Hibernate的二级缓存 下

HibernateTest.java

代码：

/**  *  */ package com.b510.examplex;

import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction;

/**  *  * @author XHW  *  * @date 2011-7-15  *  */ public class HibernateTest {

 /**   * @param args   */  public static void main(String[] args) {   new HibernateTest().getGuestbooks();  }

 public void getGuestbooks(){   SessionFactory sessionFactory=HibernateSessionFactoryUtil.getSessionFactory();   Session session1=sessionFactory.getCurrentSession();   System.out.println("Session1:"+session1);   Transaction tx=session1.beginTransaction();

//Guestbook gb=(Guestbook)session1.load(Guestbook.class, 1);   Guestbook gb=(Guestbook)session1.get(Guestbook.class, 1);   System.out.println("ID : "+gb.getId()+"name:  "+gb.getName());   tx.commit();   System.out.println("----------------------------------");   Session session2=sessionFactory.getCurrentSession();   System.out.println("Session2:"+session2);   System.out.println("session1是否等于session2  ？");   System.out.println(session1==session2);   session2.beginTransaction();

//  gb=(Guestbook)session2.load(Guestbook.class, 1);      load和get方法在这里可以随便使用，不会影响运行的结果   gb=(Guestbook)session2.get(Guestbook.class, 1);   System.out.println("ID : "+gb.getId()+"name:  "+gb.getName());   session2.getTransaction().commit();  }  }

运行效果：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment). log4j:WARN Please initialize the log4j system properly. Session1:SessionImpl(PersistenceContext[entityKeys=[],collectionKeys=[]];ActionQueue[insertions=[] updates=[] deletions=[] collectionCreations=[] collectionRemovals=[] collectionUpdates=[]]) Hibernate:     select         guestbook0_.id as id0_0_,         guestbook0_.version as version0_0_,         guestbook0_.name as name0_0_,         guestbook0_.email as email0_0_,         guestbook0_.phone as phone0_0_,         guestbook0_.title as title0_0_,         guestbook0_.content as content0_0_,         guestbook0_.created_time as created8_0_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 1name:  liuwei ---------------------------------- Session2:SessionImpl(PersistenceContext[entityKeys=[],collectionKeys=[]];ActionQueue[insertions=[] updates=[] deletions=[] collectionCreations=[] collectionRemovals=[] collectionUpdates=[]]) session1是否等于session2  ？ false ID : 1name:  liuwei

我们看到，session1和session2两个session对象是不想等的，也就是说两个session的地址不是同一个内存地址

然而我们却看到，这里却只用了一条select语句，我们明明是查询两条记录啊！这说明HIbernate的缓存是跨session的

也就是一个session对象被干掉了以后，他的缓存还是存在的，不因session被干掉二干掉。这样在我们读取相同的数据

的时候就不会再却读数据库，而是直接从缓存中读取出来。

测试代码二：

HibernateTest.java

代码；

/**  *  */ package com.b510.examplex;

import java.util.List;

import org.hibernate.Query; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction;

/**  *  * @author XHW  *  * @date 2011-7-15  *  */ public class HibernateTest {

 /**   * @param args   */  public static void main(String[] args) {   new HibernateTest().getGuestbooks();  }

 public void getGuestbooks(){   SessionFactory sessionFactory=HibernateSessionFactoryUtil.getSessionFactory();   Session session1=sessionFactory.getCurrentSession();   Transaction tx=session1.beginTransaction(); Query query=session1.createQuery("from Guestbook");   List<Guestbook> list=query.list();   for(Guestbook gb:list){    System.out.println("ID : "+gb.getId()+"name:  "+gb.getName());   }   tx.commit();   System.out.println("----------------------------------");   Session session2=sessionFactory.getCurrentSession();   session2.beginTransaction();  query=session2.createQuery("from Guestbook");   list=query.list();   for(Guestbook gb:list){    System.out.println("ID : "+gb.getId()+"name:  "+gb.getName());   }   session2.getTransaction().commit();  }  }

