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); } }