1. 一級快取 ​ MyBatis一級快取上SqlSession快取，即在統一SqlSession中，在不執行增刪改操作提交事務的前提下，對同一條資料進行多次查詢時，第一次查詢從資料庫中查詢，完成後會存入快取，其餘從快取中直接讀取。MyBatis一級快取預設開啟。  2. 二級快取 ​ MyBatis二級快取是名稱空間NameSpace快取，也可理解為二級快取被多個SqlSession共享，是一個全域性變數。 ​ 二級快取預設是關閉的，需要手動配置進行開啟。開啟二級快取後，資料查詢流程為：二級快取->一級快取->資料庫。 - 二級快取開啟 1. 實體類需要實現Serializable介面 2. 核心配置檔案增加標籤 ```xml ``` 3. 在需要開啟的mapper中新增標籤 ```xml ``` > - `eviction`屬性可以設定快取回收策略，預設LRU策略 > > - `LRU` - 最近最少回收，移除最長時間不被使用的物件 > - `FIFO` - 先進先出，按照快取進入的順序來移除它們 >

- `SOFT` - 軟引用，移除基於垃圾回收器狀態和軟引用規則的物件 > - `WEAK` - 弱引用，更積極的移除基於垃圾收集器和弱引用規則的物件 > > - `flushinterval` 快取重新整理間隔，快取多長時間重新整理一次，預設不清空，設定一個毫秒值 > > - `readOnly`: 是否只讀； > > false讀寫(預設)：MyBatis 覺得資料可能會被修改 > > true 只讀，MyBatis 認為所有從快取中獲取資料的操作都是隻讀操作，不會修改資料。MyBatis 為了加快獲取資料，直接就會將資料在快取中的引用交給使用者。不安全，速度快。 >

> - `size` : 快取存放多少個元素 > > - `type`: 指定自定義快取的全類名(實現Cache 介面即可) > > - `blocking`： 若快取中找不到對應的key，是否會一直blocking，直到有對應的資料進入快取。 > > 例如： > > ```xml > > ``` - 二級快取測試，多個SqlSession ```java 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(); ``` 控制檯檢視輸出日誌，發現快取已命中—Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.5 ```xml 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'} ``` - 二級快取失效條件 - 首次SqlSession未提交事務時，二級快取無法命中 SqlSession未提交時，執行結果未放入二級快取中，這時候第二個SqlSession在查詢時候是無法命中的 例如：調整sqlSession1.close();在sqlSession2執行之後，就不會命中快取 ```java 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(); ``` 檢視控制檯輸出，發現兩次快取命中都是0.0，Cache Hit Ratio [com.rangers.dao.IUserDao]: 0.0 ```xml 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. ``` - 更新提交事務，與一級快取一樣，增刪改操作提交事務會清空快取 ```java 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查詢結果："+user1); sqlSession1.close(); SqlSession sqlSession2 = sqlSessionFactory.openSession(); IUserDao userDao2 = sqlSession2.getMapper(IUserDao.class); param.setName("遊騎兵"); param.setAddress("西安"); Boolean flag = userDao2.updateUser(param) > 0; System.out.println("執行更新操作結果："+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查詢結果："+user3); ``` 檢視控制檯輸出，經過sqlSession2提交事務後，sqlSession3的查詢快取命中率為0.0 ```xml 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查詢結果：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: 遊騎兵(String), 西安(String), 1(Integer) 15:00:34,334 DEBUG updateUser:159 - <== Updates: 1 執行更新操作結果：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查詢結果：com.rangers.entity.User{id=1, name='遊騎兵', address='西安'} ``` - 多表操作時，對MyBatis二級快取的影響 1. 問題：未同一namespace下進行增刪改提交操作時，其他namespace的快取是無法感知的 例如：兩個Mapper檔案，AMapper.xml和BMapper.xml，B修改了user表中的內容，A是感知不到的，那麼再從A裡查詢如果用到了快取，就是舊的資料。 ​ 新增UserMapper1.xml ,修改名稱空間為com.rangers.dao.IUserDao1 ```java 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查詢結果："+user1); sqlSession1.close(); SqlSession sqlSession2 = sqlSessionFactory.openSession(); param.setName("遊騎兵"); param.setAddress("西安"); Boolean flag = sqlSession2.update("com.rangers.dao.IUserDao1.updateUser",param) > 0; System.out.println("執行更新操作結果："+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查詢結果："+user3); ``` ​ 檢視控制檯輸出，發現sqlSession2成功更新使用者資訊後，sqlSession3進行查詢時二級快取命中，依舊是sqlSession1中的結果，查詢結果錯誤 ```xml 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查詢結果：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: 遊騎兵(String), 西安(String), 1(Integer) 15:33:55,233 DEBUG updateUser:159 - <== Updates: 1 執行更新操作結果：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查詢結果：com.rangers.entity.User{id=1, name='Rangers', address='杭州'} ``` 2. 解決 在查詢的Mapper檔案中引入cache-ref標籤，指向執行增刪改操作表的名稱空間，例如在UserMapper.xml中增加一下標籤 ```xml ``` 在此執行，檢視控制檯輸出，發現第三次查詢已直接從資料庫中進行查詢，結果正確 ```xml 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查詢結果：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: 遊騎兵(String), 西安(String), 1(Integer) 15:46:05,865 DEBUG updateUser:159 - <== Updates: 1 執行更新操作結果：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查詢結果：com.rangers.entity.User{id=1, name='遊騎兵', address='西安'} ``` - 是否應該使用二級快取 > 二級快取的注意事項： > > 1. 快取是以`namespace`為單位的，不同`namespace`下的操作互不影響。 > > 2. insert,update,delete操作會清空所在`namespace`下的全部快取。 > > 3. 通常使用MyBatis Generator生成的程式碼中，都是各個表獨立的，每個表都有自己的`namespace`。 > > 4. 多表操作一定不要使用二級快取，因為多表操作進行更新操作，一定會產生髒資料。 > > 如果你遵守二級快取的注意事項，那麼你就可以使用二級快取。 > > 如果不能使用多表操作，二級快取不就可以用一級快取來替換掉嗎？而且二級快取是表級快取，開銷大，沒有一級快取直接使用 HashMap 來儲存的效率更高，所以二級快取並不推薦使用。 引用內容參考：https://www.cnblogs.com/cxuanBlog/p/11333021.html