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Hibernate之关联关系（一对多）

news 2026/7/28 18:01:07

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:
t_hibernate_order
t_hibernate_order_item
一个订单对应多个订单项

class Order{ private List<OrderItem> orderItems; } class OrderItem{ private Order order; } class B{ private String name; } public class A{ private B b = new B; public A(){} }

测试代码如下：
DemoDao.java：

package com.LHJ.three.dao; import java.util.List; import org.hibernate.Hibernate; import org.hibernate.Session; import org.hibernate.Transaction; import org.junit.Test; import com.LHJ.three.entity.Order; import com.LHJ.three.entity.OrderItem; import com.LHJ.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; } /** * 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.java：

package com.LHJ.three.dao; import static org.junit.Assert.*; import java.util.List; import org.junit.Test; import com.LHJ.three.entity.Order; import com.LHJ.three.entity.OrderItem; public class DemoDaoTest { private DemoDao demoDao = new DemoDao(); @Test public void testAddOrder() { Order order = new Order(); order.setOrderNo("LHJ"); for (int i = 0; i < 6; i++) { OrderItem oi = new OrderItem(); oi.setProductId(10+i); oi.setQuantity(20+i); // demoDao.addOrderItem(oi); order.getOrderItems().add(oi); oi.setOrder(order); } demoDao.addOrder(order); } @Test public void testAddOrderItem() { OrderItem oi = new OrderItem(); oi.setProductId(66); oi.setQuantity(66); Order order = new Order(); order.setOrderId(11); oi.setOrder(order); demoDao.addOrderItem(oi); } @Test public void testGetOrder() { Order order = new Order(); order.setOrderId(11); order.setInitChildren(1); Order o = demoDao.getOrder(order); //failed to lazily initialize a collection of role: //com.LHJ.three.entity.Order.orderItems, could not initialize proxy - no Session System.out.println(o.getOrderNo()); System.out.println(o.getOrderItems().size()); } @Test public void testGetOrderList() { List<Order> orderList = demoDao.getOrderList(); for (Order order: orderList) { System.out.println(order.getOrderNo()); // System.out.println(order.getOrderItems().size()); } } @Test public void testDelOrder() { Order order = new Order(); order.setOrderId(11); this.demoDao.delOrder(order); } }

然后两个实体类：
Order.java：

package com.LHJ.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<OrderItem>(); private Integer initChildren = 0;//0代表默认懒加载，1代表强制加载 public Integer getInitChildren() { return initChildren; } public void setInitChildren(Integer initChildren) { this.initChildren = initChildren; } 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 List<OrderItem> getOrderItems() { return orderItems; } public void setOrderItems(List<OrderItem> orderItems) { this.orderItems = orderItems; } @Override public String toString() { return "Order [orderId=" + orderId + ", orderNo=" + orderNo + ", orderItems=" + orderItems + "]"; } }

OrderItem.java：

package com.LHJ.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 + "]"; } }

然后对应的两个映射文件：
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.LHJ.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: name:当前要映射的类的关联属性 cascade:维护关联关系 inverse:决定由谁来维护关联关系 key标签: column:表的外键对应的列段 one-to-many: class:关联属性对应的类的全路径名 --> <bag name="orderItems" cascade="save-update" inverse="true"> <key column="oid"></key> <one-to-many class="com.LHJ.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.LHJ.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:多对一的配置(因为多个订单项对应一个订单) name:指的是当前所要映射的类com.LHJ.three.entity.OrderItem的关联属性order class:指的是关联属性的类别(全路径名) column:这里填的是外键 --> <many-to-one name="order" class="com.LHJ.three.entity.Order" column="oid"></many-to-one> </class> </hibernate-mapping>

然后在主配置文件hibernate.cfg.xml中配置：

因为在OrderItem.hbm.xml中有两个对oid进行了维护所以我们需要放弃一个

然后解决？

加入箭头所指的代码就行了。

增加测试：

结果：

增加订单项：

结果：

懒加载：

会出现错误：

有两种解决方案：
第一种：
在bag标签里面添加箭头所指代码就行了

结果：

这样子虽然能处理问题但是对性能影响很不好因为它有多少条数据 sql语句就会执行多少条+1。所以不推荐使用