1. 什么是緩存？
數據庫的緩存指的是應用程序和物理數據源之間的數據。即把物理數據源的數據復制到緩存。有了緩存，可以降低應用程序對物理數據源的訪問頻率，從而提高效率。緩存的介質一般是內存，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自動進行管理，不需要程序進行干預。一級緩存根據對象的ID進行加載和緩存。如下面的代碼：
@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(); }運行結果：
Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?Name:計算機原理Name:計算機原理
第1次查詢時生成了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，在一級緩存里面直接找到了這個對象，就不需要再次生成SQL語句了。
再看一個例子：
@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(); }由于一級緩存是Session級別的緩存，所以Session關閉以后，一級緩存也就不存在了，第2次查詢也要生成SQL語句：
Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?Name:計算機原理Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?Name:計算機原理
3. 二級緩存
二級緩存即SessionFactory緩存，和一級緩存類似，也是根據對象的ID進行加載和緩存，區別就在于一級緩存只在Session內有效，而二級緩存在SessionFactory內有效。在訪問某個ID的對象時，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行管理。首先，配置Maven下載相關的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>
創建EHCache配置文件ehcache.xml：
<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是默認的設置，下面一個cache指明了對哪一個類進行二級緩存。里面設置了最大緩存的對象數量，是否永久有效、最大空閑秒數、最大生存秒數、內存滿時是否寫到硬盤，寫到硬盤的路徑等等。
修改需要緩存的類的hbm文件：
<class name="com.hzhi.course.entity.Course" table="clas"> <cache usage="read-only"/> ...... </class>
usage設置了并發訪問策略，一般設置成read-only。
修改applicationContext.xml中的SessionFactory的配置，增加二級緩存的一些屬性：
<!-- 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(); }結果：
Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?Name:計算機原理Name:計算機原理
雖然關閉了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：
@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=?計算機原理Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?計算機網絡Hibernate: select course0_.ID as ID0_0_, course0_.NAME as NAME0_0_, course0_.COMMENT as COMMENT0_0_ from clas course0_ where course0_.ID=?數據庫原理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=?高等數學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_計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程
當使用Query的list()方法時，只生成一次SQL語句查詢出所有的對象，使用iterate()方法時，會先得到所有對象的ID，然后根據每個ID生成一次SQL語句查詢。第二個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，并沒有再根據每個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：
@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(); }運行結果：
Hibernate: select course0_.NAME as col_0_0_ from clas course0_計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程----------Hibernate: select course0_.NAME as col_0_0_ from clas course0_計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程----------
雖然開啟了二級緩存，但是查詢的結果不是對象，是屬性，所以并沒有緩存，第2次查詢仍然生成了查詢語句。要解決這個問題，就需要查詢緩存。
3. 查詢緩存
在配置了二級緩存的基礎上，可以設置查詢緩存，在SessionFactory的設置里面加上一行：
<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SessionFactory里面都是有效的。
關閉二級緩存，運行下面的例子，在Query后面添加setCacheable(true)打開查詢緩存：
@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(); }結果：
Hibernate: select course0_.NAME as col_0_0_ from clas course0_計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程----------計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程----------
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：
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("----------");結果：
Hibernate: select course0_.NAME as col_0_0_ from clas course0_計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程----------Hibernate: select course0_.NAME as col_0_0_ from clas course0_ where course0_.ID>5大學英語AJavaLinux高等數學語文大學物理軟件工程----------
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，但是當緩存對象時，只緩存對象的ID，不會緩存整個對象。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(); }結果：
Hibernate: select course0_.ID as ID0_, course0_.NAME as NAME0_, course0_.COMMENT as COMMENT0_ from clas course0_計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程----------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=?計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程----------
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，沒有緩存整個對象。所以在第2個Session里面"from Course"這個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，沒有緩存整個對象，只能根據每個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，而第二次是根據查詢緩存里面的ID一個一個的生成SQL語句。
如果同時打開查詢緩存和二級緩存，第2個Session里面就不用再根據ID去生成SQL語句了：
Hibernate: select course0_.ID as ID0_, course0_.NAME as NAME0_, course0_.COMMENT as COMMENT0_ from clas course0_計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程----------計算機原理計算機網絡數據庫原理C語言大學英語AJavaLinux高等數學語文大學物理軟件工程----------
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