1. 什么是关联(association)
1.1 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
首先建2个实体类
Order.java
package com.jt3.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，默认。0则代表关闭懒加载 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) { 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.java
package com.jt3.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 + ", 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(); // TODO Auto-generated constructor stub } }
然后是映射文件
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.jt3.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> <!--需要在映射文件中进行关联关系的维护 这里描述的是一对多的关系 --> <bag name="orderItems" cascade="save-update" inverse="true"> <!-- 从表的外键 --> <key column="oid"></key> <!-- 查询从表数据，然后形成list集合填充到orderItems属性中 --> <one-to-many class="com.jt3.entity.OrderItem"></one-to-many> <!-- selct * from t_hibernate_order_item where oid=1 --> </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.jt3.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> <!--需要在映射文件中进行关联关系的维护 这里描述的是多对一的关系 --> <!--Repeated column in mapping for entity: com.jt3.entity.OrderItem column: oid (should be mapped with insert="false" update="false") --> <many-to-one name="order" class="com.jt3.entity.Order" column="oid"></many-to-one> </class> </hibernate-mapping>
要把映射文件配置进
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/sys?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/jt/entity/User.hbm.xml"/> <!--主键生成策略 --> <mapping resource="com/jt2/entity/Student.hbm.xml"/> <mapping resource="com/jt2/entity/Worker.hbm.xml"/> <!-- 一对多 --> <mapping resource="com/jt3/entity/Order.hbm.xml"/> <mapping resource="com/jt3/entity/OrderItem.hbm.xml"/> </session-factory> </hibernate-configuration>
然后就是写dao层
DemoDao.java
package com.jt3.dao; import java.util.List; import org.hibernate.Hibernate; import org.hibernate.Session; import org.hibernate.Transaction; import com.jt2.util.SessionFactoryUtils; import com.jt3.entity.Order; import com.jt3.entity.OrderItem; 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(0).equals(order.getInitChildren())) { //0代表程序员想要关闭懒加载，也就意味着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; } /** * 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(); } }
然后就是使用junit来测试CTRL+N建一个测试类
选择我们需要测试的DAO方法，就会自动生成一个类DemoDaoTest.java
package com.jt3.dao; import java.util.List; import org.junit.Test; import com.jt3.entity.Order; import com.jt3.entity.OrderItem; public class DemoDaoTest { private DemoDao demoDao = new DemoDao(); /** * 新增订单（单个对应多个订单项） */ @Test public void testAddOrder() { Order order = new Order(); order.setOrderNo("jt111111"); OrderItem oi = null; for (int i = 0; i < 3; i++) { oi = new OrderItem(); oi.setProductId(10+i); oi.setQuantity(20+i); oi.setOrder(order); order.getOrderItems().add(oi); } demoDao.addOrder(order); } /** * 新增订单项（多个） */ @Test public void testAddOrderItem() { Order order = new Order(); order.setOrderId(5); OrderItem oi = null; for (int i = 0; i < 3; i++) { oi = new OrderItem(); oi.setProductId(10+i); oi.setQuantity(20+i); oi.setOrder(order); order.getOrderItems().add(oi); demoDao.addOrderItem(oi); } } /** * 查询单个 * 重点： * 关于懒加载的问题 * hibernate3之后出现的； * 错误信息： * failed to lazily initialize a collection of role: * com.yuan.three.entity.Order.orderItems, * could not initialize proxy - no Session * * orderNo数据来源于t_hibernate_order表 * orderItems数据来源于t_hibernate_order_item表 * * 因为两表有关联关系所以不可以分开查询， * 而hibernate默认的策略是不查询关联关系对应关联表数据的 * * 处理方式：（第一种） * 在对应实体映射文件中的关联关系的维护中添加 lazy="false" * 如：<bag lazy="false" name="orderItems" cascade="save-update" inverse="true"> */ @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()); } /** * 查询多个 * 在查询单个中懒加载问题的解决方案 * 会导致在查询多个的时候sql语句也会增多， * 当查询数据过多的时候导致代码性能变低 * * 总：如果将懒加载的功能关闭，即：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 * * 查单个需要关闭懒加载 目的是需要加载出关联表的数据 * 查多个不能关闭懒加载 加载关联表的数据的查询次数过多 * * 处理方式：（第二种） * 在一对多的实体类中添加属性是否打开懒加载 * */ @Test public void testGetOrderList() { List<Order> orderList = this.demoDao.getOrderList(); for (Order order : orderList) { System.out.println(order.getOrderNo()); // System.out.println(order.getOrderItems()); } } /** * 删除订单，关联关系同时删除 */ @Test public void testDelOrder() { Order order = new Order(); order.setOrderId(10); this.demoDao.delOrder(order); } }
测试结果
再来演示下删除12
