MyBatis(七):MyBatis缓存详解(一级缓存/二级缓存)

一级缓存

MyBatis一级缓存上SqlSession缓存，即在统一SqlSession中，在不执行增删改操作提交事务的前提下，对同一条数据进行多次查询时，第一次查询从数据库中查询，完成后会存入缓存，其余从缓存中直接读取。MyBatis一级缓存默认开启。

二级缓存

MyBatis二级缓存是命名空间NameSpace缓存，也可理解为二级缓存被多个SqlSession共享，是一个全局变量。

二级缓存默认是关闭的，需要手动配置进行开启。开启二级缓存后，数据查询流程为：二级缓存->一级缓存->数据库。

二级缓存开启
1. 实体类需要实现Serializable接口
2. 核心配置文件增加标签
```
<settings>

  <setting name="cacheEnabled" value="true"/>

</settings>
```
1. 在需要开启的mapper中添加标签
```


<cache/>
```
- eviction属性可以设置缓存回收策略，默认LRU策略
  - LRU - 最近最少回收，移除最长时间不被使用的对象
  - FIFO - 先进先出，按照缓存进入的顺序来移除它们
  - SOFT - 软引用，移除基于垃圾回收器状态和软引用规则的对象
  - WEAK - 弱引用，更积极的移除基于垃圾收集器和弱引用规则的对象
- flushinterval 缓存刷新间隔，缓存多长时间刷新一次，默认不清空，设置一个毫秒值
- readOnly: 是否只读；
  
  false读写(默认)：MyBatis 觉得数据可能会被修改
  
  true 只读，MyBatis 认为所有从缓存中获取数据的操作都是只读操作，不会修改数据。MyBatis 为了加快获取数据，直接就会将数据在缓存中的引用交给用户。不安全，速度快。
- size : 缓存存放多少个元素
- type: 指定自定义缓存的全类名(实现Cache 接口即可)
- blocking：若缓存中找不到对应的key，是否会一直blocking，直到有对应的数据进入缓存。
  
  例如：
```
<cache eviction="LRU" flushInterval="1000*60*60*24*7" readOnly="true" blocking="false" type="MyCache" size="1000"/>
```

二级缓存测试，多个SqlSession

InputStream inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");

SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

User param = new User();

param.setId(1);

SqlSession sqlSession1 = sqlSessionFactory.openSession();

IUserDao userDao1 = sqlSession1.getMapper(IUserDao.class);

User user1 = userDao1.findOne(param);

System.out.println(user1);

sqlSession1.close();

SqlSession sqlSession2 = sqlSessionFactory.openSession();

IUserDao userDao2 = sqlSession2.getMapper(IUserDao.class);

User user2 = userDao2.findOne(param);

System.out.println(user2);

sqlSession2.close();

控制台查看输出日志，发现缓存已命中—Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.5

14:26:48,608 DEBUG IUserDao:62 - Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.0

14:26:48,611 DEBUG JdbcTransaction:137 - Opening JDBC Connection

14:26:48,952 DEBUG PooledDataSource:406 - Created connection 543846639.

14:26:48,952 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

14:26:48,957 DEBUG findOne:159 - ==>  Preparing: select * from user where id=?

14:26:48,980 DEBUG findOne:159 - ==> Parameters: 1(Integer)

14:26:49,004 DEBUG findOne:159 - <==      Total: 1

com.rangers.entity.User{id=1, name='rangers', address='杭州'}

14:26:49,007 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

14:26:49,010 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

14:26:49,011 DEBUG PooledDataSource:363 - Returned connection 543846639 to pool.

