一级缓存

• 一级缓存也叫本地缓存:SqlSession
  • 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，没必须再去查询数据库;

测试：

1.开启日志

<settings><setting name="logImpl" value="STDOUT_LOGGING"/>
</settings>

2.测试在一个session中查询两次相同的日志

3.查看日志输出

代码演示

核心配置文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd">
<!--核心配置环境-->
<configuration>
<!--  引入外部配置文件  优先使用  --><properties resource="db.properties" />
<settings><setting name="logImpl" value="STDOUT_LOGGING"/>
</settings><!--可以给实体类取别名--><typeAliases><typeAlias type="com.tian.pojo.User" alias="User"/></typeAliases><environments default="development"><environment id="development"><transactionManager type="JDBC"/><dataSource type="POOLED"><property name="driver" value="${driver}"/><property name="url" value="${url}"/><property name="username" value="${username}"/><property name="password" value="${password}"/></dataSource></environment></environments>
<!--每一个Mapper.xml都需要在mybatis核心配置文件中注册！-->
<mappers><mapper class="com.tian.dao.UserMapper"/>
</mappers></configuration>

UserMapper

//根据id查询User queryUserById(@Param("id") int id);

UserMapper.xml

<select id="queryUserById" resultType="User">select * from user where id=#{id}
</select>

测试类

   @Testpublic void test(){SqlSession sqlSession = MybatisUtils.getSqlSession();UserMapper userMapper = sqlSession.getMapper(UserMapper.class);User user1 = userMapper.queryUserById(1);System.out.println(user1);System.out.println("--------------");User user2 = userMapper.queryUserById(1);System.out.println(user2);System.out.println(user1==user2);sqlSession.close();}

运行结果

Opening JDBC Connection
Created connection 1026055550.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@3d285d7e]
==>  Preparing: select * from user where id=? 
==> Parameters: 1(Integer)
<==    Columns: id, name, pwd
<==        Row: 1, 狂神, 123456
<==      Total: 1
User(id=1, name=狂神, pwd=123456)
--------------
User(id=1, name=狂神, pwd=123456)
true
Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@3d285d7e]
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@3d285d7e]
Returned connection 1026055550 to pool.

缓存失效的情况

1.查询不同的东西

   @Testpublic void test(){SqlSession sqlSession = MybatisUtils.getSqlSession();UserMapper userMapper = sqlSession.getMapper(UserMapper.class);User user1 = userMapper.queryUserById(1);System.out.println(user1);System.out.println("--------------");User user2 = userMapper.queryUserById(2);System.out.println(user2);System.out.println(user1==user2);sqlSession.close();}

⒉.增删改操作，可能会改变原来的数据，所以必定会刷新缓存!

   @Testpublic void test(){SqlSession sqlSession = MybatisUtils.getSqlSession();UserMapper userMapper = sqlSession.getMapper(UserMapper.class);User user1 = userMapper.queryUserById(1);System.out.println(user1);userMapper.updateUser(new User(1,"haha","123123"));System.out.println("--------------");User user2 = userMapper.queryUserById(1);System.out.println(user2);System.out.println(user1==user2);sqlSession.close();}

3.查询不同的Mapper.xml

4.手动清理缓存！

sqlSession.clearCache();//手动清理缓存

小结:

• 一级缓存默认是开启的，只在一次SqlSession中有效，也就是拿到连接到关闭连接这个区间段!