mybatis缓存之一级缓存

概述一级缓存：与数据库同一次会话期间查询到的数据会放在本地缓存中，以后如果需要获取相同的数据，直接从缓存中拿，没必要再去查询数据库。 package com.gong.mybatis.test; impo

 

 

 一级缓存：与数据库同一次会话期间查询到的数据会放在本地缓存中，以后如果需要获取相同的数据，直接从缓存中拿，没必要再去查询数据库。

package com.gong.mybatis.test;import java.io.IOException; java.io.inputStream; java.util.ArrayList; java.util.Arrays; java.util.HashMap; java.util.List; java.util.Map; org.apache.ibatis.io.Resources; org.apache.ibatis.session.sqlSession; org.apache.ibatis.session.sqlSessionFactory; org.apache.ibatis.session.sqlSessionFactoryBuilder; org.junit.Test; com.gong.mybatis.bean.Department; com.gong.mybatis.bean.Employee; com.gong.mybatis.dao.EmployeeMapper; com.gong.mybatis.mapper.EmployeeMapperDynamicsql;public class TestMybatis4 {        public sqlSessionFactory getsqlSessionFactory() throws IOException {        String resource = "mybatis-config.xml";        inputStream is = Resources.getResourceAsstream(resource);        return new sqlSessionFactoryBuilder().build(is);    }    @Test    voID test()  IOException {        sqlSessionFactory sqlSessionFactory = getsqlSessionFactory();        sqlSession openSession = sqlSessionFactory.openSession();        try {            EmployeeMapper mapper = openSession.getMapper(EmployeeMapper.);            Employee em = mapper.getEmpByID(1);            System.out.println(em);            Employee em2 = mapper.getEmpByID(1);            System.out.println(em2);            System.out.println(em == em2);            openSession.commit();        } finally {            // Todo: handle finally clause            openSession.close();        }                    }    }

输出结果：

DEBUG 01-22 10:18:00,037 ==>  Preparing: select ID,last_name lastname,email,gender from tbl_employee where ID = ?   (BaseJdbcLogger.java:145) DEBUG 01-22 10:18:00,108 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) DEBUG 01-22 10:18:00,175 <==      Total: 1  (BaseJdbcLogger.java:145) Employee [ID=1,lastname=dema,1)">genderemail=dema@qq.com,1)">dept=null]Employee [IDtrue

说明：一级缓存的作用：在一次会话中，使用了相同的查询方法，所以系统只会发送一次sql请求，同时可以看到，em和em2是同一个对象，说明em2并不是重新发送sql请求得到的，而是直接从缓存中获得的。

一级缓存失效的四种情况：

1、使用不同的sqlSession对象

);            Employee em = mapper.getEmpByID(1);            System.out.println(em);                        sqlSession openSession2 = sqlSessionFactory.openSession();            EmployeeMapper mapper2 = openSession2.getMapper(EmployeeMapper.class);            Employee em2 = mapper2.getEmpByID(1);            System.out.println(em2);                        System.out.println(em == em2);            openSession.commit();        }             openSession.close();        }                            }    }

结果：

DEBUG 01-22 10:23:05,279 ==>  Preparing: select ID,gender from tbl_employee where ID = ?   (BaseJdbcLogger.java:145) DEBUG 01-22 10:23:05,323 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) DEBUG 01-22 10:23:05,367 ==>  Preparing: select ID,gender from tbl_employee where ID = ?   (BaseJdbcLogger.java:145) DEBUG 01-22 10:23:05,388 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) DEBUG 01-22 10:23:05,393 <==      Total: 1  (BaseJdbcLogger.java:145) Employee [IDfalse

2、同一个sqlSession对象，但查询条件不一样：

);            System.out.println(em);                        sqlSession openSession2 = sqlSessionFactory.openSession();            EmployeeMapper mapper2 = openSession2.getMapper(EmployeeMapper.class);            Employee em2 = mapper.getEmpByID(3            openSession.close();        }                            }    }

输出：

DEBUG 01-22 10:25:29,976 ==>  Preparing: select ID,gender from tbl_employee where ID = ?   (BaseJdbcLogger.java:145) DEBUG 01-22 10:25:30,021 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) DEBUG 01-22 10:25:30,086 DEBUG 01-22 10:25:30,088 (BaseJdbcLogger.java:145) DEBUG 01-22 10:25:30,093 Parameters: 3(Integer)  (BaseJdbcLogger.java:145) DEBUG 01-22 10:25:30,096 <=3,1)">=小红,1)">=0,1)">=xiaohong@qq.com,1)">false

3、同一个sqlSession，但在两次查询之间执行了增删改 *** 作

            EmployeeMapper mapper2 = openSession2.getMapper(EmployeeMapper.class);            mapper.deleteEmp(5);            Employee em2 = mapper.getEmpByID(1DEBUG 01-22 10:27:54,536 ==>  Preparing: select ID,gender from tbl_employee where ID = ?   (BaseJdbcLogger.java:145) DEBUG 01-22 10:27:54,586 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) DEBUG 01-22 10:27:54,636 Preparing: delete from tbl_employee where ID=?   (BaseJdbcLogger.java:145) DEBUG 01-22 10:27:54,640 Parameters: 5(Integer)  (BaseJdbcLogger.java:145) DEBUG 01-22 10:27:54,700 <==    Updates: 1  (BaseJdbcLogger.java:145) DEBUG 01-22 10:27:54,702 Parameters: 1(Integer)  (BaseJdbcLogger.java:145) DEBUG 01-22 10:27:54,712 <false

4、同一个sqlSession，但手动清除了一级缓存

即在两次查询之间使用了：

openSession.clearCache();

总结

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