Mybatis的一级缓存机制简介

1.接口

public interface MemberMapperCache {

    public Members selectMembersById(Integer id);

}

2.配置文件xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.dao.MemberMapperCache">

    <select id="selectMembersById" resultType="members">

        select * from members where id = #{id}

    </select>

</mapper>

3.测试类

    /**
     * Mybatis的缓存机制
     * 一级缓存(本地缓存)也叫sqlSession级别的缓存: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,以后再获取相同的数据就没必要再去数据库中拿了
     *         一级缓存失效的4种情况
     *         1.sqlSession不同
     *         2.sqlSession相同但是查询条件不同(很好理解)
     *         3.sqlSession相同但是两次查询之间掺杂了相同数据的增删改操作
     *         4.sqlSession相同但是两次查询之间清除了缓存(session.clearCache();)
     */
    public static void main(String[] args) throws IOException {
        SqlSession session = getSqlSession();
        try {
            // 面向接口方式
            MemberMapperCache mapper = session.getMapper(MemberMapperCache.class);
            Members member1 = mapper.selectMembersById(1);
            Members member2 = mapper.selectMembersById(2);
            System.out.println(member1.toString());
            System.out.println(member2.toString());
            session.commit();
        } finally {
            // 关闭session
            session.close();
        }
    }

    public static SqlSession getSqlSession() throws IOException {
        // 读取全局配置文件
        String resource = "conf/mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        // 获取sqlSessionFactory对象
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        // 打开session
        SqlSession session = sqlSessionFactory.openSession();
        return session;
    }

4.打印结果

==>  Preparing: select * from members where id = ?
==> Parameters: 1(Integer)
<==    Columns: id, member_name, pass_word, age
<==        Row: 1, 张三, 123456, 25
<==      Total: 1
Member [id=1, member_name=张三, password=123456, age=25]
Member [id=1, member_name=张三, password=123456, age=25]

两次相同查询可见只打印了一条sql语句,这就是本地缓存机制

5.清除缓存操作

            MemberMapperCache mapper = session.getMapper(MemberMapperCache.class);
            Members member1 = mapper.selectMembersById(1);
            session.clearCache();
            Members member2 = mapper.selectMembersById(1);
            System.out.println(member1.toString());
            System.out.println(member2.toString());

结果

==>  Preparing: select * from members where id = ?
==> Parameters: 1(Integer)
<==    Columns: id, member_name, pass_word, age
<==        Row: 1, 张三, 123456, 25
<==      Total: 1
==>  Preparing: select * from members where id = ?
==> Parameters: 1(Integer)
<==    Columns: id, member_name, pass_word, age
<==        Row: 1, 张三, 123456, 25
<==      Total: 1
Member [id=1, member_name=张三, password=123456, age=25]
Member [id=1, member_name=张三, password=123456, age=25]

发送了两条sql语句

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