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MyBatis学习——缓存机制_sqlsession.getmapper有缓存作用吗

作者：不正经 | 2024-02-28 15:33:27

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sqlsession.getmapper有缓存作用吗

mybatis提供查询缓存，用于减轻数据库的压力，提高数据库的性能

一级缓存

Mybatis的一级缓存是SQLSession级别的缓存、mybatis默认是开启一级缓存的。在SQLSession实例对象下存在一个数据结构（HashMap），用户进行存储缓存数据，不同的SQLSession有各自的缓存数据区域，它们之间互不影响。

用法：

  1.     在同一个sqlSession中两次执行相同的sql语句，第一次执行完毕会将数据库中查询的数据写到缓存（内
  2.  
  3. 存），第二次会从缓存中获取数据将不再从数据库查询，从而提高查询效率。当一个sqlSession结束后该
  4.  
  5. sqlSession中的一级缓存也就不存在了。当数据库发生变更操作（insert、update、delete）操作时，会将缓
  6.  
  7. 存数据删除。这样做的目标是避免缓存数据和数据库数据不一致，读取脏数据
  8.

 

实际操作：

测试

  1.              SqlSession sqlSession = sqlSessionFactory.openSession();
  2.             UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
  3.  
  4.             System.out.println("第一次开始");
  5.             User user = userMapper.getUserByID(1);
  6.             System.out.println(user.toString());
  7.             System.out.println("第一次结束");
  8.  
  9.             System.out.println("第二次开始");
  10.             User user1 = userMapper.getUserByID(1);
  11.             System.out.println(user.toString());
  12.             System.out.println("第二次结束");

结果：

  1. 第一次开始
  2. 2019-06-16-17:28:54,299 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Opening JDBC Connection
  3. 2019-06-16-17:28:54,828 [main]  [org.apache.ibatis.datasource.pooled.PooledDataSource] [DEBUG] - Created connection 182738614.
  4. 2019-06-16-17:28:54,829 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@ae45eb6]
  5. 2019-06-16-17:28:54,833 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==>  Preparing: select * from user where id = ? 
  6. 2019-06-16-17:28:54,951 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==> Parameters: 1(Integer)
  7. 2019-06-16-17:28:54,986 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - <==      Total: 1
  8. User{id=1, name='wj', sex='19', address='西安'}
  9. 第一次结束
  10. 第二次开始
  11. User{id=1, name='wj', sex='19', address='西安'}
  12. 第二次结束

从结果可以看出，第二次查询的时候并没有去数据库里面进行查询。

测试：进行了添加操作后

  1.              SqlSession sqlSession = sqlSessionFactory.openSession();
  2.             UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
  3.  
  4.             System.out.println("第一次开始");
  5.             User user = userMapper.getUserByID(1);
  6.             System.out.println(user.toString());
  7.             System.out.println("第一次结束");
  8.  
  9.             User user2 = new User(6,"w","男","陕西");
  10.             userMapper.InsertUser(user2);
  11.  
  12.             System.out.println("第二次开始");
  13.             User user1 = userMapper.getUserByID(1);
  14.             System.out.println(user1.toString());
  15.             System.out.println("第二次结束");

结果：

  1. 第一次开始
  2. 2019-06-16-17:32:12,109 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Opening JDBC Connection
  3. 2019-06-16-17:32:12,629 [main]  [org.apache.ibatis.datasource.pooled.PooledDataSource] [DEBUG] - Created connection 182738614.
  4. 2019-06-16-17:32:12,630 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@ae45eb6]
  5. 2019-06-16-17:32:12,633 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==>  Preparing: select * from user where id = ? 
  6. 2019-06-16-17:32:12,748 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==> Parameters: 1(Integer)
  7. 2019-06-16-17:32:12,787 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - <==      Total: 1
  8. User{id=1, name='wj', sex='19', address='西安'}
  9. 第一次结束
  10. 2019-06-16-17:32:12,789 [main]  [com.wj.dao.UserMapper.InsertUser] [DEBUG] - ==>  Preparing: insert into user (id, name, sex, address ) values ( ?,?, ?, ? ) 
  11. 2019-06-16-17:32:12,791 [main]  [com.wj.dao.UserMapper.InsertUser] [DEBUG] - ==> Parameters: 6(Integer), w(String), 男(String), 陕西(String)
  12. 2019-06-16-17:32:12,792 [main]  [com.wj.dao.UserMapper.InsertUser] [DEBUG] - <==    Updates: 1
  13. 第二次开始
  14. 2019-06-16-17:32:12,793 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==>  Preparing: select * from user where id = ? 
  15. 2019-06-16-17:32:12,793 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==> Parameters: 1(Integer)
  16. 2019-06-16-17:32:12,795 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - <==      Total: 1
  17. User{id=1, name='wj', sex='19', address='西安'}
  18. 第二次结束

