1. 什么是緩存？
數(shù)據(jù)庫的緩存指的是應(yīng)用程序和物理數(shù)據(jù)源之間的數(shù)據(jù)。即把物理數(shù)據(jù)源的數(shù)據(jù)復(fù)制到緩存。有了緩存，可以降低應(yīng)用程序?qū)ξ锢頂?shù)據(jù)源的訪問頻率，從而提高效率。緩存的介質(zhì)一般是內(nèi)存，也可以是硬盤。
Hibernate的緩存有三種類型：一級緩存、二級緩存和查詢緩存。
2. 一級緩存
一級緩存即Session緩存，由Session自動進(jìn)行管理，不需要程序進(jìn)行干預(yù)。一級緩存根據(jù)對象的ID進(jìn)行加載和緩存。如下面的代碼：
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@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(); } |
運(yùn)行結(jié)果：
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Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?Name:計(jì)算機(jī)原理Name:計(jì)算機(jī)原理 |
第1次查詢時(shí)生成了SQL語句，并將查詢出來的對象放在一級緩存里面，第2次查詢時(shí)，在一級緩存里面直接找到了這個(gè)對象，就不需要再次生成SQL語句了。
再看一個(gè)例子：
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@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(); } |
由于一級緩存是Session級別的緩存，所以Session關(guān)閉以后，一級緩存也就不存在了，第2次查詢也要生成SQL語句：
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Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?Name:計(jì)算機(jī)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:計(jì)算機(jī)原理 |
3. 二級緩存
二級緩存即SessionFactory緩存，和一級緩存類似，也是根據(jù)對象的ID進(jìn)行加載和緩存，區(qū)別就在于一級緩存只在Session內(nèi)有效，而二級緩存在SessionFactory內(nèi)有效。在訪問某個(gè)ID的對象時(shí)，先到一級緩存里面去找，如果沒有找到就到二級緩存里面去找。二級緩存包括EHCache，OSCache，SwarmCache和JBossCache等。這里以EHCache作為例子。
二級緩存需要程序進(jìn)行管理。首先，配置Maven下載相關(guān)的Jar，在pom文件里面添加：
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<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> |
創(chuàng)建EHCache配置文件ehcache.xml：
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<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是默認(rèn)的設(shè)置，下面一個(gè)cache指明了對哪一個(gè)類進(jìn)行二級緩存。里面設(shè)置了最大緩存的對象數(shù)量，是否永久有效、最大空閑秒數(shù)、最大生存秒數(shù)、內(nèi)存滿時(shí)是否寫到硬盤，寫到硬盤的路徑等等。
修改需要緩存的類的hbm文件：
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<class name="com.hzhi.course.entity.Course" table="clas"> <cache usage="read-only"/> ...... </class> |
usage設(shè)置了并發(fā)訪問策略，一般設(shè)置成read-only。
修改applicationContext.xml中的SessionFactory的配置，增加二級緩存的一些屬性：
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<!-- SessionFactory --> <bean id="sessionFactory" class="org.springframework.orm.hibernate4.LocalSessionFactoryBean"> <property name="dataSource" > <ref local="dataSource"/> </property> <!-- 配置Hibernate的屬性 --> <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> <!-- 二級緩存 --> <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> |
運(yùn)行下面的例子：
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@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(); } |
結(jié)果：
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Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?Name:計(jì)算機(jī)原理Name:計(jì)算機(jī)原理 |
雖然關(guān)閉了Session，但是二級緩存仍然存在，所以只生成了一次SQL語句。
下面的例子：
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@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(); } |
結(jié)果：
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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=?計(jì)算機(jī)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=?計(jì)算機(jī)網(wǎng)絡(luò)Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?數(shù)據(jù)庫原理Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?C語言Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?大學(xué)英語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=?高等數(shù)學(xué)Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?語文Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?大學(xué)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=?軟件工程Hibernate: select course0_.ID as col_0_0_ from clas course0_計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程 |