14:26:49,014 DEBUG IUserDao:62 - Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.5

com.rangers.entity.User{id=1, name='rangers', address='杭州'}

二级缓存失效条件

首次SqlSession未提交事务时，二级缓存无法命中

SqlSession未提交时，执行结果未放入二级缓存中，这时候第二个SqlSession在查询时候是无法命中的

例如：调整sqlSession1.close();在sqlSession2执行之后，就不会命中缓存

InputStream inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");

SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

User param = new User();

param.setId(1);

SqlSession sqlSession1 = sqlSessionFactory.openSession();

IUserDao userDao1 = sqlSession1.getMapper(IUserDao.class);

User user1 = userDao1.findOne(param);

System.out.println(user1);

SqlSession sqlSession2 = sqlSessionFactory.openSession();

IUserDao userDao2 = sqlSession2.getMapper(IUserDao.class);

User user2 = userDao2.findOne(param);

System.out.println(user2);

sqlSession1.close();

sqlSession2.close();

查看控制台输出，发现两次缓存命中都是0.0，Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.0

14:49:39,073 DEBUG IUserDao:62 - Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.0

14:49:39,075 DEBUG JdbcTransaction:137 - Opening JDBC Connection

14:49:39,385 DEBUG PooledDataSource:406 - Created connection 543846639.

14:49:39,385 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

14:49:39,389 DEBUG findOne:159 - ==>  Preparing: select * from user where id=?

14:49:39,408 DEBUG findOne:159 - ==> Parameters: 1(Integer)

14:49:39,433 DEBUG findOne:159 - <==      Total: 1

com.rangers.entity.User{id=1, name='rangers', address='杭州'}

14:49:39,434 DEBUG IUserDao:62 - Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.0

14:49:39,434 DEBUG JdbcTransaction:137 - Opening JDBC Connection

14:49:39,489 DEBUG PooledDataSource:406 - Created connection 2079179914.

14:49:39,489 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@7bedc48a]

14:49:39,491 DEBUG findOne:159 - ==>  Preparing: select * from user where id=?

14:49:39,492 DEBUG findOne:159 - ==> Parameters: 1(Integer)

14:49:39,495 DEBUG findOne:159 - <==      Total: 1

com.rangers.entity.User{id=1, name='rangers', address='杭州'}

14:49:39,497 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

14:49:39,499 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

14:49:39,500 DEBUG PooledDataSource:363 - Returned connection 543846639 to pool.

14:49:39,500 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@7bedc48a]

14:49:39,502 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@7bedc48a]

14:49:39,502 DEBUG PooledDataSource:363 - Returned connection 2079179914 to pool.

更新提交事务，与一级缓存一样，增删改操作提交事务会清空缓存

InputStream inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");

SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

User param = new User();

param.setId(1);

SqlSession sqlSession1 = sqlSessionFactory.openSession();

IUserDao userDao1 = sqlSession1.getMapper(IUserDao.class);

User user1 = userDao1.findOne(param);

System.out.println("sqlSession1查询结果："+user1);

sqlSession1.close();

SqlSession sqlSession2 = sqlSessionFactory.openSession();

IUserDao userDao2 = sqlSession2.getMapper(IUserDao.class);

param.setName("游骑兵");

param.setAddress("西安");

Boolean flag = userDao2.updateUser(param) > 0;

System.out.println("执行更新操作结果："+flag);

sqlSession2.commit();

sqlSession2.close();

SqlSession sqlSession3 = sqlSessionFactory.openSession();

IUserDao userDao3 = sqlSession3.getMapper(IUserDao.class);

User user3 = userDao3.findOne(param);

sqlSession3.close();

System.out.println("sqlSession3查询结果："+user3);

查看控制台输出，经过sqlSession2提交事务后，sqlSession3的查询缓存命中率为0.0

15:00:33,901 DEBUG IUserDao:62 - Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.0

15:00:33,904 DEBUG JdbcTransaction:137 - Opening JDBC Connection

15:00:34,263 DEBUG PooledDataSource:406 - Created connection 543846639.

15:00:34,263 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,267 DEBUG findOne:159 - ==>  Preparing: select * from user where id=?

15:00:34,290 DEBUG findOne:159 - ==> Parameters: 1(Integer)

15:00:34,314 DEBUG findOne:159 - <==      Total: 1

sqlSession1查询结果：com.rangers.entity.User{id=1, name='rangers', address='杭州'}

15:00:34,317 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,321 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,321 DEBUG PooledDataSource:363 - Returned connection 543846639 to pool.

15:00:34,321 DEBUG JdbcTransaction:137 - Opening JDBC Connection

15:00:34,322 DEBUG PooledDataSource:398 - Checked out connection 543846639 from pool.

