hibernate默认有一个一级缓存，就是默认的Session缓存，当我们做了一次查询，hibernate会把这个结果缓存起来，叫做一级缓存，当我们接着在这个Session会话里面再做一次同样条件的查询，hibernate不会再次查询数据库，而是直接在一级缓存中获取结果并返回。一级缓存是内置的，他也不能被卸载。一级缓存是指在同一个Session会话内的查询做的缓存，如果跨了Session，或者当前Session关闭，重新开启了一个新的Session，这时候一级缓存是失效的。

二级缓存是一个可插拔的缓存，可有可无，如果你配置了，他就有，没有配置就没有。二级缓存的实现，默认是用ehcache,需要增加一个配置文件ehcache.xml，如下所示：

<?xml version="1.0" encoding="utf-8" ?>
<ehcache><diskStore path="java.io.tmpdir"/><defaultCache maxElementsInMemory="10000"eternal="false"timeToIdleSeconds="120"timeToLiveSeconds="120"overflowToDisk="true" />
</ehcache>

实体类如下：

BaseEntity.java

package com.xxx.hibernate.domain;
import java.util.Date;import javax.persistence.Column;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;
@MappedSuperclass
public class BaseEntity {@Id@GeneratedValue(strategy=GenerationType.IDENTITY)protected Integer id;@Column(name="modifydate")protected Date modifyDate;@Column(name="createdate")protected Date createDate;public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public Date getModifyDate() {return modifyDate;}public void setModifyDate(Date modifyDate) {this.modifyDate = modifyDate;}public Date getCreateDate() {return createDate;}public void setCreateDate(Date createDate) {this.createDate = createDate;}
}

User.java 

package com.xxx.hibernate.domain;
import javax.persistence.Entity;
import javax.persistence.Table;import org.hibernate.annotations.Cache;
import org.hibernate.annotations.CacheConcurrencyStrategy;
@Table(name="xx_user")
@Entity
@Cache(usage=CacheConcurrencyStrategy.READ_WRITE)
public class User extends BaseEntity{private String name;private int age;private String email;private String mobile;private String password;public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String getEmail() {return email;}public void setEmail(String email) {this.email = email;}public String getMobile() {return mobile;}public void setMobile(String mobile) {this.mobile = mobile;}public String getPassword() {return password;}public void setPassword(String password) {this.password = password;}@Overridepublic String toString() {return "user:[id="+id+",name="+name+",age="+age+",email="+email+",mobile="+mobile+"]";}
}

hibernate.cfg.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" 
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd" >
<hibernate-configuration><session-factory><property name="connection.driver_class">com.mysql.jdbc.Driver</property><property name="connection.url">jdbc:mysql:///hibernate?useUnicode=true&amp;characterEncoding=utf-8&amp;useSSL=false</property><property name="connection.username">root</property><property name="connection.password">root</property><property name="connection.pool_size">20</property><property name="dialect">org.hibernate.dialect.MySQL57InnoDBDialect</property><property name="show_sql">true</property><property name="format_sql">true</property><property name="hbm2ddl.auto">update</property><property name="hibernate.cache.use_second_level_cache">true</property><property name="hibernate.cache.provider_class">org.hibernate.cache.EhCacheProvider</property><property name="hibernate.cache.region.factory_class">org.hibernate.cache.ehcache.EhCacheRegionFactory</property><property name="hibernate.cache.use_query_cache">true</property><mapping class="com.xxx.hibernate.domain.User"/></session-factory>
</hibernate-configuration>

 测试类代码：

package com.xxx.hibernate.test;
import java.util.Date;
import java.util.List;import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;import com.xxx.hibernate.domain.User;
public class HibernateAppTest {private static SessionFactory sessionFactory;private static final ThreadLocal<Session> session = new ThreadLocal<Session>();static{try {Configuration cfg = new Configuration().configure();sessionFactory = cfg.buildSessionFactory();} catch (Exception e) {e.printStackTrace();}}public static Session getSession(){Session sess = session.get();if(sess==null){sess = sessionFactory.openSession();session.set(sess);}return sess;}public static void closeSession(){Session sess = session.get();if(sess!=null){sess.close();}session.set(null);sessionFactory.close();}public static void add(String name,int age,String email,String mobile,String password){User user = new User();user.setName(name);user.setAge(age);user.setEmail(email);user.setMobile(mobile);user.setPassword(password);user.setCreateDate(new Date());user.setModifyDate(new Date());//Session sess = getSession();Transaction tx = sess.beginTransaction();sess.save(user);tx.commit();closeSession();}public static void search(Integer id){Session sess = getSession();User user = sess.get(User.class, id);if(user!=null)System.out.println(user);}public static void testDefaultCache(){long start = System.currentTimeMillis();search(2);long end = System.currentTimeMillis();System.out.println("it takes "+(end-start)+" .");//======================start = System.currentTimeMillis();search(2);end = System.currentTimeMillis();System.out.println("it takes "+(end-start)+" .");start = System.currentTimeMillis();search(1);end = System.currentTimeMillis();System.out.println("it takes "+(end-start)+" .");closeSession();}@SuppressWarnings("unchecked")public static void testL2Cache(){Session sess = sessionFactory.openSession();List<User> list = sess.createQuery("from User").setCacheable(true).list();for(User user:list){System.out.println(user);}sess.close();System.out.println("=======================分割线======================");Session sess2 = sessionFactory.openSession();list = sess2.createQuery("from User").setCacheable(true).list();for(User user:list){System.out.println(user);}sess2.close();closeSession();}public static void main(String[] args) {//add("xxx", 18, "xxx@hibernate.cn", "15902020509", "123456");//add("aaa", 33, "aaa@hibernate.cn", "15902020520", "123456");	//testDefaultCache();testL2Cache();}
}

运行testL2Cache()方法，打印关键信息如下：

第一次查询，打印了sql语句，说明从数据库中做了查询，第二次查询，直接使用了二级缓存，因此，并没有打印sql语句。这个结果说明hibernate二级缓存配置正确并且生效了。