當(dāng)使用Query的list()方法時(shí)，只生成一次SQL語句查詢出所有的對象，使用iterate()方法時(shí)，會先得到所有對象的ID，然后根據(jù)每個(gè)ID生成一次SQL語句查詢。第二個(gè)Session里面使用的也是iterate()方法，首先生成一次SQL語句，得到ID，然后根據(jù)ID查找對象，由于開啟了二級緩存，在二級緩存里面找到了對象，所以就直接輸出了，并沒有再根據(jù)每個(gè)ID生成SQL語句。
不論是一級緩存還是二級緩存，都只能緩存對象，不能緩存屬性的值。下面的例子：
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@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(); } |
運(yùn)行結(jié)果：
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Hibernate: select course0_.NAME as col_0_0_ from clas course0_計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程----------Hibernate: select course0_.NAME as col_0_0_ from clas course0_計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程---------- |
雖然開啟了二級緩存，但是查詢的結(jié)果不是對象，是屬性，所以并沒有緩存，第2次查詢?nèi)匀簧闪瞬樵冋Z句。要解決這個(gè)問題，就需要查詢緩存。
3. 查詢緩存
在配置了二級緩存的基礎(chǔ)上，可以設(shè)置查詢緩存，在SessionFactory的設(shè)置里面加上一行：
<prop key="hibernate.cache.use_query_cache">true</prop>
即打開了查詢緩存。查詢緩存也是SessionFactory級別的緩存，在整個(gè)SessionFactory里面都是有效的。
關(guān)閉二級緩存，運(yùn)行下面的例子，在Query后面添加setCacheable(true)打開查詢緩存：
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@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(); } |
結(jié)果：
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Hibernate: select course0_.NAME as col_0_0_ from clas course0_計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程----------計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程---------- |
由于兩次查詢的HQL語句是一致的，所以只生成一次SQL語句。但是如果把第二次查詢改一下：
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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("----------"); |
結(jié)果：
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Hibernate: select course0_.NAME as col_0_0_ from clas course0_計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程----------Hibernate: select course0_.NAME as col_0_0_ from clas course0_ where course0_.ID>5大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程---------- |
由于HQL語句變了，所以第二次也生成了SQL語句。
查詢緩存可以緩存屬性，也可以緩存對象，但是當(dāng)緩存對象時(shí)，只緩存對象的ID，不會緩存整個(gè)對象。下面的例子：
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@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(); } |
結(jié)果：
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Hibernate: select course0_.ID as ID0_, course0_.NAME as NAME0_, course0_.COMMENT as COMMENT0_ from clas course0_計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程----------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=?計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程---------- |
由于開了查詢緩存，沒有開二級緩存，雖然使用的是list()方法一次查詢出了所有的對象，但是查詢緩存只緩存了對象ID，沒有緩存整個(gè)對象。所以在第2個(gè)Session里面"from Course"這個(gè)HQL由于和前面的相同，并沒有生成SQL語句，但是由于沒有開二級緩存，沒有緩存整個(gè)對象，只能根據(jù)每個(gè)ID去生成一次SQL語句。雖然兩次用的都是list()方法，但是第一次是生成SQL語句去一次查詢出所有的對象，而第二次是根據(jù)查詢緩存里面的ID一個(gè)一個(gè)的生成SQL語句。
如果同時(shí)打開查詢緩存和二級緩存，第2個(gè)Session里面就不用再根據(jù)ID去生成SQL語句了：
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Hibernate: select course0_.ID as ID0_, course0_.NAME as NAME0_, course0_.COMMENT as COMMENT0_ from clas course0_計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程----------計(jì)算機(jī)原理計(jì)算機(jī)網(wǎng)絡(luò)數(shù)據(jù)庫原理C語言大學(xué)英語AJavaLinux高等數(shù)學(xué)語文大學(xué)物理軟件工程---------- |
以上就是本文的全部內(nèi)容，希望本文的內(nèi)容對大家的學(xué)習(xí)或者工作能帶來一定的幫助，同時(shí)也希望多多支持服務(wù)器之家！
原文鏈接：http://www.cnblogs.com/mstk/p/6363351.html
