Hibernate中的一级缓存

HibernateTest.java

代码：

/**
 * 
 */
package com.b510.examplex;

import org.hibernate.Session;


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

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

 public void getGuestbooks(){
  Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();
  session.beginTransaction();
  
  Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);
  Guestbook gb2=(Guestbook)session.get(Guestbook.class, 2);
  
  System.out.println(gb==gb2);
  session.getTransaction().commit();
 }

}

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：

/**
 * 
 */
package com.b510.examplex;

import org.hibernate.Session;


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

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

 public void getGuestbooks(){
  Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();
  session.beginTransaction();
  
  Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);   
  session.getTransaction().commit();

  session=HibernateSessionFactoryUtil.getSessionFactory().openSession();
  session.beginTransaction(); 
  
  Guestbook gb2=(Guestbook)session.get(Guestbook.class, 2);    
  session.getTransaction().commit();
  
  System.out.println(gb==gb2);
 }

}

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

/**
 * 
 */
package com.b510.examplex;

import org.hibernate.Session;


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

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

 public void getGuestbooks(){
  Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();
  session.beginTransaction();
  
  Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);
  Guestbook gb2=(Guestbook)session.get(Guestbook.class, 2);
   
  gb.setName("Hongtenzone@foxmail.com");
  gb2.setEmail("Helloworld@sina.com");
  session.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_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：

/**
 * 
 */
package com.b510.examplex;

import org.hibernate.Session;


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

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

 public void getGuestbooks(){
  Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();
  session.beginTransaction();
  
  Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);
  //移除gb的缓存
  session.evict(gb);  
  Guestbook gb2=(Guestbook)session.get(Guestbook.class, 2);   
  
  System.out.println(gb==gb2);
  session.getTransaction().commit();

 }

}

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：

/**
 * 
 */
package com.b510.examplex;

import org.hibernate.Session;


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

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

 public void getGuestbooks(){
  Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();
  session.beginTransaction();
  
  Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);
  Guestbook gb2=(Guestbook)session.get(Guestbook.class, 3);   
  //清楚缓存，把缓存中的所有东西都给干掉，不留下任何东西
  session.clear();
  Guestbook gb3=(Guestbook)session.get(Guestbook.class, 2);
  Guestbook gb4=(Guestbook)session.get(Guestbook.class, 3); 
  
  System.out.println(gb==gb3);
  System.out.println(gb2==gb4);
  session.getTransaction().commit();

 }

}

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=?
false
false

测试三：

HibernateTest.java

代码：

/**
 * 
 */
package com.b510.examplex;

import org.hibernate.Session;


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

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

 public void test(){
  Session session=HibernateSessionFactoryUtil.getSessionFactory().openSession();
  session.beginTransaction();
  
  Guestbook gb=(Guestbook)session.get(Guestbook.class, 2);
  Guestbook gb2=(Guestbook)session.get(Guestbook.class, 3);
  //移除干净缓存中的数据
  session.clear(); 
  
  //判断gb2对象的是否在缓存中
  System.out.println(session.contains(gb2));

//session.flush();              session刷新

//session.close();              session关闭

说明一下，在session执行下面的commit()方法后，会自动调用

flush()方法和close()方法。这两步是隐含在里面，所以不用显示出来

都是可以的。
  session.getTransaction().commit();

 }

}

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