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Hibernate缓存研究

1. 什么是缓存?

数据库的缓存指的是应用程序和物理数据源之间的数据。即把物理数据源的数据复制到缓存。有了缓存,可以降低应用程序对物理数据源的访问频率,从而提高效率。缓存的介质一般是内存,也可以是硬盘。

Hibernate的缓存有三种类型:一级缓存、二级缓存和查询缓存。

2. 一级缓存

一级缓存即Session缓存,由Session自动进行管理,不需要程序进行干预。一级缓存根据对象的ID进行加载和缓存。如下面的代码:

    @Override    public void testCache() {        // TODO Auto-generated method stub        Session session = sessionFactory.openSession();        Transaction tx = session.beginTransaction();          Course c = (Course) session.get(Course.class, 1);        System.out.println("Name:" + c.getName());        c = (Course) session.get(Course.class, 1);        System.out.println("Name:" + c.getName());        tx.commit();        session.close();    }

运行结果:

Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Name:计算机原理Name:计算机原理

第1次查询时生成了SQL语句,并将查询出来的对象放在一级缓存里面,第2次查询时,在一级缓存里面直接找到了这个对象,就不需要再次生成SQL语句了。

再看一个例子:

    @Override    public void testCache() {        // TODO Auto-generated method stub        Session session = sessionFactory.openSession();        Transaction tx = session.beginTransaction();          Course c = (Course) session.get(Course.class, 1);        System.out.println("Name:" + c.getName());        tx.commit();        session.close();                session = sessionFactory.openSession();        tx = session.beginTransaction();         c = (Course) session.get(Course.class, 1);        System.out.println("Name:" + c.getName());        tx.commit();        session.close();    }

由于一级缓存是Session级别的缓存,所以Session关闭以后,一级缓存也就不存在了,第2次查询也要生成SQL语句:

Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Name:计算机原理Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Name:计算机原理

 3. 二级缓存

二级缓存即SessionFactory缓存,和一级缓存类似,也是根据对象的ID进行加载和缓存,区别就在于一级缓存只在Session内有效,而二级缓存在SessionFactory内有效。在访问某个ID的对象时,先到一级缓存里面去找,如果没有找到就到二级缓存里面去找。二级缓存包括EHCache,OSCache,SwarmCache和JBossCache等。这里以EHCache作为例子。

二级缓存需要程序进行管理。首先,配置Maven下载相关的Jar,在pom文件里面添加:

        <dependency>             <groupId>org.hibernate</groupId>             <artifactId>hibernate-ehcache</artifactId>             <version>4.1.0.Final</version>         </dependency>        <dependency>              <groupId>net.sf.ehcache</groupId>              <artifactId>ehcache</artifactId>              <version>2.8.3</version>          </dependency>

创建EHCache配置文件ehcache.xml:

<ehcache>    <diskStore path="E:\Eclipse\MyWorkspace\Cache"/>    <defaultCache        maxElementsInMemory="10000"        eternal="true"        timeToIdleSeconds="120"        timeToLiveSeconds="120"        overflowToDisk="true"    />    <cache name="com.hzhi.course.entity.Course"        maxElementsInMemory="10000"        eternal="true"        timeToIdleSeconds="300"        timeToLiveSeconds="600"        overflowToDisk="true"    /></ehcache>

defaultCache是默认的设置,下面一个cache指明了对哪一个类进行二级缓存。里面设置了最大缓存的对象数量,是否永久有效、最大空闲秒数、最大生存秒数、内存满时是否写到硬盘,写到硬盘的路径等等。

修改需要缓存的类的hbm文件:

    <class name="com.hzhi.course.entity.Course" table="clas">        <cache usage="read-only"/>                ......    </class>

usage设置了并发访问策略,一般设置成read-only。

修改applicationContext.xml中的SessionFactory的配置,增加二级缓存的一些属性:

