本文介绍Hibernate框架中一对多关联关系的实现方法，包括实体类设计、XML配置、懒加载策略等关键技术点。
Hibernate一对多关联关系
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
案例(订单与订单项的关系)
建实体类:
Order
package com.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<>();
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 + "]";
}
}
OrderItem
package com.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;
}
@Override
public String toString() {
return "OrderItem [orderItemId=" + orderItemId + ", productId=" + productId + ", quantity=" + quantity
+ ", oid=" + oid + "]";
}
}
配置xml:
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.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="true" name="orderItems" cascade="save-update" inverse="true">
<key column="oid"></key>
<one-to-many class="com.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.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.three.entity.Order" column="oid"></many-to-one>
</class>
</hibernate-mapping>
hibernate.cfg.xml
<?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/db_school?useUnicode=true&characterEncoding=UTF-8
</property>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property>
<property name="hbm2ddl.auto">update</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/one/entity/User.hbm.xml" />
<!-- 4.主键生成策略 -->
<mapping resource="com/two/entity/Student.hbm.xml" />
<mapping resource="com/two/entity/Worker.hbm.xml" />
<!-- 5.一对多 -->
<mapping resource="com/three/entity/OrderItem.hbm.xml"/>
<mapping resource="com/three/entity/Order.hbm.xml"/>
</session-factory>
</hibernate-configuration>
SessionFactoryUtils.java工具类
package com.three.uitl;
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());
}
}
DemoDao
package com.three.dao;
import java.util.List;
import org.hibernate.Hibernate;
import org.hibernate.Session;
import org.hibernate.Transaction;
import org.junit.Test;
import com.three.entity.Order;
import com.three.entity.OrderItem;
import com.three.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(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();
}
}
数据库:
package com.three.dao;
import static org.junit.Assert.*;
import org.hibernate.Session;
import org.hibernate.Transaction;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import com.three.entity.Order;
import com.three.entity.OrderItem;
import com.three.uitl.SessionFactoryUtils;
/**
* @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 86135
*
*/
public class DemoDaoTest {
private DemoDao demoDao = new DemoDao();
// @Before
// public void setUp() throws Exception {
// System.out.println("加载资源的");
// Session session = SessionFactoryUtils.openSession();
// Transaction transaction = session.beginTransaction();
// }
//
// @After
// public void tearDown() throws Exception {
// System.out.println("释放资源的");
// transaction.commit();
// session.close();
// }
@Test
public void testAddOrder() {
Order order = new Order();
order.setOrderNo("53地方");
OrderItem orderItem = null;
for (int i = 0; i < 6; i++) {
orderItem = new OrderItem();
orderItem.setProductId(30+i);
orderItem.setQuantity(40+i);
//维护关联关系
orderItem.setOrder(order);
order.getOrderItems().add(orderItem);
}
demoDao.addOrder(order );
}
}
执行后:
已成功加入:
//往一个订单里加一个订单项
@Test
public void testAddOrderItem() {
OrderItem orderItem = new OrderItem();
orderItem.setProductId(21);
orderItem.setQuantity(323);
Order order = new Order();
order.setOrderId(3);
order.getOrderItems().add(orderItem);
orderItem.setOrder(order);
demoDao.addOrderItem(orderItem);
}
执行后:
//往一个订单里加多个订单项
@Test
public void testAddOrderItem() {
OrderItem orderItem = null;
for (int i = 0; i < 6; i++) {
orderItem = new OrderItem();
orderItem.setProductId(30+i);
orderItem.setQuantity(40+i);
//维护关联关系
Order order = new Order();
order.setOrderId(4);
order.getOrderItems().add(orderItem);
orderItem.setOrder(order);
demoDao.addOrderItem(orderItem);
}
}
//查单个订单
@Test
public void testGetOrder() {
Order order = new Order();
order.setOrderId(3);
Order o = this.demoDao.getOrder(order);
System.out.println(o);
}
要查订单及这个订单下面的所有订单项呢?
把Order.hbm.xml中的bag标签中的lazy属性改为false:
<bag lazy="false" name="orderItems" cascade="save-update" inverse="true">
<key column="oid"></key>
<one-to-many class="com.three.entity.OrderItem"/>
</bag>
@Test
public void testGetOrder() {
Order order = new Order();
order.setOrderId(3);
Order o = this.demoDao.getOrder(order);
List<OrderItem> orderItems = o.getOrderItems();
for (OrderItem orderItem : orderItems) {
System.out.println(orderItem);
}
System.out.println(o);
}
出来了:
//查全部:
@Test
public void testGetOrderList() {
List<Order> orderList = this.demoDao.getOrderList();
for (Order order : orderList) {
System.out.println(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
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
但当lazy 为true时,我们查询会报错:
这时候通过字段控制，强制加载:
成功:
删除操作:
删除订单号为1的订单及其下的订单项:
@Test
public void testDelOrder() {
Order order = new Order();
order.setOrderId(1);
this.demoDao.delOrder(order );
}
删除前:
删除后:
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