https://www.heckjj.com/index.php zh-cn https://www.heckjj.com/hibernate-cache-manage-description/ Heck <@hecks.tk> Fri, 06 Jul 2012 03:09:45 +0000 https://www.heckjj.com/hibernate-cache-manage-description/ 在hibernate中缓存分为三种：一级缓存、二级缓存、查询缓存（三级缓存）
1、一级缓存
Session级缓存，只能在Session内部使用。它是内置的，不能被卸载（不能被卸载的意思就是这种缓存不具有可选性，必须有的功能，不可以取消session缓存）。由于Session对象的生命周期通常对应一个数据库事务或者一个应用事务，因此它的缓存是事务范围的缓存。第一级缓存是必需的，不允许而且事实上也无法卸除。在第一级缓存中，持久化类的每个实例都具有唯一的OID。

2、二级缓存
sessionFactory级别，所有Session可以共用。由于SessionFactory对象的生命周期和应用程序的整个过程对应，因此Hibernate二级缓存是进程范围或者集群范围的缓存，有可能出现并发问题，因此需要采用适当的并发访问策略，该策略为被缓存的数据提供了事务隔离级别。第二级缓存是可选的，是一个可配置的插件，在默认情况下，SessionFactory不会启用这个插件。
以配置EHCache为例：
a)新建/src/ehcache.xml文件，内容如下：
<ehcache>     <!-- Sets the path to the directory where cache .data files are created.          If the path is a Java System Property it is replaced by          its value in the running VM.          The following properties are translated:          user.home - User's home directory          user.dir - User's current working directory          java.io.tmpdir - Default temp file path -->     <diskStore path="java.io.tmpdir"/>     <!--Default Cache configuration. These will applied to caches programmatically created through         the CacheManager.         The following attributes are required for defaultCache:         maxInMemory       - Sets the maximum number of objects that will be created in memory         eternal           - Sets whether elements are eternal. If eternal,  timeouts are ignored and the element                             is never expired.         timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only used                             if the element is not eternal. Idle time is now - last accessed time         timeToLiveSeconds - Sets the time to live for an element before it expires. Is only used                             if the element is not eternal. TTL is now - creation time         overflowToDisk    - Sets whether elements can overflow to disk when the in-memory cache                             has reached the maxInMemory limit.         -->     <defaultCache         maxElementsInMemory="10000"         eternal="false"         timeToIdleSeconds="120"         timeToLiveSeconds="1200"         overflowToDisk="true"         />     <!--Predefined caches.  Add your cache configuration settings here.         If you do not have a configuration for your cache a WARNING will be issued when the         CacheManager starts         The following attributes are required for defaultCache:         name              - Sets the name of the cache. This is used to identify the cache. It must be unique.         maxInMemory       - Sets the maximum number of objects that will be created in memory         eternal           - Sets whether elements are eternal. If eternal,  timeouts are ignored and the element                             is never expired.         timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only used                             if the element is not eternal. Idle time is now - last accessed time         timeToLiveSeconds - Sets the time to live for an element before it expires. Is only used                             if the element is not eternal. TTL is now - creation time         overflowToDisk    - Sets whether elements can overflow to disk when the in-memory cache                             has reached the maxInMemory limit.         -->     <!-- Sample cache named sampleCache1         This cache contains a maximum in memory of 10000 elements, and will expire         an element if it is idle for more than 5 minutes and lives for more than         10 minutes.         If there are more than 10000 elements it will overflow to the         disk cache, which in this configuration will go to wherever java.io.tmp is         defined on your system. On a standard Linux system this will be /tmp"         -->     <cache name="sampleCache1"         maxElementsInMemory="10000"         eternal="false"         timeToIdleSeconds="300"         timeToLiveSeconds="600"         overflowToDisk="true"         />     <!-- Sample cache named sampleCache2         This cache contains 1000 elements. Elements will always be held in memory.         They are not expired. -->     <cache name="sampleCache2"         maxElementsInMemory="1000"         eternal="true"         timeToIdleSeconds="0"         timeToLiveSeconds="0"         overflowToDisk="false"         />     <!-- Place configuration for your caches following --> </ehcache>

b)hibernate.cfg.xml
        <property name="cache.use_second_level_cache">true</property>         <property name="cache.provider_class">org.hibernate.cache.EhCacheProvider</property>

c)在需要使用缓存的实体类上加标注
@Cache(usage=CacheConcurrencyStrategy.READ_WRITE)

d)测试用例：    @Test     public void testCache1() {         Session session = sf.openSession();         session.beginTransaction();         Category c = (Category) session.load(Category.class, 1);         System.out.println(c.getName());         session.getTransaction().commit();         session.close();         Session session2 = sf.openSession();         session2.beginTransaction();         Category c2 = (Category) session2.load(Category.class, 1);         System.out.println(c2.getName());         session2.getTransaction().commit();         session2.close();     }

3、查询缓存（三级缓存）
Query缓存，查询语句相同时会使用。
a)hibernate.cfg.xml
        <property name="cache.provider_class">org.hibernate.cache.EhCacheProvider</property>         <property name="cache.use_query_cache">true</property>

b)测试用例：    @Test     public void testQueryCache() {         Session session = sf.openSession();         session.beginTransaction();         List<Category> q = session.createQuery("from Category c where c.id>?")                 .setCacheable(true)                 .setInteger(0, 3)                 .list();         session.getTransaction().commit();         session.close();         Session session2 = sf.openSession();         session2.beginTransaction();         List<Category> q2 = session2                 .createQuery("from Category c where c.id>?")                 .setCacheable(true)                 .setInteger(0, 3)                 .list();         session2.getTransaction().commit();         session2.close();     }

Hibernate查找对象如何应用缓存？
当Hibernate根据ID访问数据对象的时候，首先从Session一级缓存中查；查不到，如果配置了二级缓存，那么从二级缓存中查；如果都查不到，再查询数据库，把结果按照ID放入到缓存
删除、更新、增加数据的时候，同时更新缓存

Hibernate管理缓存实例
无论何时，我们在管理Hibernate缓存（Managing the caches）时，当你给save()、update()或saveOrUpdate()方法传递一个对象时，或使用load()、 get()、list()、iterate() 或scroll()方法获得一个对象时, 该对象都将被加入到Session的内部缓存中。
当随后flush()方法被调用时，对象的状态会和数据库取得同步。 如果你不希望此同步操作发生，或者你正处理大量对象、需要对有效管理内存时，你可以调用evict() 方法，从一级缓存中去掉这些对象及其集合。

二级缓存适合放什么对象？
1、经常被访问
2、改动不大（不会被经常改动）
3、数量有限

load默认使用二级缓存，iterator默认使用二级缓存
list默认往二级缓存加数据，但是查询的时候不使用

缓存算法：
LRU(Least Recently Used）最近很少使用的对象会被删除
LFU(Least Frequently Used）命中率低的对象会被删除
FIFO(First In First Out）先进先出

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