    <!-- SessionFactory -->     <bean id="sessionFactory" class="org.springframework.orm.hibernate4.LocalSessionFactoryBean">        <property name="dataSource" >            <ref local="dataSource"/>        </property>        <!-- 配置Hibernate的属性 -->        <property name="hibernateProperties">            <props>                <prop key="hibernate.dialect">org.hibernate.dialect.MySQLDialect</prop>                <prop key="hibernate.show_sql">true</prop>                <prop key="hibernate.format_sql">true</prop>                <prop key="hibernate.connection.isolation">8</prop>                <!-- 二级缓存 -->                <prop key="hibernate.cache.use_second_level_cache">false</prop>                <prop key="hibernate.cache.region.factory_class">org.hibernate.cache.ehcache.EhCacheRegionFactory</prop>                                <prop key="hibernate.cache.provider_configuration_file_resource_path">WEB-INF/ehcache.xml</prop>                            </props>        </property>
     ......
</bean>

运行下面的例子:

    @Override    public void testCache() {        // TODO Auto-generated method stub        Session session = sessionFactory.openSession();        Transaction tx = session.beginTransaction();          Course c = (Course) session.get(Course.class, 1);        System.out.println("Name:" + c.getName());        tx.commit();        session.close();                session = sessionFactory.openSession();        tx = session.beginTransaction();         c = (Course) session.get(Course.class, 1);        System.out.println("Name:" + c.getName());        tx.commit();        session.close();    }

结果:

Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Name:计算机原理Name:计算机原理

虽然关闭了Session,但是二级缓存仍然存在,所以只生成了一次SQL语句。

下面的例子:

    @Override    public void testCache() {        // TODO Auto-generated method stub        Session session = sessionFactory.openSession();        Transaction tx = session.beginTransaction();          Query query = session.createQuery("from Course");          Iterator iter = query.iterate();          while(iter.hasNext()){                 System.out.println(((Course)iter.next()).getName());          }        tx.commit();        session.close();                session = sessionFactory.openSession();        tx = session.beginTransaction();         query = session.createQuery("from Course");          iter = query.iterate();          while(iter.hasNext()){                 System.out.println(((Course)iter.next()).getName());          }        tx.commit();        session.close();    }

结果:

Hibernate:     select        course0_.ID as col_0_0_     from        clas course0_Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?计算机原理Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?计算机网络Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?数据库原理Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?C语言Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?大学英语AHibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?JavaHibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?LinuxHibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?高等数学Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?语文Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?大学物理Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?软件工程Hibernate:     select        course0_.ID as col_0_0_     from        clas course0_计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程

当使用Query的list()方法时,只生成一次SQL语句查询出所有的对象,使用iterate()方法时,会先得到所有对象的ID,然后根据每个ID生成一次SQL语句查询。第二个Session里面使用的也是iterate()方法,首先生成一次SQL语句,得到ID,然后根据ID查找对象,由于开启了二级缓存,在二级缓存里面找到了对象,所以就直接输出了,并没有再根据每个ID生成SQL语句。

不论是一级缓存还是二级缓存,都只能缓存对象,不能缓存属性的值。下面的例子:

    @Override    public void testCache() {        // TODO Auto-generated method stub        Session session = sessionFactory.openSession();        Transaction tx = session.beginTransaction();          Query query = session.createQuery("select c.name from Course c");           List<String> names = query.list();          for(Iterator iter = names.iterator(); iter.hasNext();){              String name = (String) iter.next();              System.out.println(name);          }          System.out.println("----------");          query = session.createQuery("select c.name from Course c");           names = query.list();          for(Iterator iter = names.iterator(); iter.hasNext();){              String name = (String) iter.next();              System.out.println(name);          }         System.out.println("----------");         tx.commit();        session.close();    }

运行结果:

Hibernate:     select        course0_.NAME as col_0_0_     from        clas course0_计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程----------Hibernate:     select        course0_.NAME as col_0_0_     from        clas course0_计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程----------

虽然开启了二级缓存,但是查询的结果不是对象,是属性,所以并没有缓存,第2次查询仍然生成了查询语句。要解决这个问题,就需要查询缓存。

3. 查询缓存

在配置了二级缓存的基础上,可以设置查询缓存,在SessionFactory的设置里面加上一行:

