1.实体类:
Customer类如下:
- package com.reiyen.hibernate.domain;
- public class Customer {
- private int id;
- private String name;
- private String email;
- //setter和getter方法
- }
package com.reiyen.hibernate.domain; public class Customer { private int id; private String name; private String email; //setter和getter方法 }
2.Customer.hbm.xml映射文件如下:
- <hibernate-mapping package="com.reiyen.hibernate.domain">
- <class name="Customer">
- <id name="id">
- <generator class="native" />
- </id>
- <property name="name" />
- <property name="email" />
- </class>
- </hibernate-mapping>
<hibernate-mapping package="com.reiyen.hibernate.domain"> <class name="Customer"> <id name="id"> <generator class="native" /> </id> <property name="name" /> <property name="email" /> </class> </hibernate-mapping>
3.测试类如下:
- public class CustomerTest {
- public static void main(String[] args) {
- add();
- Customer customer = getCustomer(1);
- System.out.println(customer.getName());
- }
- static void add(){
- Session session = null;
- Transaction tran = null;
- try {
- Customer customer = new Customer();
- customer.setName("xxlong");
- customer.setEmail("[email protected]");
- session = HibernateUtil.getSession();
- tran = session.beginTransaction();
- session.save(customer);
- tran.commit();
- } finally {
- if (session != null)
- session.close();
- }
- }
- static Customer getCustomer(Integer id){
- Session session = null;
- try {
- session = HibernateUtil.getSession();
- Customer customer = (Customer)session.load(Customer.class, id);
- //System.out.println(customer.getClass()); //1
- //customer.getName();//2
- //Hibernate.initialize(customer); //3
- return customer;
- } finally {
- if (session != null)
- session.close();
- }
- }
- }
public class CustomerTest { public static void main(String[] args) { add(); Customer customer = getCustomer(1); System.out.println(customer.getName()); } static void add(){ Session session = null; Transaction tran = null; try { Customer customer = new Customer(); customer.setName("xxlong"); customer.setEmail("[email protected]"); session = HibernateUtil.getSession(); tran = session.beginTransaction(); session.save(customer); tran.commit(); } finally { if (session != null) session.close(); } } static Customer getCustomer(Integer id){ Session session = null; try { session = HibernateUtil.getSession(); Customer customer = (Customer)session.load(Customer.class, id); //System.out.println(customer.getClass()); //1 //customer.getName();//2 //Hibernate.initialize(customer); //3 return customer; } finally { if (session != null) session.close(); } } }
运行此程序会抛出如下异常:
org.hibernate.LazyInitializationException: could not initialize proxy - no Session
将注释为1的语句的注释去掉,运行可以发现load方法得到的customer对象如下所示:
class com.reiyen.hibernate.domain.Customer$$EnhancerByCGLIB$$c3543d5f
说明load得到的对象不是我们的实体类了,其实它是hibernate实现的Customer的一个子类,是Customer的一个代理类,此代理类在hibernate的Session没有关闭时能访问数据库,但一旦Session关闭,它就不能再访问数据库了。解决此异常的方法:即在Session关闭之前强迫此代理类去访问数据库。
如在程序中增加注释为2的程序即可。但写这样一句程序没有意义,所以在正式开发环境中一般增加注释为3的程序语句来解决此异常。
使用load方法获取对象时,它不会立即去访问数据库,它直接就实例化了一个此对象的代理对象返回给你,只有等到你真正需要使用此对象时,它才会去访问数据库。所以load方法在一定程度上能提高性能。注意:使用load方法时,永远不能使用if(customer != null)这种判断方法来判断load方法成功没有,因为它永远会返回给你一个代理对象。
懒加载是通过asm.jar和cglib.jar来实现的。
懒加载总结:
hibernate中懒加载是通过asm和cglib二个包实现的,