运行效果：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment). log4j:WARN Please initialize the log4j system properly. Hibernate:     select         guestbook0_.id as id0_,         guestbook0_.version as version0_,         guestbook0_.name as name0_,         guestbook0_.email as email0_,         guestbook0_.phone as phone0_,         guestbook0_.title as title0_,         guestbook0_.content as content0_,         guestbook0_.created_time as created8_0_     from         users.guestbook guestbook0_ ID : 1name:  liuwei ID : 2name:  Hongtenzone@foxmail.com ID : 3name:  Hongten ID : 4name:  HOngten ID : 5name:  HOngten ID : 6name:  HOngten ID : 7name:  HOngten ---------------------------------- Hibernate:     select         guestbook0_.id as id0_,         guestbook0_.version as version0_,         guestbook0_.name as name0_,         guestbook0_.email as email0_,         guestbook0_.phone as phone0_,         guestbook0_.title as title0_,         guestbook0_.content as content0_,         guestbook0_.created_time as created8_0_     from         users.guestbook guestbook0_ ID : 1name:  liuwei ID : 2name:  Hongtenzone@foxmail.com ID : 3name:  Hongten ID : 4name:  HOngten ID : 5name:  HOngten ID : 6name:  HOngten ID : 7name:  HOngten

我们看到的结果是用两条select语句，查出了相同的结果！

这个list对象有这么一个特点：他只是从数据库中读取数据，放到缓存中，但是据对不会到缓存中

去索要数据。也就是说list只是贡献缓存，不会索要缓存。

他不用二级缓存。

测试代码三：

HibernateTest.java

代码：

/**  *  */ package com.b510.examplex;

import java.util.List;

import java.util.Iterator;

import org.hibernate.Query; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction;

/**  *  * @author XHW  *  * @date 2011-7-15  *  */ public class HibernateTest {

 /**   * @param args   */  public static void main(String[] args) {   new HibernateTest().getGuestbooks();  }

 public void getGuestbooks(){   SessionFactory sessionFactory=HibernateSessionFactoryUtil.getSessionFactory();   Session session1=sessionFactory.getCurrentSession();   Transaction tx=session1.beginTransaction();   Query query=session1.createQuery("from Guestbook"); List<Guestbook> list=query.list();   for(Guestbook gb:list){    System.out.println("ID : "+gb.getId()+"name:  "+gb.getName());   }   tx.commit();   System.out.println("----------------------------------");   Session session2=sessionFactory.getCurrentSession();   session2.beginTransaction();   query=session2.createQuery("from Guestbook");   Iterator it=query.iterate();   while(it.hasNext()){    Guestbook gb=(Guestbook)it.next();    System.out.println("ID : "+gb.getId()+"name:  "+gb.getName());   }   session2.getTransaction().commit();  }  }

运行效果：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment). log4j:WARN Please initialize the log4j system properly. Hibernate:     select         guestbook0_.id as id0_,         guestbook0_.version as version0_,         guestbook0_.name as name0_,         guestbook0_.email as email0_,         guestbook0_.phone as phone0_,         guestbook0_.title as title0_,         guestbook0_.content as content0_,         guestbook0_.created_time as created8_0_     from         users.guestbook guestbook0_ ID : 1name:  liuwei ID : 2name:  Hongtenzone@foxmail.com ID : 3name:  Hongten ID : 4name:  HOngten ID : 5name:  HOngten ID : 6name:  HOngten ID : 7name:  HOngten ---------------------------------- Hibernate:     select         guestbook0_.id as col_0_0_     from         users.guestbook guestbook0_ ID : 1name:  liuwei ID : 2name:  Hongtenzone@foxmail.com ID : 3name:  Hongten ID : 4name:  HOngten ID : 5name:  HOngten ID : 6name:  HOngten ID : 7name:  HOngten

