mybatis:一级缓存And二级缓存

一级缓存，与hibernate一样，都是session缓存，也就是说该缓存的应用范围为同一个SqlSession.(不用废话)
二级缓存，mybatis自身也实现了缓存，但是mybatis的缓存实现并不同于我们常用的，这里介绍EhCache与MyBatis的整合。

默认的Mybatis缓存配置
只需要在mapper.xml中配置标签

<mapper namespace="com.xbz.user.mapper.UserMapper">    <cache type="org.mybatis.caches.ehcache.EhcacheCache"></cache>    <select id="selectById" parameterType="java.lang.Long" resultType="com.xbz.user.entity.User">        SELECT * FROM users WHERE id = #{id}    </select> </mapper>

test类

    @Test    public void testTwoLevelCache(){        User user1 = userMapper.selectById(1L);        System.out.println("--------------------------------------------------------------------");        User user2 = userMapper.selectById(1L);        System.out.println(user1==user2);    }

测试结果
如下日志可看出，myabtis确实使用了二级缓存，没有查询两次数据库，但是两个user却是不相等的。

原因:首先mybatis的缓存的实体类，必须实现Serializable接口，另外估计他会将数据序列化查找二级缓存时再反序列化，反正了解就好，何必纠结

2016-09-26 20:29:39,464 org.mybatis.spring.transaction.SpringManagedTransaction.openConnection JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@31148fb3] will be managed by Spring2016-09-26 20:29:39,474 com.xbz.user.mapper.UserMapper.selectById.debug ==>  Preparing: SELECT * FROM users WHERE id = ? 2016-09-26 20:29:39,511 com.xbz.user.mapper.UserMapper.selectById.debug ==> Parameters: 1(Long)2016-09-26 20:29:39,542 com.xbz.user.mapper.UserMapper.selectById.debug <==      Total: 12016-09-26 20:29:39,548 org.mybatis.spring.SqlSessionUtils.closeSqlSession Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@46603295]2016-09-26 20:29:39,549 org.mybatis.spring.SqlSessionUtils.beforeCompletion Transaction synchronization deregistering SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@46603295]2016-09-26 20:29:39,549 org.mybatis.spring.SqlSessionUtils.beforeCompletion Transaction synchronization closing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@46603295]--------------------------------------------------------------------2016-09-26 20:29:39,557 org.mybatis.spring.SqlSessionUtils.getSqlSession Creating a new SqlSession2016-09-26 20:29:39,557 org.mybatis.spring.SqlSessionUtils.registerSessionHolder Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7d230d96]2016-09-26 20:29:39,577 com.xbz.user.mapper.UserMapper.getObject Cache Hit Ratio [com.xbz.user.mapper.UserMapper]: 0.52016-09-26 20:29:39,578 org.mybatis.spring.SqlSessionUtils.closeSqlSession Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7d230d96]2016-09-26 20:29:39,578 org.mybatis.spring.SqlSessionUtils.beforeCompletion Transaction synchronization deregistering SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7d230d96]2016-09-26 20:29:39,578 org.mybatis.spring.SqlSessionUtils.beforeCompletion Transaction synchronization closing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7d230d96]false

EhCache才是我们的重点

• 导入EhCache.jar
• 导入mybtatis-ehcache.jar
• org.mybatis.caches.ehcache.EhcacheCache
  maven方式：

<dependency>    <groupId>net.sf.ehcache</groupId>    <artifactId>ehcache</artifactId>    <version>2.10.2</version></dependency><dependency>    <groupId>org.mybatis</groupId>    <artifactId>mybatis-ehcache</artifactId>    <version>1.0.0</version></dependency>

然后再配置之前的Cache标签

<cache type="org.mybatis.caches.ehcache.EhcacheCache"></cache>

再来测试下结果（依旧是上面的测试方法）
哈哈哈，这次满意了吧！！！确实得到是同一个对象

2016-09-26 20:40:13,334 org.mybatis.spring.transaction.SpringManagedTransaction.openConnection JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@20962ef0] will be managed by Spring2016-09-26 20:40:13,344 com.xbz.user.mapper.UserMapper.selectById.debug ==>  Preparing: SELECT * FROM users WHERE id = ? 2016-09-26 20:40:13,380 com.xbz.user.mapper.UserMapper.selectById.debug ==> Parameters: 1(Long)2016-09-26 20:40:13,403 com.xbz.user.mapper.UserMapper.selectById.debug <==      Total: 12016-09-26 20:40:13,409 org.mybatis.spring.SqlSessionUtils.closeSqlSession Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@504148d4]2016-09-26 20:40:13,409 org.mybatis.spring.SqlSessionUtils.beforeCompletion Transaction synchronization deregistering SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@504148d4]2016-09-26 20:40:13,410 org.mybatis.spring.SqlSessionUtils.beforeCompletion Transaction synchronization closing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@504148d4]2016-09-26 20:40:13,412 net.sf.ehcache.store.disk.Segment.put put added 0 on heap--------------------------------------------------------------------2016-09-26 20:40:13,415 org.mybatis.spring.SqlSessionUtils.getSqlSession Creating a new SqlSession2016-09-26 20:40:13,415 org.mybatis.spring.SqlSessionUtils.registerSessionHolder Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@249c5c4d]2016-09-26 20:40:13,415 com.xbz.user.mapper.UserMapper.getObject Cache Hit Ratio [com.xbz.user.mapper.UserMapper]: 0.52016-09-26 20:40:13,415 org.mybatis.spring.SqlSessionUtils.closeSqlSession Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@249c5c4d]2016-09-26 20:40:13,416 org.mybatis.spring.SqlSessionUtils.beforeCompletion Transaction synchronization deregistering SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@249c5c4d]2016-09-26 20:40:13,416 org.mybatis.spring.SqlSessionUtils.beforeCompletion Transaction synchronization closing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@249c5c4d]true