15:00:34,322 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,325 DEBUG updateUser:159 - ==>  Preparing: update user set name=?,address=? where id=?

15:00:34,325 DEBUG updateUser:159 - ==> Parameters: 游骑兵(String), 西安(String), 1(Integer)

15:00:34,334 DEBUG updateUser:159 - <==    Updates: 1

执行更新操作结果：true

15:00:34,335 DEBUG JdbcTransaction:70 - Committing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,445 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,448 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,450 DEBUG PooledDataSource:363 - Returned connection 543846639 to pool.

15:00:34,450 DEBUG IUserDao:62 - Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.0

15:00:34,450 DEBUG JdbcTransaction:137 - Opening JDBC Connection

15:00:34,451 DEBUG PooledDataSource:398 - Checked out connection 543846639 from pool.

15:00:34,451 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,453 DEBUG findOne:159 - ==>  Preparing: select * from user where id=?

15:00:34,454 DEBUG findOne:159 - ==> Parameters: 1(Integer)

15:00:34,458 DEBUG findOne:159 - <==      Total: 1

15:00:34,458 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,461 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@206a70ef]

15:00:34,462 DEBUG PooledDataSource:363 - Returned connection 543846639 to pool.

sqlSession3查询结果：com.rangers.entity.User{id=1, name='游骑兵', address='西安'}

多表操作时，对MyBatis二级缓存的影响

问题：未同一namespace下进行增删改提交操作时，其他namespace的缓存是无法感知的

例如：两个Mapper文件，AMapper.xml和BMapper.xml，B修改了user表中的内容，A是感知不到的，那么再从A里查询如果用到了缓存，就是旧的数据。

新增UserMapper1.xml ,修改命名空间为com.rangers.dao.IUserDao1

InputStream inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");

SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

User param = new User();

param.setId(1);

SqlSession sqlSession1 = sqlSessionFactory.openSession();

IUserDao userDao1 = sqlSession1.getMapper(IUserDao.class);

User user1 = userDao1.findOne(param);

System.out.println("sqlSession1查询结果："+user1);

sqlSession1.close();

SqlSession sqlSession2 = sqlSessionFactory.openSession();

param.setName("游骑兵");

param.setAddress("西安");

Boolean flag = sqlSession2.update("com.rangers.dao.IUserDao1.updateUser",param) > 0;

System.out.println("执行更新操作结果："+flag);

sqlSession2.commit();

sqlSession2.close();

SqlSession sqlSession3 = sqlSessionFactory.openSession();

IUserDao userDao3 = sqlSession3.getMapper(IUserDao.class);

User user3 = userDao3.findOne(param);

sqlSession3.close();

System.out.println("sqlSession3查询结果："+user3);

查看控制台输出，发现sqlSession2成功更新用户信息后，sqlSession3进行查询时二级缓存命中，依旧是sqlSession1中的结果，查询结果错误

15:33:54,792 DEBUG JdbcTransaction:137 - Opening JDBC Connection

15:33:55,164 DEBUG PooledDataSource:406 - Created connection 1165303897.

15:33:55,165 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:33:55,168 DEBUG findOne:159 - ==>  Preparing: select * from user where id=?

15:33:55,186 DEBUG findOne:159 - ==> Parameters: 1(Integer)

15:33:55,212 DEBUG findOne:159 - <==      Total: 1

sqlSession1查询结果：com.rangers.entity.User{id=1, name='Rangers', address='杭州'}

15:33:55,215 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:33:55,218 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:33:55,218 DEBUG PooledDataSource:363 - Returned connection 1165303897 to pool.

15:33:55,219 DEBUG JdbcTransaction:137 - Opening JDBC Connection

15:33:55,220 DEBUG PooledDataSource:398 - Checked out connection 1165303897 from pool.

15:33:55,220 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:33:55,224 DEBUG updateUser:159 - ==>  Preparing: update user set name=?,address=? where id=?

