Hibernate中的一级缓存

HibernateTest.java

代码：

/** 2  *  3  */ 4 package com.b510.examplex; 5  6 import org.hibernate.Session; 7  8  9 /**10  *11  * @author XHW12  *13  * @date 2011-7-1514  *15  */16 public class HibernateTest {17 18  /**19   * @param args20   */21  public static void main(String[] args) {22   new HibernateTest().getGuestbooks();23  }24 25  public void getGuestbooks(){26   Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();27   session.beginTransaction();28   29   Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);30   Guestbook gb2=(Guestbook)session.get(Guestbook.class, 2);31   32   System.out.println(gb==gb2);33   session.getTransaction().commit();34  }35 36 }

运行效果：

我们会看到，这里只用了一条select语句。但是我们读的是两条记录啊！

为什么会出现这种情况呢？

这是因为 Hibernate使用了一级缓存，一级缓存又叫Session缓存

在一个会话的生命期里面，他所用到的数据会使用缓冲的，第一次读的是编号为2的数据记录

第一次读的时候，内存中什么都没有，这时候就要用一条select语句从数据库中读出一条记录，同时把这些数据放到session的缓存

里面，接下来又要读同样的数据，这时候他就不会再向数据库读取数据，而是向缓存中读取数据。

得到这个对象，把这个对象返回，所以我们看到一条select语句

Sysout(true);即：gb=gb2

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=?true

但是，如果我们在session对象结束的时候再来sysout(gb=gb2)，

会出现什么样的结果呢？

我们看看下面的运行效果就会看到，这时候会看到两条select语句，

一个session对象结束后，这是缓存中的数据会清空。

所以我们看到两条select语句。

代码：

/** 2  *  3  */ 4 package com.b510.examplex; 5  6 import org.hibernate.Session; 7  8  9 /**10  *11  * @author XHW12  *13  * @date 2011-7-1514  *15  */16 public class HibernateTest {17 18  /**19   * @param args20   */21  public static void main(String[] args) {22   new HibernateTest().getGuestbooks();23  }24 25  public void getGuestbooks(){26   Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();27   session.beginTransaction();28   29   Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);   30   session.getTransaction().commit();31 32   session=HibernateSessionFactoryUtil.getSessionFactory().openSession();33   session.beginTransaction(); 34   35   Guestbook gb2=(Guestbook)session.get(Guestbook.class, 2);    36   session.getTransaction().commit();37   38   System.out.println(gb==gb2);39  }40 41 }

运行效果：

我们看到，我们要跟新两条记录，但是这里只是执行了一条update语句。

这是Hibernate利用缓存，用一个update就可以了。

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=?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=?false

 HibernateTest.java

/** 2  *  3  */ 4 package com.b510.examplex; 5  6 import org.hibernate.Session; 7  8  9 /**10  *11  * @author XHW12  *13  * @date 2011-7-1514  *15  */16 public class HibernateTest {17 18  /**19   * @param args20   */21  public static void main(String[] args) {22   new HibernateTest().getGuestbooks();23  }24 25  public void getGuestbooks(){26   Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();27   session.beginTransaction();28   29   Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);30   Guestbook gb2=(Guestbook)session.get(Guestbook.class, 2);31    32   gb.setName("Hongtenzone@foxmail.com");33   gb2.setEmail("Helloworld@sina.com");34   session.getTransaction().commit();35 36  }37 38 }

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=?Hibernate:     update        users.guestbook     set        version=?,        name=?,        email=?,        phone=?,        title=?,        content=?,        created_time=?     where        id=?         and version=?

HibernateTest.java

代码：

/** 2  *  3  */ 4 package com.b510.examplex; 5  6 import org.hibernate.Session; 7  8  9 /**10  *11  * @author XHW12  *13  * @date 2011-7-1514  *15  */16 public class HibernateTest {17 18  /**19   * @param args20   */21  public static void main(String[] args) {22   new HibernateTest().getGuestbooks();23  }24 25  public void getGuestbooks(){26   Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();27   session.beginTransaction();28   29   Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);30   //移除gb的缓存31   session.evict(gb);  32   Guestbook gb2=(Guestbook)session.get(Guestbook.class, 2);   33   34   System.out.println(gb==gb2);35   session.getTransaction().commit();36 37  }38 39 }

运行效果：

首先查出gb对象的数据，放入缓存中，当执行session.evict(gb);语句时，就会把

缓存中的数据移除掉。

这次要查gb2对象的数据，就得从新生成一条select语句。

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=?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=?false

测试二：

HIbernateTest.java

代码：

/** 2  *  3  */ 4 package com.b510.examplex; 5  6 import org.hibernate.Session; 7  8  9 /**10  *11  * @author XHW12  *13  * @date 2011-7-1514  *15  */16 public class HibernateTest {17 18  /**19   * @param args20   */21  public static void main(String[] args) {22   new HibernateTest().getGuestbooks();23  }24 25  public void getGuestbooks(){26   Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();27   session.beginTransaction();28   29   Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);30   Guestbook gb2=(Guestbook)session.get(Guestbook.class, 3);   31   //清楚缓存，把缓存中的所有东西都给干掉，不留下任何东西32   session.clear();33   Guestbook gb3=(Guestbook)session.get(Guestbook.class, 2);34   Guestbook gb4=(Guestbook)session.get(Guestbook.class, 3); 35   36   System.out.println(gb==gb3);37   System.out.println(gb2==gb4);38   session.getTransaction().commit();39 40  }41 42 }

运行效果：

我们看到session.clear();的执行效果果然是“心狠手辣”，不让敌人有丝毫反抗的有生力量……

这个方法会让缓存中的所有东西清除的干干净净。通通的，大大的，全部干掉的……

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=?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=?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=?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=?falsefalse

测试三：

HibernateTest.java

代码：

/** 2  *  3  */ 4 package com.b510.examplex; 5  6 import org.hibernate.Session; 7  8  9 /**10  *11  * @author XHW12  *13  * @date 2011-7-1514  *15  */16 public class HibernateTest {17 18  /**19   * @param args20   */21  public static void main(String[] args) {22   new HibernateTest().test();23  }24 25  public void test(){26   Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();27   session.beginTransaction();28   29   Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);30   Guestbook gb2=(Guestbook)session.get(Guestbook.class, 3);31   //移除干净缓存中的数据32   session.clear(); 33   34   //判断gb2对象的是否在缓存中35   System.out.println(session.contains(gb2));36 37 //session.flush();              session刷新38 39 //session.close();              session关闭40 41 说明一下，在session执行下面的commit()方法后，会自动调用42 43 flush()方法和close()方法。这两步是隐含在里面，所以不用显示出来44 45 都是可以的。46   session.getTransaction().commit();47 48  }49 50 }

运行效果：

session.contains(gb2);返回的是一个boolean值，判断缓存中是否缓存在gb2对象。

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=?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=?false