我们看到了进行插入操作后，第二次查询只能去数据库进行查询了。

二级缓存

二级缓存是mapper级别的缓存，多个SqlSession去操作同一个Mapper的sql语句，多个SqlSession去操作数据库得到数据会存在二级缓存区域，多个SqlSession可以共用二级缓存，二级缓存是跨SqlSession的。

当开一个会话时，一个SqlSession对象会使用一个Executor对象来完成会话操作，MyBatis的二级缓存机制的关键就是对这个Executor对象做文章 如果用户配置了cacheEnabled=true，那么MyBatis在为SqlSession对象创建Executor对象时，会对Executor对象加上一个装饰者：CachingExecutor，这时SqlSession使用CachingExecutor对象来完成操作请求 CachingExecutor对于查询请求，会先判断该查询请求在Application级别的二级缓存中是否有缓存结果

  • 如果有查询结果，则直接返回缓存结果

  • 如果缓存中没有，再交给真正的Executor对象来完成查询操作，之后CachingExecutor会将真正Executor返回的查询结果放置到缓存中，然后在返回给用户

mybatis二级缓存，必须sqlsession.close 或者commit后，才能把第一次查询的结果提交，才会写入缓存里面

 

在全局配置中配置开启二级缓存

  1.   <settings>
  2.         <!--开启二级缓存-->
  3.     <setting name="cacheEnabled" value="true"/>
  4.     </settings>

在mapper.xml文件中打开缓存

  1. <cache eviction="LRU" flushInterval="1000" size="100"></cache>
  2. eviction表示缓存淘汰策略
  3. LRU：最近最少使用

测试：

  1.              SqlSession sqlSession1 = sqlSessionFactory.openSession();
  2.             SqlSession sqlSession = sqlSessionFactory.openSession();
  3.             SqlSession sqlSession2 = sqlSessionFactory.openSession();
  4.             UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
  5.             UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
  6.             UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
  7.  
  8.             System.out.println("第一次开始");
  9.             User user = userMapper.getUserByID(1);
  10.             System.out.println(user.toString());
  11.             System.out.println("第一次结束");
  12.             sqlSession.close();
  13.  
  14.  
  15.             System.out.println("第二次开始");
  16.             User user1 = userMapper1.getUserByID(1);
  17.             System.out.println(user1.toString());
  18.             System.out.println("第二 次结束");

结果：

  1. 第一次开始
  2. 2019-06-16-17:40:56,362 [main]  [com.wj.dao.UserMapper] [DEBUG] - Cache Hit Ratio [com.wj.dao.UserMapper]: 0.0
  3. 2019-06-16-17:40:56,379 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Opening JDBC Connection
  4. 2019-06-16-17:40:56,978 [main]  [org.apache.ibatis.datasource.pooled.PooledDataSource] [DEBUG] - Created connection 87765719.
  5. 2019-06-16-17:40:56,978 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@53b32d7]
  6. 2019-06-16-17:40:56,983 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==>  Preparing: select * from user where id = ? 
  7. 2019-06-16-17:40:57,038 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==> Parameters: 1(Integer)
  8. 2019-06-16-17:40:57,119 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - <==      Total: 1
  9. User{id=1, name='wj', sex='19', address='西安'}
  10. 第一次结束
  11. 2019-06-16-17:40:57,146 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@53b32d7]
  12. 2019-06-16-17:40:57,146 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@53b32d7]
  13. 2019-06-16-17:40:57,147 [main]  [org.apache.ibatis.datasource.pooled.PooledDataSource] [DEBUG] - Returned connection 87765719 to pool.
  14. 第二次开始
  15. 2019-06-16-17:40:57,266 [main]  [com.wj.dao.UserMapper] [DEBUG] - Cache Hit Ratio [com.wj.dao.UserMapper]: 0.5
  16. User{id=1, name='wj', sex='19', address='西安'}
  17. 第二 次结束

