Mybatis之缓存

缓存

2.一级缓存

SqlSession sqlSession = MybatisUnit.getSqlSession ();
        BlogMapper mapper = sqlSession.getMapper (BlogMapper.class);
        Map<String, Object> map = new HashMap<> ();
        map.put ("author","queen");
        Blog blog = mapper.getBlog (map);
        System.out.println (blog);
        System.out.println ("****************************");
        map.put ("author","queen");
        Blog blog1 = mapper.getBlog (map);
        System.out.println (blog1);
        sqlSession.commit ();
        sqlSession.close ();

一级缓存是在sqlsession层面上的，在sqlsession产生到关闭之间的涉及增删改查的数据都会被保存下来

效果：

==>  Preparing: select * from mybatis.blog where author=?
==> Parameters: queen(String)
<==    Columns: id, title, author, create_time, views
<==        Row: 566993470e944aa2ba4003c7ccbfaf5d, 我是最帅的, queen, 2020-08-09 10:23:10, 10
<==      Total: 1
Blog{author='queen', createTime=null, id='566993470e944aa2ba4003c7ccbfaf5d', title='我是最帅的', views=10}
****************************
Blog{author='queen', createTime=null, id='566993470e944aa2ba4003c7ccbfaf5d', title='我是最帅的', views=10}

由于存在缓存，我们的第二次查询就没有执行查询语句，而是直接从缓存中取出来；

可以利用sqlSession.clearCache()来清除缓存；清除以后的话，就会在加载一次数据库；

缓存消失的原因：

自己清除，利用函数
查询语句会被我们的增删改语句覆盖，目的就是我们的增删改已经修改了数据，再次访问缓存里面的就会导致取出的数据不真实；
查询的是不同的mapper.xml下面的东西
查询不同的东西，比如查询一号和二号

3.二级缓存

当你使用二级缓存的时候，只有一级缓存会话消失后才会缓存到二级缓存；

@org.junit.Test
    public void test2 () {
        SqlSession sqlSession = MybatisUnit.getSqlSession ();
        SqlSession sqlSession2 = MybatisUnit.getSqlSession ();
        BlogMapper mapper = sqlSession.getMapper (BlogMapper.class);
        Map<String, Object> map = new HashMap<> ();
        map.put ("author","Queen and King");
        Blog blog = mapper.getBlog (map);
        System.out.println (blog);
        sqlSession.close ();//会话关闭，一级缓存变成二级缓存

        System.out.println ("++++++++++++++++++++++++++++++++++++++++++++++++");

        BlogMapper mapper2 = sqlSession2.getMapper (BlogMapper.class);
        Map<String, Object> map2 = new HashMap<> ();
        map2.put ("author","Queen and King");
        Blog blog2 = mapper2.getBlog (map);
        System.out.println (blog2);
        sqlSession2.close ();
    }