二级缓存也称进程级的缓存或SessionFactory级的缓存，二级缓存可以被所有的session共享
二级缓存的生命周期和SessionFactory的生命周期一致，SessionFactory可以管理二级缓存
二级缓存的配置和使用：
* 将echcache.xml文件拷贝到src下
* 开启二级缓存，修改hibernate.cfg.xml文件
<property name="hibernate.cache.use_second_level_cache">true</property>
* 指定缓存产品提供商，修改hibernate.cfg.xml文件
<property name="hibernate.cache.provider_class">org.hibernate.cache.EhCacheProvider</property>
* 指定那些实体类使用二级缓存（两种方法）
* 在映射文件中采用<cache>标签
<!--
<cache usage="read-only"/>
-->
* 在hibernate.cfg.xml文件中，采用<class-cache>标签
<class-cache class="com.bjsxt.hibernate.Student" usage="read-only"/>
二级缓存是缓存实体对象的
了解一级缓存和二级缓存的交互
package com.bjsxt.hibernate;
import java.io.Serializable;
import org.hibernate.CacheMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import junit.framework.TestCase;
public class CacheLevel2Test extends TestCase {
/**
* 开启两个session，分别调用load
*/
public void testCache1() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
Student student = (Student)session.load(Student.class, 1);
System.out.println("student.name=" + student.getName());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//不会发出sql，因为开启了二级缓存，session是共享二级缓存的
Student student = (Student)session.load(Student.class, 1);
System.out.println("student.name=" + student.getName());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
/**
* 开启两个session，分别调用get
*/
public void testCache2() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
Student student = (Student)session.get(Student.class, 1);
System.out.println("student.name=" + student.getName());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//不会发出sql，因为开启了二级缓存，session是共享二级缓存的
Student student = (Student)session.get(Student.class, 1);
System.out.println("student.name=" + student.getName());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
/**
* 开启两个session，分别调用load,在使用SessionFactory清除二级缓存
*/
public void testCache3() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
Student student = (Student)session.load(Student.class, 1);
System.out.println("student.name=" + student.getName());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
//管理二级缓存
SessionFactory factory = HibernateUtils.getSessionFactory();
//factory.evict(Student.class);
factory.evict(Student.class, 1);
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//会发出查询sql，因为二级缓存中的数据被清除了
Student student = (Student)session.load(Student.class, 1);
System.out.println("student.name=" + student.getName());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
/**
* 一级缓存和二级缓存的交互
*/
public void testCache4() {
Session session = null;
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//仅向二级缓存读数据，而不向二级缓存写数据
session.setCacheMode(CacheMode.GET);
Student student = (Student)session.load(Student.class, 1);
System.out.println("student.name=" + student.getName());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//发出sql语句，因为session设置了CacheMode为GET，所以二级缓存中没有数据
Student student = (Student)session.load(Student.class, 1);
System.out.println("student.name=" + student.getName());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
try {
session = HibernateUtils.getSession();
session.beginTransaction();
//只向二级缓存写数据，而不从二级缓存读数据
session.setCacheMode(CacheMode.PUT);
//会发出查询sql，因为session将CacheMode设置成了PUT
Student student = (Student)session.load(Student.class, 1);
System.out.println("student.name=" + student.getName());
session.getTransaction().commit();
}catch(Exception e) {
e.printStackTrace();
session.getTransaction().rollback();
}finally {
HibernateUtils.closeSession(session);
}
}
}
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