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MyBatis缓存

2020-09-22 03:01:12  阅读：171  来源： 互联网

标签：缓存 System class println MyBatis id out


一级缓存
  • 一级缓存也叫本地缓存，SqlSession开启到关闭之间缓存查询结果
  • 缓存select结果，指的是查询相同的数据
  • 通过==可以看出，两次查询返回的是同一个对象
  • insert，update，delete会刷新缓存，无论是否更新查询的数据，都会刷新缓存
  • 一级缓存默认开启

连续查询两次相同数据

SqlSession sqlSession = MyBatisUtil.getSqlSession();
EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);

System.out.println("第一次查询");
Employee employee = mapper.selectById(1);
System.out.println(employee);

System.out.println("第二次查询");
Employee employee2 = mapper.selectById(1);
System.out.println(employee2);

sqlSession.close();

运行结果

  • 可以看出第一次查询执行了sql，第二次查询并没有执行sql
  • 这个时候employee和employee2实际上是同一个对象
第一次查询
Opening JDBC Connection
Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
Created connection 436532993.
==>  Preparing: select id, name, age, gender, birthday, phone, dept_id from employee where id = ?
==> Parameters: 1(Integer)
<==    Columns: id, name, age, gender, birthday, phone, dept_id
<==        Row: 1, 张三二, 80, 男, 2020-09-01, 13323332331, 2
<==      Total: 1
Employee{id=1, name='张三二', age=80, gender='男', birthday=Tue Sep 01 00:00:00 CST 2020, phone='13323332331', deptId=2}
第二次查询
Employee{id=1, name='张三二', age=80, gender='男', birthday=Tue Sep 01 00:00:00 CST 2020, phone='13323332331', deptId=2}
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@1a04f701]
Returned connection 436532993 to pool.

两次查询之间执行更新语句

SqlSession sqlSession = MyBatisUtil.getSqlSession();
EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);

System.out.println("第一次查询");
Employee employee = mapper.selectById(1);
System.out.println(employee);

System.out.println("更新数据");
mapper.update(2, "13399887766");

System.out.println("第二次查询");
Employee employee2 = mapper.selectById(1);
System.out.println(employee2);

sqlSession.close();

运行结果

  • 更新的是与查询无关的数据，但是也刷新了缓存
第一次查询
Opening JDBC Connection
Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
Created connection 436532993.
==>  Preparing: select id, name, age, gender, birthday, phone, dept_id from employee where id = ?;
==> Parameters: 1(Integer)
<==    Columns: id, name, age, gender, birthday, phone, dept_id
<==        Row: 1, 张三二, 80, 男, 2020-09-01, 13323332331, 2
<==      Total: 1
Employee{id=1, name='张三二', age=80, gender='男', birthday=Tue Sep 01 00:00:00 CST 2020, phone='13323332331', deptId=2}
更新数据
==>  Preparing: update employee set phone = ? where id = ?;
==> Parameters: 13399887766(String), 2(Integer)
<==    Updates: 1
第二次查询
==>  Preparing: select id, name, age, gender, birthday, phone, dept_id from employee where id = ?;
==> Parameters: 1(Integer)
<==    Columns: id, name, age, gender, birthday, phone, dept_id
<==        Row: 1, 张三二, 80, 男, 2020-09-01, 13323332331, 2
<==      Total: 1
Employee{id=1, name='张三二', age=80, gender='男', birthday=Tue Sep 01 00:00:00 CST 2020, phone='13323332331', deptId=2}
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@1a04f701]
Returned connection 436532993 to pool.