此时已经命中缓存了

测试:进行添加操作

  1.             SqlSession sqlSession1 = sqlSessionFactory.openSession();
  2.             SqlSession sqlSession = sqlSessionFactory.openSession();
  3.             SqlSession sqlSession2 = sqlSessionFactory.openSession();
  4.             UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
  5.             UserMapper userMapper2 = sqlSession2.getMapper(UserMapper.class);
  6.             UserMapper userMapper1 = sqlSession1.getMapper(UserMapper.class);
  7.  
  8.             System.out.println("第一次开始");
  9.             User user = userMapper.getUserByID(1);
  10.             System.out.println(user.toString());
  11.             System.out.println("第一次结束");
  12.             sqlSession.close();
  13.  
  14.  
  15.             System.out.println("第二次开始");
  16.             User user1 = userMapper1.getUserByID(1);
  17.             System.out.println(user1.toString());
  18.             System.out.println("第二 次结束");
  19.  
  20.             User user2 = new User(10,"w","男","陕西");
  21.             userMapper1.InsertUser(user2);
  22.             sqlSession1.commit();
  23.  
  24.  
  25.             System.out.println("第三次开始");
  26.             User user3 = userMapper2.getUserByID(1);
  27.             System.out.println(user3.toString());
  28.             System.out.println("第三 次结束");

结果：

  1. 第一次开始
  2. 2019-06-16-17:46:34,598 [main]  [com.wj.dao.UserMapper] [DEBUG] - Cache Hit Ratio [com.wj.dao.UserMapper]: 0.0
  3. 2019-06-16-17:46:34,611 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Opening JDBC Connection
  4. 2019-06-16-17:46:35,086 [main]  [org.apache.ibatis.datasource.pooled.PooledDataSource] [DEBUG] - Created connection 87765719.
  5. 2019-06-16-17:46:35,087 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@53b32d7]
  6. 2019-06-16-17:46:35,091 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==>  Preparing: select * from user where id = ? 
  7. 2019-06-16-17:46:35,154 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==> Parameters: 1(Integer)
  8. 2019-06-16-17:46:35,211 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - <==      Total: 1
  9. User{id=1, name='wj', sex='19', address='西安'}
  10. 第一次结束
  11. 2019-06-16-17:46:35,235 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@53b32d7]
  12. 2019-06-16-17:46:35,235 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@53b32d7]
  13. 2019-06-16-17:46:35,236 [main]  [org.apache.ibatis.datasource.pooled.PooledDataSource] [DEBUG] - Returned connection 87765719 to pool.
  14. 第二次开始
  15. 2019-06-16-17:46:35,358 [main]  [com.wj.dao.UserMapper] [DEBUG] - Cache Hit Ratio [com.wj.dao.UserMapper]: 0.5
  16. User{id=1, name='wj', sex='19', address='西安'}
  17. 第二 次结束
  18. 2019-06-16-17:46:35,359 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Opening JDBC Connection
  19. 2019-06-16-17:46:35,359 [main]  [org.apache.ibatis.datasource.pooled.PooledDataSource] [DEBUG] - Checked out connection 87765719 from pool.
  20. 2019-06-16-17:46:35,359 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@53b32d7]
  21. 2019-06-16-17:46:35,360 [main]  [com.wj.dao.UserMapper.InsertUser] [DEBUG] - ==>  Preparing: insert into user (id, name, sex, address ) values ( ?,?, ?, ? ) 
  22. 2019-06-16-17:46:35,361 [main]  [com.wj.dao.UserMapper.InsertUser] [DEBUG] - ==> Parameters: 10(Integer), w(String), 男(String), 陕西(String)
  23. 2019-06-16-17:46:35,362 [main]  [com.wj.dao.UserMapper.InsertUser] [DEBUG] - <==    Updates: 1
  24. 2019-06-16-17:46:35,362 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@53b32d7]
  25. 第三次开始
  26. 2019-06-16-17:46:35,408 [main]  [com.wj.dao.UserMapper] [DEBUG] - Cache Hit Ratio [com.wj.dao.UserMapper]: 0.3333333333333333
  27. 2019-06-16-17:46:35,408 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Opening JDBC Connection
  28. 2019-06-16-17:46:35,428 [main]  [org.apache.ibatis.datasource.pooled.PooledDataSource] [DEBUG] - Created connection 1033490990.
  29. 2019-06-16-17:46:35,429 [main]  [org.apache.ibatis.transaction.jdbc.JdbcTransaction] [DEBUG] - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d99d22e]
  30. 2019-06-16-17:46:35,430 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==>  Preparing: select * from user where id = ? 
  31. 2019-06-16-17:46:35,430 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - ==> Parameters: 1(Integer)
  32. 2019-06-16-17:46:35,432 [main]  [com.wj.dao.UserMapper.getUserByID] [DEBUG] - <==      Total: 1
  33. User{id=1, name='wj', sex='19', address='西安'}
  34. 第三 次结束

可以看到第三次就需要去数据库中查找了。

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