7、hibernate中的关联关系

hbm.xml文件可以完成多种映射关系的映射

- 创建具有一对多关联关系的域模型
-创建具有一对多参照关系的关系模型
-映射一对多关联关系
-通过Hibernate API级联操纵具有关联关系的域对象。（级联操纵：一的一方删除，多的一方同时删除）

1、一对多的关联关系（一对一是一对多的特殊情况）

Customer和Order的一对多双向关联（双向：你中有我，我中有你），一个用户可以有多个订单，一个订单只能属于一个用户

创建两个类：

import java.util.Set;public class Customer{private Long id;private String name;private Set<Order> orders; //一对多，通过该变量可以引用到对应的Order集合对象public Long getId(){return id;}public void setId(Long id){this.id = id;}public String getName(){return name;}public void setName(String name){this.name = name;}public Set<Order> getOrders(){return orders;}public void setOrders(Set<Order> orders){this.orders = orders;}}

 

public class Order{private Long id;private String orderNumber;private Customer customer; //多对一，通过该变量可以引用到对应的customer对象public Long getId(){return id;}public void setId(Long id){this.id = id;}public String getOrderNumber(){return orderNumber;}public void setOrderNumber(String orderNumber){this.orderNumber = orderNumber;}public Customer getCustomer(){return customer;}public void setCustomer(Customer customer){this.customer = customer;}}

创建两个数据库表：

CREATE TABLE `customers` (  `ID` bigint(20) NOT NULL,  `NAME` varchar(50) default NULL,  PRIMARY KEY  (`ID`)) ENGINE=InnoDB DEFAULT CHARSET=latin1;

 

CREATE TABLE `orders` (  `ID` bigint(20) NOT NULL,  `ORDER_NUMBER` varchar(15) default NULL,  `CUSTOMER_ID` bigint(20) NOT NULL) ENGINE=InnoDB DEFAULT CHARSET=latin1;

创建对应类的hbm.xml文件：

Customer.hbm.xml

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping><class name="com.cdtax.hibernate.Customer" table="customers"><id name="id" column="id" type="long"><generator class="increment"></generator></id><property name="name" type="string"><column name="name" length="50"></column></property><set name="orders" cascade="save-update" inverse="true"><key column="customer_id"></key><one-to-many class="com.cdtax.hibernate.Order"/></set></class></hibernate-mapping>

Order.hbm.xml

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping><class name="com.cdtax.hibernate.Order" table="orders"><id name="id" column="id" type="long"><generator class="increment"></generator></id><property name="orderNumber" type="string"><column name="order_number" length="15"></column></property><many-to-one name="customer" class="com.cdtax.hibernate.Customer" column="customer_id"></many-to-one></class></hibernate-mapping>

创建测试类：

import java.util.HashSet;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.Transaction;import org.hibernate.cfg.Configuration;public class HibernateTest{private static SessionFactory sessionFactory;static{try{sessionFactory = new Configuration().configure().buildSessionFactory();} catch (Exception ex){ex.printStackTrace();}}public static void main(String[] args) throws Exception{Session session = sessionFactory.openSession();Transaction tx = null;try{tx = session.beginTransaction();Customer customer = new Customer();customer.setName("zhangsan");customer.setOrders(new HashSet());Order order1 = new Order();order1.setOrderNumber("order1");Order order2 = new Order();order2.setOrderNumber("order2");Order order3 = new Order();order3.setOrderNumber("order3");order1.setCustomer(customer);order2.setCustomer(customer);order3.setCustomer(customer);customer.getOrders().add(order1);customer.getOrders().add(order2);customer.getOrders().add(order3);session.save(customer);tx.commit();}catch(Exception ex){if(null != tx){tx.rollback();}ex.printStackTrace();}finally{session.close();}}}

重点要理解的是hbm.xml文件

对于Customer.hbm.xml：

<hibernate-mapping>
 
 <class name="com.cdtax.hibernate.Customer" table="customers"> ： 这一句class标签表示对类整体进行映射，name指出映射的域模型，即类名，table指出类映射到的关系模型即与哪张表映射。
 
  <id name="id" column="id" type="long">
   <generator class="increment"></generator>
  </id>
  id标签定义关键字，name是类的OID，column是表的主键，generator指出主键的生成方式。

  <property name="name" type="string">
   <column name="name" length="50"></column>
  </property>
  property标签定义类的普通属性映射关系，name是类的属性名，column是对应表的列名

