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:EclipseMyWorkspaceCache"/>
    <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A
Hibernate: 
    select
        course0_.ID as ID0_0_,
        course0_.NAME as NAME0_0_,
        course0_.COMMENT as COMMENT0_0_ 
    from
        clas course0_ 
    where
        course0_.ID=?
Java
Hibernate: 
    select
        course0_.ID as ID0_0_,
        course0_.NAME as NAME0_0_,
        course0_.COMMENT as COMMENT0_0_ 
    from
        clas course0_ 
    where
        course0_.ID=?
Linux
Hibernate: 
    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A
Java
Linux
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A
Java
Linux
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A
Java
Linux
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A
Java
Linux
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A
Java
Linux
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A
Java
Linux
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A
Java
Linux
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A
Java
Linux
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A
Java
Linux
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A
Java
Linux
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A
Java
Linux
高等数学
语文
大学物理
软件工程
----------
 

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