15:33:55,225 DEBUG updateUser:159 - ==> Parameters: 游骑兵(String), 西安(String), 1(Integer)

15:33:55,233 DEBUG updateUser:159 - <==    Updates: 1

执行更新操作结果：true

15:33:55,233 DEBUG JdbcTransaction:70 - Committing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:33:55,351 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:33:55,354 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:33:55,355 DEBUG PooledDataSource:363 - Returned connection 1165303897 to pool.

15:33:55,357 DEBUG IUserDao:62 - Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.5

sqlSession3查询结果：com.rangers.entity.User{id=1, name='Rangers', address='杭州'}

解决

在查询的Mapper文件中引入cache-ref标签，指向执行增删改操作表的命名空间，例如在UserMapper.xml中增加一下标签

<cache-ref namespace="com.rangers.dao.IUserDao1"/>

在此执行，查看控制台输出，发现第三次查询已直接从数据库中进行查询，结果正确

15:46:05,486 DEBUG IUserDao1:62 - Cache Hit Ratio [com.rangers.dao.IUserDao1]: 0.0

15:46:05,489 DEBUG JdbcTransaction:137 - Opening JDBC Connection

15:46:05,788 DEBUG PooledDataSource:406 - Created connection 1165303897.

15:46:05,789 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,793 DEBUG findOne:159 - ==>  Preparing: select * from user where id=?

15:46:05,822 DEBUG findOne:159 - ==> Parameters: 1(Integer)

15:46:05,842 DEBUG findOne:159 - <==      Total: 1

sqlSession1查询结果：com.rangers.entity.User{id=1, name='rangers', address='杭州'}

15:46:05,845 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,848 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,848 DEBUG PooledDataSource:363 - Returned connection 1165303897 to pool.

15:46:05,849 DEBUG JdbcTransaction:137 - Opening JDBC Connection

15:46:05,849 DEBUG PooledDataSource:398 - Checked out connection 1165303897 from pool.

15:46:05,849 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,852 DEBUG updateUser:159 - ==>  Preparing: update user set name=?,address=? where id=?

15:46:05,852 DEBUG updateUser:159 - ==> Parameters: 游骑兵(String), 西安(String), 1(Integer)

15:46:05,865 DEBUG updateUser:159 - <==    Updates: 1

执行更新操作结果：true

15:46:05,865 DEBUG JdbcTransaction:70 - Committing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,970 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,974 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,976 DEBUG PooledDataSource:363 - Returned connection 1165303897 to pool.

15:46:05,976 DEBUG IUserDao1:62 - Cache Hit Ratio [com.rangers.dao.IUserDao1]: 0.0

15:46:05,976 DEBUG JdbcTransaction:137 - Opening JDBC Connection

15:46:05,976 DEBUG PooledDataSource:398 - Checked out connection 1165303897 from pool.

15:46:05,976 DEBUG JdbcTransaction:101 - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,979 DEBUG findOne:159 - ==>  Preparing: select * from user where id=?

15:46:05,979 DEBUG findOne:159 - ==> Parameters: 1(Integer)

15:46:05,985 DEBUG findOne:159 - <==      Total: 1

15:46:05,985 DEBUG JdbcTransaction:123 - Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,989 DEBUG JdbcTransaction:91 - Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@45752059]

15:46:05,990 DEBUG PooledDataSource:363 - Returned connection 1165303897 to pool.

sqlSession3查询结果：com.rangers.entity.User{id=1, name='游骑兵', address='西安'}

是否应该使用二级缓存
二级缓存的注意事项：
1. 缓存是以namespace为单位的，不同namespace下的操作互不影响。
2. insert,update,delete操作会清空所在namespace下的全部缓存。
3. 通常使用MyBatis Generator生成的代码中，都是各个表独立的，每个表都有自己的namespace。
4. 多表操作一定不要使用二级缓存，因为多表操作进行更新操作，一定会产生脏数据。
  
  如果你遵守二级缓存的注意事项，那么你就可以使用二级缓存。
  
  如果不能使用多表操作，二级缓存不就可以用一级缓存来替换掉吗？而且二级缓存是表级缓存，开销大，没有一级缓存直接使用 HashMap 来存储的效率更高，所以二级缓存并不推荐使用。

引用内容参考：https://www.cnblogs.com/cxuanBlog/p/11333021.html

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