  <set name="orders" cascade="save-update" inverse="true">
   <key column="customer_id"></key>
   <one-to-many class="com.cdtax.hibernate.Order"/>
  </set>
  set是一个很关键的标签，在一对多的映射中，一的一方（这里就是指Customer类）要包含多的一方（Order类），那么一的一方的属性就应该是一个集合，所以用set来表示，set的name属性是一的一方的属性名，这里就是Customer类中的orders属性，cascade是级联关系，就是指出当我对一的一方进行保存或更新时，多的一方是否同时进行操作，可以有很多值，save-update表示同时操作，none表示不级联，等等，inverse是反转的意思，默认值为false，就是说一对多的关系由一的一方维护，如果改为true，则由多的一方维护，一般一对多的一方都设为true，即反转。
  one-to-many标签指出一的一方包含多的一方的属性（应该是一个集合类型）集合中对象的类型，用class来指出集合对象的类型，对于Customer的orders属性，集合中的对象类型为Order类型。可以这样理解，看源代码定义：
                  private Set<Order> orders;这里使用了泛型，所以我们一看就知道orders中保存的是Order对象，如果不使用泛型，我们看不出来，机器也无从分辨，所以class就是指出泛型的具体类型。
  key子标签的column属性是指出多的一方（这里就是Order）是根据多方对应的表（orders表）的哪一列来查询的，也就是多的一方对应一的一方的外键，如我查询id为1的一方（从customers表中），因为还有一个orders属性，这个属性的值就是根据相应的查询id关联到多的一方的外键，即customer_id查询出来的。
  用SQL来表示，就是第一步：select * from customers where id = 1，因为这个表里没有Order信息，所以还要：select * from orders where customer_id = 1，这里的<key column="customer_id"></key>的column值就是orders表的customer_id，外键。

 </class>
</hibernate-mapping>

 

对于Order.hbm.xml，需要注意的是

<many-to-one name="customer" class="com.cdtax.hibernate.Customer" column="customer_id">
  </many-to-one>

这是关系映射中的多的一方对包含的一的一方属性的映射定义，name指出多的一方（Order）的属性名，即Order类中private Customer customer;，class相当于普通属性映射property标签中的type，定义属性类型，column定义与orders表的哪一列关联。

执行测试类HibernateTest，结果：

Hibernate: select max(id) from customers
Hibernate: select max(id) from orders
Hibernate: insert into customers (name, id) values (?, ?)
Hibernate: insert into orders (order_number, customer_id, id) values (?, ?, ?)
Hibernate: insert into orders (order_number, customer_id, id) values (?, ?, ?)
Hibernate: insert into orders (order_number, customer_id, id) values (?, ?, ?)
可以看出，我们只是执行了save（customer），而插入表时进行级联操作，同时将关联的3个order插入了orders表。

 

如果我们将Customer.hbm.xml中的set进行修改，将cascade值改为none，

<set name="orders" cascade="none" inverse="true">
   <key column="customer_id"></key>
   <one-to-many class="com.cdtax.hibernate.Order"/>
  </set>

执行结果：

Hibernate: select max(id) from customers
Hibernate: insert into customers (name, id) values (?, ?)

只进行了customers表的插入。

2、延迟加载

对customer进行查询：

修改HibernateTest的main方法

Session session = sessionFactory.openSession();Customer customer = null;Transaction tx = null;try{tx = session.beginTransaction();customer = (Customer)session.get(Customer.class, new Long(1));tx.commit();}catch(Exception ex){if(null != tx){tx.rollback();}ex.printStackTrace();}finally{session.close();}System.out.println(customer.getName());}

执行结果：

Hibernate: select customer0_.id as id0_0_, customer0_.name as name0_0_ from customers customer0_ where customer0_.id=?
zhangsan
只执行了一次查询

继续修改：

Session session = sessionFactory.openSession();Transaction tx = null;try{tx = session.beginTransaction();Customer customer = (Customer)session.get(Customer.class, new Long(1));System.out.println(customer.getName());tx.commit();}catch(Exception ex){if(null != tx){tx.rollback();}ex.printStackTrace();}finally{session.close();}}

执行结果：

Hibernate: select customer0_.id as id0_0_, customer0_.name as name0_0_ from customers customer0_ where customer0_.id=?
zhangsan

执行了一次查询

继续修改：

try{tx = session.beginTransaction();Customer customer = (Customer)session.get(Customer.class, new Long(1));System.out.println(customer.getName());System.out.println(customer.getOrders());tx.commit();}

执行结果：
Hibernate: select customer0_.id as id0_0_, customer0_.name as name0_0_ from customers customer0_ where customer0_.id=?
zhangsan
Hibernate: select orders0_.customer_id as customer3_1_, orders0_.id as id1_, orders0_.id as id1_0_, orders0_.order_number as order2_1_0_, orders0_.customer_id as customer3_1_0_ from orders orders0_ where orders0_.customer_id=?
[com.cdtax.hibernate.Order@110003, com.cdtax.hibernate.Order@12d96f2, com.cdtax.hibernate.Order@17e4ca]

修改：

Session session = sessionFactory.openSession();Transaction tx = null;try{tx = session.beginTransaction();Customer customer = (Customer)session.get(Customer.class, new Long(1));System.out.println(customer.getName());Set<Order> orders = customer.getOrders();for(Iterator iter = orders.iterator();iter.hasNext();){Order order =(Order) iter.next();System.out.println(order.getOrderNumber());}tx.commit();}catch(Exception ex){if(null != tx){tx.rollback();}ex.printStackTrace();}finally{session.close();}

