MyBatis的二级缓存

二级缓存的原理和一级缓存一样，第一次查询会将数据放到缓存中，然后第二次查询直接去缓存读取。文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

但是一级缓存是基于 SqlSession 的，二级缓存是基于 mapper 的 namespace 的。文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

也就是说，多个 SqlSession 可以共享同一个二级缓存区域。如果两个 mapper 的 namespace 相同，那么即使是两个 mapper，这两个 mapper 执行 sql 查询的数据也将保存在相同的二级缓存区域中。文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

​文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

注意：User 需要实现序列化接口。文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

和一级缓存不一样，二级缓存需要手动开启。文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

首先在全局配置文件 sqlMapConfig.xml 中加入如下代码文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

<settings>
    <!-- 开启二级缓存 -->
    <setting name="cacheEnabled" value="true"/>
</settings>

然后在 UserMapper.xml 中开启二级缓存。文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

<!-- 开启二级缓存 -->
<cache />

如果是使用注解方式，在 IUserMapper 上写上注解 @CacheNamespace文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

@CacheNamespace
public interface IUserMapper {
}

测试代码文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

public class SecondCacheTest {
    private SqlSessionFactory sqlSessionFactory;

    @Before
    public void before() throws Exception {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
    }

    @Test
    public void secondLevelCache() {
        SqlSession sqlSession1 = sqlSessionFactory.openSession();
        SqlSession sqlSession2 = sqlSessionFactory.openSession();

        IUserMapper userMapper1 = sqlSession1.getMapper(IUserMapper.class);
        IUserMapper userMapper2 = sqlSession2.getMapper(IUserMapper.class);

        User user1 = userMapper1.findUserById(1);
        // 清空一级缓存
        sqlSession1.close();
        User user2 = userMapper2.findUserById(1);
        System.out.println(user1 == user2);
    }
}

效果如下文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@56528192]
==>  Preparing: select * from user where id=?
==> Parameters: 1(Integer)
<==    Columns: id, username, password, birthday
<==        Row: 1, lucy, 123, 2019-12-12
<==      Total: 1
Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@56528192]
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@56528192]
Returned connection 1448247698 to pool.
As you are using functionality that deserializes object streams, it is recommended to define the JEP-290 serial filter. Please refer to https://docs.oracle.com/pls/topic/lookup?ctx=javase15&id=GUID-8296D8E8-2B93-4B9A-856E-0A65AF9B8C66
Cache Hit Ratio [com.terwergreen.mapper.IUserMapper]: 0.5
false

可以看到，两次请求，只有第一次输出了 sql ， 并且输出了缓存命中率是 0.5 。文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

• useCache 用来设置是否禁用二级缓存。在 statement 中设置 useCache="false" 可以禁用当前 select 语句的二级缓存

• <select id="selectUserByUserId" useCache="false" resultType="space.terwer.pojo.User"
  parameterType="int">
  select * from user where id=#{id}
  </select>
  

• 设置 statement 的 flushCache="true" 即 刷新缓存 ，默认情况下就是 true。因为一般不设置，默认即可。文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

• 注解方式使用文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

  /**
  * 根据ID查询用户
  *
  * @param id
  * @return
  */
  @Options(useCache = false, flushCache = Options.FlushCachePolicy.TRUE)
  @Select("select * from user where id=#{id}")
  User findUserById(Integer id);
  

文章更新历史文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html

2022-12-25 feat: 初稿 文章源自浅海拾贝-https://blog.terwergreen.com/mybatis-s-secondary-cache-z2ssaab.html