空間站

1.EhCache是什么
    EhCache是Hibernate的二級緩存技術之一，可以把查詢出來的數據存儲在內存或者磁盤，節省下次同樣查詢語句再次查詢數據庫，大幅減輕數據庫壓力；

2.EhCache的使用注意點
    當用Hibernate的方式修改表數據(save,update,delete等等)，這時EhCache會自動把緩存中關于此表的所有緩存全部刪除掉(這樣能達到同步)。但對于數據經常修改的表來說，可能就失去緩存的意義了(不能減輕數據庫壓力)；

3.EhCache使用的場合
    3.1比較少更新表數據
        EhCache一般要使用在比較少執行write操作的表(包括update,insert,delete等)[Hibernate的二級緩存也都是這樣]；
    3.2對并發要求不是很嚴格的情況
        兩臺機子中的緩存是不能實時同步的；

4.在項目做的實現
    4.1在工程的src目錄下添加ehcache.xml文件，內容如下：
        <?xml version="1.0" encoding="UTF-8"?>
        <ehcache>   　
            <diskStore path="java.io.tmpdir" />
          <defaultCache maxElementsInMemory="5"<!--緩存可以存儲的總記錄量-->
            eternal="false"<!--緩存是否永遠不銷毀-->
            overflowToDisk="true"<!--當緩存中的數據達到最大值時，是否把緩存數據寫入磁盤-->
            timeToIdleSeconds="15"<!--當緩存閑置時間超過該值，則緩存自動銷毀-->
                timeToLiveSeconds="120"<!--緩存創建之后，到達該緩存自動銷毀-->
          />
        </ehcache>
    4.2在Hibernate.cfg.xml中的mapping標簽上面加以下內容：
        <property name="show_sql">true</property>
        <property name="hibernate.cache.provider_class">org.hibernate.cache.EhCacheProvider</property>
        <property name="hibernate.cache.use_query_cache">true</property>
    4.3在要緩存的bean的hbm.xml文件中的class標簽下加入以下內容：
       <cache usage="read-only" /><!--也可讀寫-->
    4.4創建DAO，內容如下：
        Session s = HibernateSessionFactory.getSession();
        Criteria c = s.createCriteria(Xyz.class);
        c.setCacheable(true);//這句必須要有
        System.out.println("第一次讀取");
        List l = c.list();
        System.out.println(l.size());
        HibernateSessionFactory.closeSession();

        s = HibernateSessionFactory.getSession();
        c = s.createCriteria(Xyz.class);
        c.setCacheable(true);//這句必須要有
        System.out.println("第二次讀取");
        l = c.list();
        System.out.println(l.size());
        HibernateSessionFactory.closeSession();
   4.5這時你會看到打印出來的信息為（表示第二次并沒有去讀庫）：
        第一次讀取
        Hibernate: *******
        13
        第二次讀取
        13

配置Spring+hibernate使用ehcache作為second-level cache

大量數據流動是web應用性能問題常見的原因，而緩存被廣泛的用于優化數據庫應用。cache被設計為通過保存從數據庫里load的數據來減少應用和數據 庫之間的數據流動。數據庫訪問只有當檢索的數據不在cache里可用時才必要。hibernate可以用兩種不同的對象緩存：first-level cache 和 second-level cache。first-level cache和Session對象關聯，而second-level cache是和Session Factory對象關聯。

        缺省地，hibernate已經使用基于每個事務的first-level cache。 Hibernate用first-level cache主要是減少在一個事務內的sql查詢數量。例如，如果一個對象在同一個事務內被修改多次，hibernate將只生成一個包括所有修改的 UPDATE SQL語句。為了減少數據流動，second-level cache在Session Factory級的不同事務之間保持load的對象，這些對象對整個應用可用，不只是對當前用戶正在運行的查詢。這樣，每次查詢將返回已經load在緩存 里的對象，避免一個或更多潛在的數據庫事務。

下載ehcache，hibernate3.2必須要ehcache1.2以上才能支持。可以修改log4j配置文件log4j.logger.net.sf.hibernate.cache=debug查看日志

1.在類路徑上ehcache.xml：

<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:

         maxElementsInMemory             - 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.
         overflowToDisk                  - Sets whether elements can overflow to disk when the in-memory cache
                                          has reached the maxInMemory limit.

