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MyBatis(七):MyBatis緩存詳解(一級緩存/二級緩存)

本文轉載自   查看原文   2021-03-11 16:05   565    MyBatis

  1. 一級緩存

    ​ MyBatis一級緩存上SqlSession緩存，即在統一SqlSession中，在不執行增刪改操作提交事務的前提下，對同一條數據進行多次查詢時，第一次查詢從數據庫中查詢，完成后會存入緩存，其余從緩存中直接讀取。MyBatis一級緩存默認開啟。

  1. 二級緩存

    ​ MyBatis二級緩存是命名空間NameSpace緩存，也可理解為二級緩存被多個SqlSession共享，是一個全局變量。

    ​ 二級緩存默認是開啟的，使用時需要手動配置條件。配置二級緩存后，數據查詢流程為：二級緩存->一級緩存->數據庫。

    • 二級緩存使用配置

      1. 實體類需要實現Serializable接口

      2. 核心配置文件增加標簽

      <settings> 
  <setting name="cacheEnabled" value="true"/> 
</settings>
      1. 在需要開啟的mapper中添加標簽
      <!--使用二級緩存--> 
<cache/>

      • eviction屬性可以設置緩存回收策略，默認LRU策略

        • LRU - 最近最少回收，移除最長時間不被使用的對象
        • FIFO - 先進先出，按照緩存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對象
        • WEAK - 弱引用，更積極的移除基於垃圾收集器和弱引用規則的對象

      • flushinterval 緩存刷新間隔，緩存多長時間刷新一次，默認不清空，設置一個毫秒值

      • readOnly: 是否只讀；

        false讀寫(默認)：MyBatis 覺得數據可能會被修改

        true 只讀，MyBatis 認為所有從緩存中獲取數據的操作都是只讀操作，不會修改數據。MyBatis 為了加快獲取數據，直接就會將數據在緩存中的引用交給用戶。不安全，速度快。

      • size : 緩存存放多少個元素

      • type: 指定自定義緩存的全類名(實現Cache 接口即可)

      • blocking： 若緩存中找不到對應的key，是否會一直blocking，直到有對應的數據進入緩存。

        例如：

        <cache eviction="LRU" flushInterval="1000*60*60*24*7" readOnly="true" blocking="false" type="MyCache" size="1000"/>

    • 二級緩存測試，多個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();

      控制台查看輸出日志，發現緩存已命中—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'}

    • 二級緩存失效條件

      • 首次SqlSession未提交事務時，二級緩存無法命中

        SqlSession未提交時，執行結果未放入二級緩存中，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執行之后，就不會命中緩存

        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();

        查看控制台輸出，發現兩次緩存命中都是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.

      • 更新提交事務，與一級緩存一樣，增刪改操作提交事務會清空緩存

        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查詢結果："+user1);
sqlSession1.close();

SqlSession sqlSession2 = sqlSessionFactory.openSession();
IUserDao userDao2 = sqlSession2.getMapper(IUserDao.class);
param.setName("游騎兵");
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);

        查看控制台輸出，經過sqlSession2提交事務后，sqlSession3的查詢緩存命中率為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: 游騎兵(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='游騎兵', address='西安'}

    • 多表操作時，對MyBatis二級緩存的影響

      1. 問題：未同一namespace下進行增刪改提交操作時，其他namespace的緩存是無法感知的

      例如：兩個Mapper文件，AMapper.xml和BMapper.xml，B修改了user表中的內容，A是感知不到的，那么再從A里查詢如果用到了緩存，就是舊的數據。

      ​ 新增UserMapper1.xml ,修改命名空間為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查詢結果："+user1);
sqlSession1.close();

SqlSession sqlSession2 = sqlSessionFactory.openSession();
param.setName("游騎兵");
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);

      ​ 查看控制台輸出，發現sqlSession2成功更新用戶信息后，sqlSession3進行查詢時二級緩存命中，依舊是sqlSession1中的結果，查詢結果錯誤

      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: 游騎兵(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'}

      1. 解決

        在查詢的Mapper文件中引入cache-ref標簽，指向執行增刪改操作表的命名空間，例如在UserMapper.xml中增加一下標簽

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

        在此執行，查看控制台輸出，發現第三次查詢已直接從數據庫中進行查詢，結果正確

        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: 游騎兵(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='游騎兵', address='西安'}

    • 是否應該使用二級緩存

      二級緩存的注意事項：

      1. 緩存是以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下的全部緩存。

      3. 通常使用MyBatis Generator生成的代碼中，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緩存，因為多表操作進行更新操作，一定會產生臟數據。

        如果你遵守二級緩存的注意事項，那么你就可以使用二級緩存。

        如果不能使用多表操作，二級緩存不就可以用一級緩存來替換掉嗎？而且二級緩存是表級緩存，開銷大，沒有一級緩存直接使用 HashMap 來存儲的效率更高，所以二級緩存並不推薦使用。

    引用內容參考：https://www.cnblogs.com/cxuanBlog/p/11333021.html


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