详细解读Hibernate的缓存机制

public class Client
{
 public static void main(String[] args)
 {
  Session session = HibernateUtil.getSessionFactory().openSession();
  Transaction tx = null;
  try
  {
   /*开启一个事务*/
   tx = session.beginTransaction();
   /*从数据库中获取id="402881e534fa5a440134fa5a45340002"的Customer对象*/
   Customer customer1 = (Customer)session.get(Customer.class, "402881e534fa5a440134fa5a45340002");
   System.out.println("customer.getUsername is"+customer1.getUsername());
   /*事务提交*/
   tx.commit();
   
   System.out.println("-------------------------------------");
   
   /*开启一个新事务*/
   tx = session.beginTransaction();
   /*从数据库中获取id="402881e534fa5a440134fa5a45340002"的Customer对象*/
   Customer customer2 = (Customer)session.get(Customer.class, "402881e534fa5a440134fa5a45340002");
   System.out.println("customer2.getUsername is"+customer2.getUsername());
   /*事务提交*/
   tx.commit();
   
   System.out.println("-------------------------------------");
   
   /*比较两个get()方法获取的对象是否是同一个对象*/
   System.out.println("customer1 == customer2 result is "+(customer1==customer2));
  }
  catch (Exception e)
  {
   if(tx!=null)
   {
    tx.rollback();
   }
  }
  finally
  {
   session.close();
  }
 }
}

结果
Hibernate: 
 select
  customer0_.id as id0_0_,
  customer0_.username as username0_0_,
  customer0_.balance as balance0_0_ 
 from
  customer customer0_ 
 where
  customer0_.id=?
customer.getUsername islisi
-------------------------------------
customer2.getUsername islisi
-------------------------------------
customer1 == customer2 result is true

<!-- EHCache的配置,hibernate.cfg.xml --> 
<hibernate-configuration>
 <session-factory>
  <!-- 设置二级缓存插件EHCache的Provider类-->
  <property name="hibernate.cache.provider_class">
   org.hibernate.cache.EhCacheProvider
  </property>
  <!-- 启动"查询缓存" -->
  <property name="hibernate.cache.use_query_cache">
   true
  </property>
 </session-factory>
 </hibernate-configuration>

<!-- ehcache.xml -->
<?xml version="1.0" encoding="UTF-8"?>
<ehcache>
 <!--
  缓存到硬盘的路径
 -->
 <diskStore path="d:/ehcache"></diskStore>
 <!--
  默认设置
  maxElementsInMemory : 在內存中最大緩存的对象数量。
  eternal : 缓存的对象是否永远不变。
  timeToIdleSeconds ：可以操作对象的时间。
  timeToLiveSeconds ：缓存中对象的生命周期，时间到后查询数据会从数据库中读取。
  overflowToDisk ：内存满了，是否要缓存到硬盘。
 -->
 <defaultCache maxElementsInMemory="200" eternal="false" 
  timeToIdleSeconds="50" timeToLiveSeconds="60" overflowToDisk="true"></defaultCache>
 <!--
  指定缓存的对象。
  下面出现的的属性覆盖上面出现的，没出现的继承上面的。
 -->
 <cache name="com.suxiaolei.hibernate.pojos.Order" maxElementsInMemory="200" eternal="false" 
  timeToIdleSeconds="50" timeToLiveSeconds="60" overflowToDisk="true"></cache>
</ehcache>

<!-- *.hbm.xml -->
<?xml version="1.0" encoding='UTF-8'?>
<!DOCTYPE hibernate-mapping PUBLIC
 "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >
<hibernate-mapping>
 <class>
  <!-- 设置该持久化类的二级缓存并发访问策略 read-only read-write nonstrict-read-write transactional-->
  <cache usage="read-write"/> 
 </class>
</hibernate-mapping>

<hibernate-mapping>
  <class name="com.suxiaolei.hibernate.pojos.Customer" table="customer">
   <!-- 主键设置 -->
   <id name="id" type="string">
    <column name="id"></column>
    <generator class="uuid"></generator>
   </id>
   <!-- 属性设置 -->
   <property name="username" column="username" type="string"></property>
   <property name="balance" column="balance" type="integer"></property>
   <set name="orders" inverse="true" cascade="all" lazy="false" fetch="join">
    <cache usage="read-only"/>
    <key column="customer_id" ></key>
    <one-to-many class="com.suxiaolei.hibernate.pojos.Order"/>
   </set>
  </class>
 </hibernate-mapping>