大量數(shù)據(jù)流動(dòng)是web應(yīng)用性能問(wèn)題常見的原因，而緩存被廣泛的用于優(yōu)化數(shù)據(jù)庫(kù)應(yīng)用。cache被設(shè)計(jì)為通過(guò)保存從數(shù)據(jù)庫(kù)里load的數(shù)據(jù)來(lái)減少應(yīng)用和數(shù)據(jù)庫(kù)之間的數(shù)據(jù)流動(dòng)。數(shù)據(jù)庫(kù)訪問(wèn)只有當(dāng)檢索的數(shù)據(jù)不在cache里可用時(shí)才必要。hibernate可以用兩種不同的對(duì)象緩存：first-level cache 和 second-level cache。first-level cache和Session對(duì)象關(guān)聯(lián)，而second-level cache是和Session Factory對(duì)象關(guān)聯(lián)。
缺省地，hibernate已經(jīng)使用基于每個(gè)事務(wù)的first-level cache。Hibernate用first-level cache主要是減少在一個(gè)事務(wù)內(nèi)的sql查詢數(shù)量。例如，如果一個(gè)對(duì)象在同一個(gè)事務(wù)內(nèi)被修改多次，hibernate將只生成一個(gè)包括所有修改的UPDATE SQL語(yǔ)句。為了減少數(shù)據(jù)流動(dòng)，second-level cache在Session Factory級(jí)的不同事務(wù)之間保持load的對(duì)象，這些對(duì)象對(duì)整個(gè)應(yīng)用可用，不只是對(duì)當(dāng)前用戶正在運(yùn)行的查詢。這樣，每次查詢將返回已經(jīng)load在緩存里的對(duì)象，避免一個(gè)或更多潛在的數(shù)據(jù)庫(kù)事務(wù)。
下載ehcache，hibernate3.2必須要ehcache1.2以上才能支持?？梢孕薷膌og4j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(shuō)明：如果不設(shè)置“查詢緩存”，那么hibernate只會(huì)緩存使用load()方法獲得的單個(gè)持久化對(duì)象，如果想緩存使用findall()、 list()、Iterator()、createCriteria()、createQuery()等方法獲得的數(shù)據(jù)結(jié)果集的話，就需要設(shè)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(biāo)簽，即<cache usage="read-only"/>
/**
* @hibernate.class table="WF_WORKITEM_HIS"
* @hibernate.cache usage="read-write"
*
*/
4.對(duì)于"query cache"，需要在程序里編碼：
getHibernateTemplate().setCacheQueries(true);
return getHibernateTemplate().find(hql);
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