<prop key="hibernate.cache.use_query_cache">true</prop>

即打开了查询缓存。查询缓存也是SessionFactory级别的缓存,在整个SessionFactory里面都是有效的。

关闭二级缓存,运行下面的例子,在Query后面添加setCacheable(true)打开查询缓存:

    @Override    public void testCache() {        // TODO Auto-generated method stub        Session session = sessionFactory.openSession();        Transaction tx = session.beginTransaction();          Query query = session.createQuery("select c.name from Course c");          query.setCacheable(true);          List<String> names = query.list();          for(Iterator iter = names.iterator(); iter.hasNext();){              String name = (String) iter.next();              System.out.println(name);          }          System.out.println("----------");          query = session.createQuery("select c.name from Course c");          query.setCacheable(true);          names = query.list();          for(Iterator iter = names.iterator(); iter.hasNext();){              String name = (String) iter.next();              System.out.println(name);          }         System.out.println("----------");         tx.commit();        session.close();    }

结果:

Hibernate:     select        course0_.NAME as col_0_0_     from        clas course0_计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程----------计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程----------

由于两次查询的HQL语句是一致的,所以只生成一次SQL语句。但是如果把第二次查询改一下:

        System.out.println("----------");         query = session.createQuery("select c.name from Course c where c.id > 5");          query.setCacheable(true);          names = query.list();          for(Iterator iter = names.iterator(); iter.hasNext();){              String name = (String) iter.next();              System.out.println(name);          }         System.out.println("----------"); 

结果:

Hibernate:     select        course0_.NAME as col_0_0_     from        clas course0_计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程----------Hibernate:     select        course0_.NAME as col_0_0_     from        clas course0_     where        course0_.ID>5大学英语AJavaLinux高等数学语文大学物理软件工程----------

由于HQL语句变了,所以第二次也生成了SQL语句。

查询缓存可以缓存属性,也可以缓存对象,但是当缓存对象时,只缓存对象的ID,不会缓存整个对象。下面的例子:

    @Override    public void testCache() {        // TODO Auto-generated method stub        Session session = sessionFactory.openSession();        Transaction tx = session.beginTransaction();          Query query = session.createQuery("from Course");        query.setCacheable(true);        List<Course> list = query.list();        for (int i=0; i<list.size(); i++){            System.out.println(list.get(i).getName());          }        System.out.println("----------");         tx.commit();        session.close();                session = sessionFactory.openSession();        tx = session.beginTransaction();                query = session.createQuery("from Course");         query.setCacheable(true);        list = query.list();        for (int i=0; i<list.size(); i++){            System.out.println(list.get(i).getName());          }        System.out.println("----------");         tx.commit();        session.close();    }

 结果:

Hibernate:     select        course0_.ID as ID0_,        course0_.NAME as NAME0_,        course0_.COMMENT as COMMENT0_     from        clas course0_计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程----------Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?Hibernate:     select        course0_.ID as ID0_0_,        course0_.NAME as NAME0_0_,        course0_.COMMENT as COMMENT0_0_     from        clas course0_     where        course0_.ID=?计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程----------

由于开了查询缓存,没有开二级缓存,虽然使用的是list()方法一次查询出了所有的对象,但是查询缓存只缓存了对象ID,没有缓存整个对象。所以在第2个Session里面"from Course"这个HQL由于和前面的相同,并没有生成SQL语句,但是由于没有开二级缓存,没有缓存整个对象,只能根据每个ID去生成一次SQL语句。虽然两次用的都是list()方法,但是第一次是生成SQL语句去一次查询出所有的对象,而第二次是根据查询缓存里面的ID一个一个的生成SQL语句。

如果同时打开查询缓存和二级缓存,第2个Session里面就不用再根据ID去生成SQL语句了:

Hibernate:     select        course0_.ID as ID0_,        course0_.NAME as NAME0_,        course0_.COMMENT as COMMENT0_     from        clas course0_计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程----------计算机原理计算机网络数据库原理C语言大学英语AJavaLinux高等数学语文大学物理软件工程----------

 

Hibernate缓存研究