手动关闭缓存

SqlSession sqlSession = MyBatisUtil.getSqlSession();
EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);

System.out.println("第一次查询");
Employee employee = mapper.selectById(1);
System.out.println(employee);

System.out.println("清理缓存");
sqlSession.clearCache();

System.out.println("第二次查询");
Employee employee2 = mapper.selectById(1);
System.out.println(employee2);

sqlSession.close();

运行结果

  • 手动清理完缓存，查询相同数据又一次执行了sql
第一次查询
Opening JDBC Connection
Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
Created connection 436532993.
==>  Preparing: select id, name, age, gender, birthday, phone, dept_id from employee where id = ?;
==> Parameters: 1(Integer)
<==    Columns: id, name, age, gender, birthday, phone, dept_id
<==        Row: 1, 张三二, 80, 男, 2020-09-01, 13323332331, 2
<==      Total: 1
Employee{id=1, name='张三二', age=80, gender='男', birthday=Tue Sep 01 00:00:00 CST 2020, phone='13323332331', deptId=2}
清理缓存
第二次查询
==>  Preparing: select id, name, age, gender, birthday, phone, dept_id from employee where id = ?;
==> Parameters: 1(Integer)
<==    Columns: id, name, age, gender, birthday, phone, dept_id
<==        Row: 1, 张三二, 80, 男, 2020-09-01, 13323332331, 2
<==      Total: 1
Employee{id=1, name='张三二', age=80, gender='男', birthday=Tue Sep 01 00:00:00 CST 2020, phone='13323332331', deptId=2}
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@1a04f701]
Returned connection 436532993 to pool.
二级缓存
  • 会话关闭时，会把一级缓存中的数据保存到二级缓存中
  • 再次开启会话时，会读取二级缓存中的数据

核心配置文件

  • cacheEnabled默认值是true
<setting name="cacheEnabled" value="true"/>

mapper中设置

  • eviction清除策略，默认LRU
    • LRU 最近最少使用，移除最长时间不被使用的对象
    • FIFO 先进先出，按照对象进入缓存的顺序来移除
    • SOFI 软引用
    • WEAK 弱引用
  • flushInterval刷新时间ms，默认不刷新
  • size缓存引用（对象或列表）的最大值，默认1024
  • readOnly只读，默认读写缓存。如果readOnly为false，实体类必须序列号。如果该值为true，返回的是同一个引用；如果该值为false，返回的是对象的拷贝，不是同一个引用。
<mapper namespace="com.example.mapper.EmployeeMapper">
    <cache
        eviction="FIFO"
        flushInterval="60000"
        size="512"
        readOnly="true"/>
</mapper>

测试代码

System.out.println("开启会话1");
SqlSession sqlSession = MyBatisUtil.getSqlSession();
EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);

System.out.println("第一次查询");
Employee employee = mapper.selectById(1);
System.out.println(employee);

System.out.println("关闭会话1");
sqlSession.close();

System.out.println("开启会话2");
SqlSession sqlSession2 = MyBatisUtil.getSqlSession();
EmployeeMapper mapper2 = sqlSession2.getMapper(EmployeeMapper.class);

System.out.println("第二次查询");
Employee employee2 = mapper2.selectById(1);
System.out.println(employee2);

System.out.println("关闭会话");
sqlSession2.close();

运行结果

  • 可以看出虽然使用的不是同一个会话连接，但是也只执行了一次sql
开启会话1
第一次查询
Cache Hit Ratio [com.example.mapper.EmployeeMapper]: 0.0
Opening JDBC Connection
Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
Created connection 2085002312.
==>  Preparing: select id, name, age, gender, birthday, phone, dept_id from employee where id = ?;
==> Parameters: 1(Integer)
<==    Columns: id, name, age, gender, birthday, phone, dept_id
<==        Row: 1, 张三二, 80, 男, 2020-09-01, 13323332331, 2
<==      Total: 1
Employee{id=1, name='张三二', age=80, gender='男', birthday=Tue Sep 01 00:00:00 CST 2020, phone='13323332331', deptId=2}
关闭会话1
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@7c469c48]
Returned connection 2085002312 to pool.
开启会话2
第二次查询
Cache Hit Ratio [com.example.mapper.EmployeeMapper]: 0.5
Employee{id=1, name='张三二', age=80, gender='男', birthday=Tue Sep 01 00:00:00 CST 2020, phone='13323332331', deptId=2}
关闭会话

标签：缓存,System,class,println,MyBatis,id,out
来源： https://www.cnblogs.com/qixioa/p/13709729.html

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