         The following attributes are optional:
         timeToIdleSeconds               - Sets the time to idle for an element before it expires.
                                          i.e. The maximum amount of time between accesses before an element expires
                                          Is only used if the element is not eternal.
                                          Optional attribute. A value of 0 means that an Element can idle for infinity.
                                          The default value is 0.
         timeToLiveSeconds               - Sets the time to live for an element before it expires.
                                          i.e. The maximum time between creation time and when an element expires.
                                          Is only used if the element is not eternal.
                                          Optional attribute. A value of 0 means that and Element can live for infinity.
                                          The default value is 0.
         diskPersistent                  - Whether the disk store persists between restarts of the Virtual Machine.
                                          The default value is false.
         diskExpiryThreadIntervalSeconds- The number of seconds between runs of the disk expiry thread. The default value
                                          is 120 seconds.
         -->

     <defaultCache
         maxElementsInMemory="10000"
         eternal="false"
         overflowToDisk="true"
         timeToIdleSeconds="120"
         timeToLiveSeconds="120"
         diskPersistent="false"
         diskExpiryThreadIntervalSeconds="120"/>
        
     <!-- See http://ehcache.sourceforge.net/documentation/#mozTocId258426 for how to configure caching for your objects -->
</ehcache>

2.applicationContext-hibernate.xml里Hibernate SessionFactory配置：

     <!-- Hibernate SessionFactory -->
     <bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
         <property name="dataSource" ref="dataSource"/>
         <property name="configLocation"><value>classpath:hibernate.cfg.xml</value></property>
         <!-- The property below is commented out b/c it doesn't work when run via
              Ant in Eclipse.   It works fine for individual JUnit tests and in IDEA ??
         <property name="mappingJarLocations">
             <list><value>file:dist/appfuse-dao.jar</value></list>
         </property>
         -->
         <property name="hibernateProperties">
             <props>
                 <prop key="hibernate.dialect">@HIBERNATE-DIALECT@</prop>
                 <!--<prop key="hibernate.show_sql">true</prop>-->
                 <prop key="hibernate.max_fetch_depth">3</prop>
                 <prop key="hibernate.hibernate.use_outer_join">true</prop>
                 <prop key="hibernate.jdbc.batch_size">10</prop>
                 <prop key="hibernate.cache.use_query_cache">true</prop>
                 <prop key="hibernate.cache.use_second_level_cache">true</prop>
                 <prop key="hibernate.cache.provider_class">org.hibernate.cache.EhCacheProvider</prop>
                 <!--
                 <prop key="hibernate.use_sql_comments">false</prop>
                 -->
                 <!-- Create/update the database tables automatically when the JVM starts up
                 <prop key="hibernate.hbm2ddl.auto">update</prop> -->
                 <!-- Turn batching off for better error messages under PostgreSQL
                 <prop key="hibernate.jdbc.batch_size">0</prop> -->
             </props>
         </property>
         <property name="entityInterceptor">
            <ref local="auditLogInterceptor"/>
         </property>
     </bean>
說明：如果不設置“查詢緩存”，那么hibernate只會緩存使用load()方法獲得的單個持久化對象，如果想緩存使用findall()、 list()、Iterator()、createCriteria()、createQuery()等方法獲得的數據結果集的話，就需要設置 hibernate.cache.use_query_cache true 才行

3.model類里采用Xdoclet生成*.hbm.xml里的cache xml標簽，即<cache usage="read-only"/>

/**
* @hibernate.class table="WF_WORKITEM_HIS"
* @hibernate.cache usage="read-write"
*
*/

4.對于"query cache"，需要在程序里編碼：

         getHibernateTemplate().setCacheQueries(true);
         return getHibernateTemplate().find(hql);