.quantity = quantity;
        this.oid = oid;
    }
    
    
}

配置对应的hbm.xml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.hxc.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，默认是懒加载的 true 懒汉模式
                name:类的关联属性名
                cascade:级联关系 级联新增与修改
                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 lazy="false" name="orderItems" cascade="save-update" inverse="true" >
            <key column="oid"></key>
            <one-to-many class="com.hxc.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.hxc.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.hxc.three.entity.Order" column="oid">
        </many-to-one>
    </class>
</hibernate-mapping>

配置hibernate.cfg.xml：

　　　　　<mapping resource="com/hxc/three/entity/Order.hbm.xml"/>
        <mapping resource="com/hxc/three/entity/OrderItem.hbm.xml"/>

在

package com.hxc.three.id;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class SessionFactoryUtils {
    private static SessionFactory sessionFactory;
    static {
        Configuration cfg = new Configuration().configure("hibernate.cfg.xml");
        sessionFactory = cfg.buildSessionFactory();
    }
    
    public static Session openSession() {
        Session session = sessionFactory.getCurrentSession();
        if(session == null) {
            session = sessionFactory.openSession();
        }
        return session;
    }
    
    public static void closeSession() {
        Session session = sessionFactory.getCurrentSession();
        if(session != null && session.isOpen()) {
            session.close();
        }
    }
    
    public static void main(String[] args) {
        Session session = SessionFactoryUtils.openSession();
        session.beginTransaction();
        System.out.println(session.isConnected());
        SessionFactoryUtils.closeSession();
        System.out.println(session.isConnected());
    }
}
SessionFactoryUtils 类中检查配置是否正确
package com.hxc.three.id;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class SessionFactoryUtils {
    private static SessionFactory sessionFactory;
    static {
        Configuration cfg = new Configuration().configure("hibernate.cfg.xml");
        sessionFactory = cfg.buildSessionFactory();
    }
    
    public static Session openSession() {
        Session session = sessionFactory.getCurrentSession();
        if(session == null) {
            session = sessionFactory.openSession();
        }
        return session;
    }
    
    public static void closeSession() {
        Session session = sessionFactory.getCurrentSession();
        if(session != null && session.isOpen()) {
            session.close();
        }
    }
    
    public static void main(String[] args) {
        Session session = SessionFactoryUtils.openSession();
        session.beginTransaction();
        System.out.println(session.isConnected());
        SessionFactoryUtils.closeSession();
        System.out.println(session.isConnected());
    }
}
创建DemoDao 
package com.hxc.three.dao;

import java.util.List;

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

import com.hxc.three.entity.Order;
import com.hxc.three.entity.OrderItem;
import com.hxc.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())) {
//            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;
    }
    
    /**
     * 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();
    }
}

快捷键ctrl+N创建测试类DemoDaoTest ：

package com.hxc.three.dao;

import static org.junit.Assert.fail;

import java.util.List;

import org.junit.Test;

import com.hxc.three.entity.Order;
import com.hxc.three.entity.OrderItem;
/**
 * @Before每测试一个@Test标记的测试方法，都会调用之前执行一次
 * @After每测试一个@Test标记的测试方法，都会调用之后执行一次
 * @author 旧城
 *
 */
public class DemoDaoTest {
    private DemoDao demoDao = new DemoDao();

    @Test
    public void testAddOrder() {
        Order order = new Order();
        order.setOrderNo("T226");
        OrderItem orderItem = null;
        for (int i = 0; i < 3; i++) {
            orderItem = new OrderItem();
            orderItem.setProductId(10 + i);
            orderItem.setQuantity(20 + i);
            orderItem.setOrder(order);
            order.getOrderItems().add(orderItem);
        }
        demoDao.addOrder(order);
    }

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

    @Test
    public void testGetOrder() {
        Order order = new Order();
        order.setOrderId(1);
//      order.setInitChildren(1);
        Order order2 = this.demoDao.getOrder(order);
//      当lazy=false的时候，会让hibernate执行完两次操作，session才会关闭
        //出于性能的考虑，所用hibernate3.0出现lazy这个属性，也就是说吧加载关联属性
//      List<OrderItem> orderItems = order2.getOrderItems();
//      for (OrderItem orderItem : orderItems) {
//          System.out.println(orderItem);
//      }
        System.out.println(order2);
    }

    @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(2);
            this.demoDao.delOrder(order);
    }

}
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原文地址：https://www.cnblogs.com/huxiaocong/p/11300439.html