hibernate中要实现懒加载功能,你的实体类必须设计成非final的。
hibernate中懒加载的分类以及实现条件:
1.session.load懒加载
2.one-to-one(元素)懒加载:必须同时满足下面的三个条件时才能实现懒加载:1).lazy!=false 2).constrained=true 3).fetch=select
(因为主表不能有constrained=true,所以主表没有懒加载功能)\
3.one-to-many(元素)懒加载:1).lazy!=false 2).fetch=select
4.many-to-one(元素)懒加载:1).lazy!=false 2).fetch=select
5.many-to-many(元素)懒加载:1).lazy!=false 2).fetch=select
6.能够懒加载的对象都是被改写过的代理对象,当相关联的session没有关闭时,访问这些懒加载对象(代理对象)的属性(getId和getClass除外)时,hibernate会初始化这些代理,或用Hibernate.initialize(proxy)来初始化代理对象;
当相关联的session关闭后,再访问懒加载的对象将会出现异常。
4.根据ID查询区别:
- Query q = session.createQuery("from Customer where id=:id");
- q.setParameter("id",id);
- Customer customer = q.uniqueResult();
Query q = session.createQuery("from Customer where id=:id"); q.setParameter("id",id); Customer customer = q.uniqueResult();
与
- Customer customer = (Customer)session.load(Customer.class, id);
- //Customer customer = (Customer)session.get(Customer.class, id);
Customer customer = (Customer)session.load(Customer.class, id); //Customer customer = (Customer)session.get(Customer.class, id);
的区别是:后者能利用Hibernate的缓存,所以相对前者来说查询效率会高些(因为Query默认是直接去数据库中查询数据的)。
5。DetachedCriteri可在session外创建(在其他层创建,如Service中创建),然后用getExecutableCriteria(session)方法创建Criteria对象来完成查询。
在持久层定义方法dcQuery如下:
- public List<Customer> dcQuery(DetachedCriteria dCriteria){
- Session session = HibernateUtil.getSession();
- Criteria c = dCriteria.getExecutableCriteria(session);
- List<Customer> list = c.list();
- session.close();
- return list;
- }
public List<Customer> dcQuery(DetachedCriteria dCriteria){ Session session = HibernateUtil.getSession(); Criteria c = dCriteria.getExecutableCriteria(session); List<Customer> list = c.list(); session.close(); return list; }
在应用层使用dcQuery方法,如下:
- String name = request.getParameter("name");
- String name = request.getParameter("email");
- DetachedCriteria dc = DetachedCriteria.forClass(Customer.class);
- if (name != null && !name.isEmpty())
- dc.add(Restrictions.ilike("name", name));
- if (email != null && !email.isEmpty())
- dc.add(Restrictions.eq("email", email));
- List<Customer> customers = dcQuery(dc);
String name = request.getParameter("name"); String name = request.getParameter("email"); DetachedCriteria dc = DetachedCriteria.forClass(Customer.class); if (name != null && !name.isEmpty()) dc.add(Restrictions.ilike("name", name)); if (email != null && !email.isEmpty()) dc.add(Restrictions.eq("email", email)); List<Customer> customers = dcQuery(dc);
6。N+1查询问题分析:
1).用Query.iterate可能会有N+1次查询(使用iterate进行查询时,它首先只是将查询的实体对象的id值查询出来,然后再根据id来查询实体)
- Query q = session.createQuery("from User");
- java.util.Iterator<User> users = q.iterate();
- while(users.hasNext()){
- System.out.println(users.next().getName());
- }
Query q = session.createQuery("from User"); java.util.Iterator<User> users = q.iterate(); while(users.hasNext()){ System.out.println(users.next().getName()); }
控制台打印的第一条SQL语句如下所示:
Hibernate:select user0.id as col_0_0_ from user user0_
说明它第一步只是将查询对象的id查询出来.然后它根据id再一个一个的将所有要查询实体查询出来,此过程如下:
首先根据id在一级缓存中查找,如果没有找到,再到二级缓存中去找(如果配置了二级缓存的话),如果在二级缓存中还没有找到,则再去数据库中查询.如果缓存中有要查询的对象,则iterate是很高效的,因为它只去数据库中查询了一次;但如果缓存中没有要查询的数据,则iterate查询效率是非常低的,因为它会产生N+1次查询!
2).懒加载时获取关联对象可能有N+1次查询
3).如果打开查询的缓存即使用list也可能有N+1次查询