执行结果：

Hibernate: select customer0_.id as id0_0_, customer0_.name as name0_0_ from customers customer0_ where customer0_.id=?
zhangsan
Hibernate: select orders0_.customer_id as customer3_1_, orders0_.id as id1_, orders0_.id as id1_0_, orders0_.order_number as order2_1_0_, orders0_.customer_id as customer3_1_0_ from orders orders0_ where orders0_.customer_id=?
order2
order3
order1

修改：

Session session = sessionFactory.openSession();Customer customer = null;Transaction tx = null;try{tx = session.beginTransaction();customer = (Customer)session.get(Customer.class, new Long(1));System.out.println(customer.getName());tx.commit();}catch(Exception ex){if(null != tx){tx.rollback();}ex.printStackTrace();}finally{session.close();}Set<Order> orders = customer.getOrders();for(Iterator iter = orders.iterator();iter.hasNext();){Order order =(Order) iter.next();System.out.println(order.getOrderNumber());}}

执行结果：

Hibernate: select customer0_.id as id0_0_, customer0_.name as name0_0_ from customers customer0_ where customer0_.id=?
zhangsan
Exception in thread "main" org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.cdtax.hibernate.Customer.orders, no session or session was closed
 at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:380)
 at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:372)
 at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:365)
 at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPersistentCollection.java:108)
 at org.hibernate.collection.PersistentSet.iterator(PersistentSet.java:186)
 at com.cdtax.hibernate.HibernateTest.main(HibernateTest.java:112)
出现异常

修改：

Session session = sessionFactory.openSession();Customer customer = null;Transaction tx = null;try{tx = session.beginTransaction();customer = (Customer)session.get(Customer.class, new Long(1));System.out.println(customer.getName());Set set = customer.getOrders();System.out.println(set);tx.commit();}catch(Exception ex){if(null != tx){tx.rollback();}ex.printStackTrace();}finally{session.close();}Set<Order> orders = customer.getOrders();for(Iterator iter = orders.iterator();iter.hasNext();){Order order =(Order) iter.next();System.out.println(order.getOrderNumber());}}

结果：

Hibernate: select customer0_.id as id0_0_, customer0_.name as name0_0_ from customers customer0_ where customer0_.id=?
zhangsan
Hibernate: select orders0_.customer_id as customer3_1_, orders0_.id as id1_, orders0_.id as id1_0_, orders0_.order_number as order2_1_0_, orders0_.customer_id as customer3_1_0_ from orders orders0_ where orders0_.customer_id=?
[com.cdtax.hibernate.Order@110003, com.cdtax.hibernate.Order@adb1d4, com.cdtax.hibernate.Order@17e4ca]
order1
order2
order3

3、异常：LazyInitializationException延迟初始化异常

Hibernate中的延迟加载（Lazy Loading），当我们在程序中获取到了一的一方，但是不需要多的一方，那么使用延迟加载就是非常适合的。

相对应的就是立即加载

修改hbm.xml文件：

<set name="orders" cascade="save-update" inverse="true" lazy="false">
   <key column="customer_id"></key>
   <one-to-many class="com.cdtax.hibernate.Order"/>
  </set>

加上lazy=“false”就可以实现立即加载。

4、查询多的一方：

Session session = sessionFactory.openSession();Transaction tx = null;Order order = null;Customer customer = null;try{tx = session.beginTransaction();order = (Order)session.get(Order.class, new Long(1));System.out.println(order.getOrderNumber());customer = order.getCustomer();tx.commit();}catch(Exception ex){if(null != tx){tx.rollback();}ex.printStackTrace();}finally{session.close();}System.out.println(customer.getName());}

结果：

Hibernate: select order0_.id as id1_0_, order0_.order_number as order2_1_0_, order0_.customer_id as customer3_1_0_ from orders order0_ where order0_.id=?
order1
Exception in thread "main" org.hibernate.LazyInitializationException: could not initialize proxy - no Session
 at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:132)
 at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:174)
 at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:190)
 at com.cdtax.hibernate.Customer_$$_javassist_0.getName(Customer_$$_javassist_0.java)
 at com.cdtax.hibernate.HibernateTest.main(HibernateTest.java:153)

如果将Order.hbm.xml修改：

<many-to-one name="customer" class="com.cdtax.hibernate.Customer" column="customer_id" lazy="false">
  </many-to-one>

结果：

Hibernate: select order0_.id as id1_0_, order0_.order_number as order2_1_0_, order0_.customer_id as customer3_1_0_ from orders order0_ where order0_.id=?
Hibernate: select customer0_.id as id0_0_, customer0_.name as name0_0_ from customers customer0_ where customer0_.id=?
Hibernate: select orders0_.customer_id as customer3_1_, orders0_.id as id1_, orders0_.id as id1_0_, orders0_.order_number as order2_1_0_, orders0_.customer_id as customer3_1_0_ from orders orders0_ where orders0_.customer_id=?
order1
zhangsan

cascade取值：