mybatis的二级缓存是mapper级别的，也就是同一个mapper下的查询，可以使用缓存里面的值
下面就写个demo记录下
没有使用缓存之前
service
@Override
public Device getUserById(Long id) {
Device byId = deviceMapper.getDeviceById(id);
log.info("======================");
Device one = deviceMapper.getDeviceById(id);
return one;
}
xml
<select id="getDeviceById" resultType="org.ongoal.tun.demos.entity.Device">
select * from device where device_id = #{i}
</select>
控制台
Creating a new SqlSession SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@568fa466] was not registered for synchronization because synchronization is not active JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@3c59cb10] will not be managed by Spring ==> Preparing: select * from device where device_id = ? ==> Parameters: 1(Long) <== Columns: device_id, device_name, source_id, sort_num, del_flag, create_by, update_by, create_time, update_time <== Row: 1, 开发数据1, 1750083747497119745, 0, 0, null, null, null, null <== Total: 1 Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@568fa466] 2024-03-14 16:45:05.679 INFO 19328 --- [nio-8080-exec-1] o.o.t.d.service.Impl.DeviceServiceImpl : ====================== Creating a new SqlSession SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@46143ad3] was not registered for synchronization because synchronization is not active JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@3c59cb10] will not be managed by Spring ==> Preparing: select * from device where device_id = ? ==> Parameters: 1(Long) <== Columns: device_id, device_name, source_id, sort_num, del_flag, create_by, update_by, create_time, update_time <== Row: 1, 开发数据1, 1750083747497119745, 0, 0, null, null, null, null <== Total: 1 Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@46143ad3]
从控制台可以注意到，执行了两次SQL查询。
使用缓存
<cache/> <!--添加此标签-->
<select id="getDeviceById" resultType="org.ongoal.tun.demos.entity.Device" useCache="true"> <!--加上useCache="true"属性 -->
select * from device where device_id = #{i} </select>
Creating a new SqlSession SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@25ff2dda] was not registered for synchronization because synchronization is not active Cache Hit Ratio [org.ongoal.tun.demos.mapper.DeviceMapper]: 0.0 JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@74786914] will not be managed by Spring ==> Preparing: select * from device where device_id = ? ==> Parameters: 1(Long) <== Columns: device_id, device_name, source_id, sort_num, del_flag, create_by, update_by, create_time, update_time <== Row: 1, 开发数据1, 1750083747497119745, 0, 0, null, null, null, null <== Total: 1 Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@25ff2dda] 2024-03-14 16:49:47.078 INFO 27412 --- [nio-8080-exec-1] o.o.t.d.service.Impl.DeviceServiceImpl : ====================== Creating a new SqlSession SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@558bce46] was not registered for synchronization because synchronization is not active Cache Hit Ratio [org.ongoal.tun.demos.mapper.DeviceMapper]: 0.5 Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@558bce46]
可以看到第一次是执行了SQL，第二次是使用了缓存里面的值返回。
实际开发中，一般用的也不多，因为它的颗粒度比较高，是mapper级别的，并发情况下，也可能导致数据不一致情况。只有特定场合，觉得有必要，那才会使用此二级缓存
标签：开启,device,二级缓存,synchronization,SqlSession,mybatis,where,id,select From： https://www.cnblogs.com/qwg-/p/18073272