第一条selelct语句是从数据库中读出数据然后将数据放入二级缓存中，接着进入第二个会话

Iterator会向二级缓存索要查询的数据，他的方式是首先将要查的实体类的主键值获取到，

这样在底层上就要使用一条selelct  id 的语句，就是我们看到的第二条selelct语句。这样就获取了

这个持久化对象的的主键值的一个集合。从 这个主键值集合中再分别读出一个主键值，将他的其他内容

读取出来。如上面显示的一样，先获取Id=1，然后获取id=1的其他数据，如name字段的值，然后再获取

id=2，再获取id=2的其他数据(name),以这种方式进行下去，直到读取完数据。还要申明的是，Iterator

都是先从二级缓存中获取数据，如果不在二级缓存中，那么要使用“n+1”方式了。

看下面的测试代码：

HibernateTest.java

代码：

/**  *  */ package com.b510.examplex;

import java.util.List;

import java.util.Iterator;

import org.hibernate.Query; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction;

/**  *  * @author XHW  *  * @date 2011-7-15  *  */ public class HibernateTest {

 /**   * @param args   */  public static void main(String[] args) {   new HibernateTest().getGuestbooks();  }

 public void getGuestbooks(){   SessionFactory sessionFactory=HibernateSessionFactoryUtil.getSessionFactory();   /*   Session session1=sessionFactory.getCurrentSession();   Transaction tx=session1.beginTransaction();   Query query=session1.createQuery("from Guestbook");   List<Guestbook> list=query.list();   for(Guestbook gb:list){    System.out.println("ID : "+gb.getId()+"name:  "+gb.getName());   }   tx.commit();   */   System.out.println("----------------------------------");   Session session2=sessionFactory.getCurrentSession();   session2.beginTransaction();   Query query=session2.createQuery("from Guestbook");   Iterator it=query.iterate();   while(it.hasNext()){    Guestbook gb=(Guestbook)it.next();    System.out.println("ID : "+gb.getId()+"name:  "+gb.getName());   }   session2.getTransaction().commit();  }  }

运行效果：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment). log4j:WARN Please initialize the log4j system properly. ---------------------------------- Hibernate:     select         guestbook0_.id as col_0_0_     from         users.guestbook guestbook0_                                          “n+1”方式中 的“1” Hibernate:     select         guestbook0_.id as id0_0_,         guestbook0_.version as version0_0_,         guestbook0_.name as name0_0_,         guestbook0_.email as email0_0_,         guestbook0_.phone as phone0_0_,         guestbook0_.title as title0_0_,         guestbook0_.content as content0_0_,         guestbook0_.created_time as created8_0_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 1name:  liuwei Hibernate:     select         guestbook0_.id as id0_0_,         guestbook0_.version as version0_0_,         guestbook0_.name as name0_0_,         guestbook0_.email as email0_0_,         guestbook0_.phone as phone0_0_,         guestbook0_.title as title0_0_,         guestbook0_.content as content0_0_,         guestbook0_.created_time as created8_0_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 2name:  Hongtenzone@foxmail.com Hibernate:     select         guestbook0_.id as id0_0_,         guestbook0_.version as version0_0_,         guestbook0_.name as name0_0_,         guestbook0_.email as email0_0_,         guestbook0_.phone as phone0_0_,         guestbook0_.title as title0_0_,         guestbook0_.content as content0_0_,         guestbook0_.created_time as created8_0_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 3name:  Hongten Hibernate:     select         guestbook0_.id as id0_0_,         guestbook0_.version as version0_0_,         guestbook0_.name as name0_0_,         guestbook0_.email as email0_0_,         guestbook0_.phone as phone0_0_,         guestbook0_.title as title0_0_,         guestbook0_.content as content0_0_,         guestbook0_.created_time as created8_0_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 4name:  HOngten Hibernate:     select         guestbook0_.id as id0_0_,         guestbook0_.version as version0_0_,         guestbook0_.name as name0_0_,         guestbook0_.email as email0_0_,         guestbook0_.phone as phone0_0_,         guestbook0_.title as title0_0_,         guestbook0_.content as content0_0_,         guestbook0_.created_time as created8_0_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 5name:  HOngten Hibernate:     select         guestbook0_.id as id0_0_,         guestbook0_.version as version0_0_,         guestbook0_.name as name0_0_,         guestbook0_.email as email0_0_,         guestbook0_.phone as phone0_0_,         guestbook0_.title as title0_0_,         guestbook0_.content as content0_0_,         guestbook0_.created_time as created8_0_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 6name:  HOngten Hibernate:     select         guestbook0_.id as id0_0_,         guestbook0_.version as version0_0_,         guestbook0_.name as name0_0_,         guestbook0_.email as email0_0_,         guestbook0_.phone as phone0_0_,         guestbook0_.title as title0_0_,         guestbook0_.content as content0_0_,         guestbook0_.created_time as created8_0_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 7name:  HOngten

