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。
案例:如何建立客户和订单一对多双向关联
先不建立客户和订单的关联关系,定义实体及映射文件,单独执行保存操作
建立客户到订单的一对多关联关系
实例代码：
Order.java：
package com.entity;
import java.util.ArrayList;
import java.util.List;
public class Order {
private Integer orderId;
private String orderNo;
private List<OrderItem> OrderItems=new ArrayList<>();
private Integer initChildren=1;//1代表开启懒加载 0则关闭
public Integer getInitChildren() {
return initChildren;
}
public void setInitChildren(Integer initChildren) {
this.initChildren = initChildren;
}
public List<OrderItem> getOrderItems() {
return OrderItems;
}
public void setOrderItems(List<OrderItem> orderItems) {
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 + "]";
}
public Order(Integer orderId, String orderNo) {
super();
this.orderId = orderId;
this.orderNo = orderNo;
}
public Order() {
super();
}
}
OrderItem.java:
package com.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) {
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=" + Order + "]";
}
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();
}
}
在同包下导入Order.hbm.xml文件以及OrderItem.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.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 name="orderItems" inverse="true" cascade="save-update" >
<!-- 从表的外键 -->
<key column="oid"></key>
<!-- 查询从表数据 -->
<one-to-many class="com.entity.OrderItem" />
</bag>
</class>
</hibernate-mapping>
OrderItme.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.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.entity.Order" column="oid">
</many-to-one>
</class>
</hibernate-mapping>
在hibernate.hbm.xml中进行配置
<?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/mysql?useUnicode=true&characterEncoding=UTF-8 </property> <property name="connection.driver_class">com.mysql.jdbc.Driver</property> <property name="dialect">org.hibernate.dialect.MySQLDialect</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/entity/user.hbm.xml" /> <!-- 主键生成策略 --> <mapping resource="com/entity/Worker.hbm.xml" /> <mapping resource="com/entity/Student.hbm.xml" /> <!-- 关联关系 --> <mapping resource="com/entity/Order.hbm.xml" /> <mapping resource="com/entity/OrderItem.hbm.xml" /> </session-factory> </hibernate-configuration>
DemoDao:
package com.dao;
import java.util.List;
import org.hibernate.Hibernate;
import org.hibernate.Session;
import org.hibernate.Transaction;
import com.entity.Order;
import com.entity.OrderItem;
import com.entity.Student;
import com.entity.Worker;
import com.util.SessionFactoryUtils;
/**
* hibernate中的主键生成策略
* 1、人工控制
* 2、数据库控制
* 3、hibernate控制
* 4、自定义主键生成策略
* @author xyls
*
*/
public class DemoDao {
/**
* 新增学生
* @param stu
*/
public void add(Student stu) {
Session session = SessionFactoryUtils.openSession();
Transaction transation=session.beginTransaction();
session.save(stu);
transation.commit();
SessionFactoryUtils.closeSession();
}
/**
* 新增工人
* @param worker
*/
public void addWorker(Worker worker) {
Session session = SessionFactoryUtils.openSession();
Transaction transation=session.beginTransaction();
session.save(worker);
transation.commit();
SessionFactoryUtils.closeSession();
}
/**
*
* @param args
*/
public static void testStudent() {
DemoDao dao=new DemoDao();
Student stu=new Student();
stu.setSname("柳长歌");
stu.setSid(97);
dao.add(stu);
}
public static void main(String[] args) {
DemoDao dao=new DemoDao();
Worker worker=new Worker();
worker.setWname("220");
dao.addWorker(worker);
}
/**
* 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())) {
//0代表需要强制加载关联关系对象
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;
}
/**
* 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();
}
}
进行测试：
DemoDaoTest.java:
package com.dao;
import static org.junit.Assert.*;
import java.util.List;
import org.junit.Test;
import com.entity.Order;
import com.entity.OrderItem;
import com.test.delDemo;
public class DemoDaoTest {
private DemoDao demoDao=new DemoDao();
@Test
public void testAddOrder() {
Order order=new Order();
order.setOrderNo("1001");
OrderItem oi=null;
for (int i = 0; i < 4; i++) {
oi=new OrderItem();
oi.setProductId(100+i);
oi.setQuantity(200+i);
oi.setOrder(order);
order.getOrderItems().add(oi);
}
demoDao.addOrder(order);
// demoDao.addOrderItem(orderItem);
}
@Test
public void testAddOrderItem() {
Order order=new Order();
order.setOrderId(5);
OrderItem oi=null;
for (int i = 0; i < 4; i++) {
oi=new OrderItem();
oi.setProductId(100+i);
oi.setQuantity(200+i);
oi.setOrder(order);
order.getOrderItems().add(oi);
demoDao.addOrderItem(oi);
}
}
@Test
public void testGetOrder() {
Order order=new Order();
order.setOrderId(10);
//关闭懒加载
order.setInitChildren(0);
Order o=this.demoDao.getOrder(order);
System.out.println(o.getOrderNo());
System.out.println(o.getOrderItems());
}
/**
* 查单个需要关闭懒加载
* 查多个不能关闭懒加载
*/
@Test
public void testGetOrderList() {
List<Order> orderList= this.demoDao.getOrderList();
for (Order order : orderList) {
System.out.println(order.getOrderNo());
// System.out.println(order.getOrderItems());
}
}
@Test
public void testDelOrder() {
Order order=new Order();
order.setOrderId(9);
this.demoDao.delOrder(order);
}
}
结果：
testAddOrder：
Order表
OrderItem表：
testAddOrderItem:
testGetOrder:
testGetOrderList:
testDelOrder:
主表的oid为5的数据删除以及从表oid为5的数据也被删除
关于hibernate的懒加载：
如果说将懒加载的功能关闭，也就是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
所以我在Order.java中加入：
private Integer initChildren=1;（1代表开启懒加载 0则关闭）
在测试类中的用法：
关闭懒加载 order.setInitChildren(0);
原文：https://www.cnblogs.com/omji0030/p/11300277.html