mybatis缓存机制（九）

一、一级缓存和二级缓存

二、一级缓存

    2.1、判断两次及以上的查询相同，必须满足以上条件：

       1）传入的statementId相同

       2）查询时要求的结果集中的结果范围相同

      3）要执行的sql语句字符串内容相同

      4）传递给java.sql.Statement要设置的参数值相同

   2.2、验证一级缓存的存在（以User实例为例）

      1、测试方法

[code]User selectOneUser(int id);

       2、对应的mapper.xml

[code] <select id="selectOneUser" parameterType="int" resultMap="user">
select * from user where id = #{id}
</select>

      3、执行selectOneUser方法两次

[code]  User user = userMapper.selectOneUser(3);
System.out.println(user);
User user1 = userMapper.selectOneUser(3);
System.out.println(user1);

     4、debug日志

[code]17:22:21,720 DEBUG - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.
17:22:21,746 DEBUG - PooledDataSource forcefully closed/removed all connections.
17:22:21,746 DEBUG - PooledDataSource forcefully closed/removed all connections.
17:22:21,746 DEBUG - PooledDataSource forcefully closed/removed all connections.
17:22:21,746 DEBUG - PooledDataSource forcefully closed/removed all connections.
17:22:21,955 DEBUG - Opening JDBC Connection
17:22:22,153 DEBUG - Created connection 1861781750.
17:22:22,155 DEBUG - ==>  Preparing: select * from user where id = ?
17:22:22,192 DEBUG - ==> Parameters: 3(Integer)
17:22:22,223 DEBUG - <==      Total: 1
User{id=3, username='阿帕奇', iMaxEmail='3214215@160.com', telephone='45436547', sex='男', age=34}
User{id=3, username='阿帕奇', iMaxEmail='3214215@160.com', telephone='45436547', sex='男', age=34}

Process finished with exit code 0

    注意：我们可以看到sql语句只执行了一次

  2.3、增删改对以及缓存的影响

[code] User user = userMapper.selectOneUser(3);
System.out.println(user);

User user2 = new User();
user2.setiMaxEmail("3214215@qq.com");
user2.setSex("女");
user2.setAge(34);
user2.setTelephone("542353");
user2.setUsername("张达");
userMapper.insertUser(user2);

User user1 = userMapper.selectOneUser(3);
System.out.println(user1);

   debug日志

[code]17:24:55,559 DEBUG - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.
17:24:55,582 DEBUG - PooledDataSource forcefully closed/removed all connections.
17:24:55,583 DEBUG - PooledDataSource forcefully closed/removed all connections.
17:24:55,583 DEBUG - PooledDataSource forcefully closed/removed all connections.
17:24:55,583 DEBUG - PooledDataSource forcefully closed/removed all connections.
17:24:55,735 DEBUG - Opening JDBC Connection
17:24:55,956 DEBUG - Created connection 1861781750.
17:24:55,957 DEBUG - ==>  Preparing: select * from user where id = ?
17:24:55,993 DEBUG - ==> Parameters: 3(Integer)
17:24:56,024 DEBUG - <==      Total: 1
User{id=3, username='阿帕奇', iMaxEmail='3214215@160.com', telephone='45436547', sex='男', age=34}
17:24:56,024 DEBUG - ==>  Preparing: insert into user( id, user_name, sex, imax_email, telephone, age )values ( null, ?, ?, ?, ?, ? );
17:24:56,026 DEBUG - ==> Parameters: 张达(String), 女(String), 3214215@qq.com(String), 542353(String), 34(Integer)
17:24:56,033 DEBUG - <==    Updates: 1
17:24:56,036 DEBUG - ==>  Preparing: select * from user where id = ?
17:24:56,036 DEBUG - ==> Parameters: 3(Integer)
17:24:56,041 DEBUG - <==      Total: 1
User{id=3, username='阿帕奇', iMaxEmail='3214215@160.com', telephone='45436547', sex='男', age=34}

我们可以看到该select语句执行了两次，证明缓存已经被清除了

三、二级缓存

    3.1、开启指定mapper的二级缓存

        1）将要开启二级缓存的实体类进行序列化

        2）在要开启二级缓存的mapper.xml中配置cache标签，如下：

[code] <cache eviction="LRU" flushInterval="100000" readOnly="true" size="1024"/>

3.2、开启全局的二级缓存

        在mybatis-config.xml全局配置文件中配置：

[code]<settings>
<!--这个配置使全局的映射器(二级缓存)启用或禁用缓存-->
<setting name="cacheEnabled" value="true" />
.....
</settings>