这就是“n+1”或者说“1+n”方式，也就是说，我们要查询的结果只有7条记录，但是，我们看到 的selelctyuj

却是8条。首先从缓存中去找，没有找到，就向数据库发出请求，查询出数据。然后执行id=2的Guestbook对象，

缓存中没有记录，有要向数据库发出请求，以这种方式进行下去。

要注意 的是：如果二级缓存中没有任何对象或者说是数据，那么这时如果我们采用Iterator方法，就会用到“n+1”

条selelct语句，这时还不如list的查询效率高。所以我们要用Iterator的时候的前提是二级缓存中要有数据，这样

的查询效率要才会高。不然会出现相反的效果。

测试代码四；

HibernateTest.java

代码：

/**  *  */ package com.b510.examplex;

import java.util.List;

import java.util.Iterator;

import org.hibernate.Query; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction; import org.hibernate.cfg.Configuration;

/**  *  * @author XHW  *  * @date 2011-7-15  *  */ public class HibernateTest {

 /**   * @param args   */  public static void main(String[] args) {   new HibernateTest().getGuestbooks();  }

 public void getGuestbooks(){   SessionFactory sessionFactory1=new Configuration().configure().buildSessionFactory();   Session session1=sessionFactory1.getCurrentSession();   Transaction tx1=session1.beginTransaction();   Guestbook gb=(Guestbook)session1.get(Guestbook.class, 1);   System.out.println("ID : "+gb.getId()+"name:  "+gb.getName());   System.out.println(sessionFactory1); sessionFactory1.close();   System.out.println("----------------------------------");   SessionFactory sessionFactory2=new Configuration().configure().buildSessionFactory();  Session session2=sessionFactory2.getCurrentSession();   Transaction tx2=session2.beginTransaction();   Guestbook gb2=(Guestbook)session2.get(Guestbook.class, 1);   System.out.println("ID : "+gb2.getId()+"name:  "+gb2.getName());   System.out.println(sessionFactory2);   sessionFactory2.close();  }  }

运行效果：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment). log4j:WARN Please initialize the log4j system properly. Hibernate:     select         guestbook0_.id as id0_0_,         guestbook0_.version as version0_0_,         guestbook0_.name as name0_0_,         guestbook0_.email as email0_0_,         guestbook0_.phone as phone0_0_,         guestbook0_.title as title0_0_,         guestbook0_.content as content0_0_,         guestbook0_.created_time as created8_0_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 1name:  liuwei org.hibernate.impl.SessionFactoryImpl@1fcc0a2 ---------------------------------- Hibernate:     select         guestbook0_.id as id2_0_,         guestbook0_.version as version2_0_,         guestbook0_.name as name2_0_,         guestbook0_.email as email2_0_,         guestbook0_.phone as phone2_0_,         guestbook0_.title as title2_0_,         guestbook0_.content as content2_0_,         guestbook0_.created_time as created8_2_0_     from         users.guestbook guestbook0_     where         guestbook0_.id=? ID : 1name:  liuwei org.hibernate.impl.SessionFactoryImpl@19b46dc

Hibernate的二级缓存是跟SessionFactory相关的，所以当一个SessionFactory对象关闭后，二级缓存中的

数据就会跟着这个SessionFactory对象的关闭而随之消失，当我们启动第二个SessionFactory对象的时候，这个

这个对象的二级缓存生效。当然当这个SessionFactory对象关闭的时候，二级缓存中的数据也会消失。

因此，我们就会看到两个select语句。

所以：Hibernate的二级缓存又被叫做SessionFactory缓存