一级缓存

一级缓存是SqlSession级别的，通过同一个SqlSession查询的数据会被缓存，下次查询相同的数据，就
会从缓存中直接获取，不会从数据库重新访问
使一级缓存失效的四种情况：
1) 不同的SqlSession对应不同的一级缓存：创建1个sqlSession进行查询操作，执行2次查询操作，控制台只打印了1次查询语句，第2次查询会从缓存中拿数据
    @Test
    public void testOneCache(){
        SqlSession sqlSession1 = SqlSessionUtils.getSqlSession();
        CacheMapper mapper1 = sqlSession1.getMapper(CacheMapper.class);
        Emp emp1 = mapper1.getEmpByEid(1);
        System.out.println(emp1);
        Emp emp2 = mapper1.getEmpByEid(1);
        System.out.println(emp2);
    }

# 创建2个sqlSession，2次执行同样的查询语句，控制台打印了2次查询语句，说明第2次查询并没有从缓存中拿数据
    @Test
    public void testOneCache(){
        SqlSession sqlSession1 = SqlSessionUtils.getSqlSession();
        CacheMapper mapper1 = sqlSession1.getMapper(CacheMapper.class);
        Emp emp1 = mapper1.getEmpByEid(1);
        System.out.println(emp1);
        SqlSession sqlSession2 = SqlSessionUtils.getSqlSession();
        CacheMapper mapper2 = sqlSession2.getMapper(CacheMapper.class);
        Emp emp2 = mapper2.getEmpByEid(1);
        System.out.println(emp2);
    }


2) 同一个SqlSession但是查询条件不同：使用同一个SqlSession进行的2次查询操作不一样时，第2次查询并不能从缓存中拿数据
    @Test
    public void testOneCache(){
        SqlSession sqlSession1 = SqlSessionUtils.getSqlSession();
        CacheMapper mapper1 = sqlSession1.getMapper(CacheMapper.class);
        Emp emp1 = mapper1.getEmpByEid(1);
        System.out.println(emp1);
        List<Emp> emp2 = mapper2.getAllEmp();
        System.out.println(emp2);
    }

3) 同一个SqlSession两次查询期间执行了任何一次增删改操作：使用同一个SqlSession进行的2次相同的查询操作，第1次查询执行完后，再执行1次（增删改）操作，这时mybatis会把缓存清空，最后执行相同的查询，就不能从缓存中拿数据了
    @Test
    public void testOneCache(){
        SqlSession sqlSession1 = SqlSessionUtils.getSqlSession();
        CacheMapper mapper1 = sqlSession1.getMapper(CacheMapper.class);
        Emp emp1 = mapper1.getEmpByEid(1);
        System.out.println(emp1);
        mapper1.insertEmp(new Emp(null,"abc",23,"男","123@qq.com"));
        Emp emp2 = mapper1.getEmpByEid(1);
        System.out.println(emp2);
    }

4) 同一个SqlSession两次查询期间手动清空了缓存
    @Test
    public void testOneCache(){
        SqlSession sqlSession1 = SqlSessionUtils.getSqlSession();
        CacheMapper mapper1 = sqlSession1.getMapper(CacheMapper.class);
        Emp emp1 = mapper1.getEmpByEid(1);
        System.out.println(emp1);
        sqlSession1.clearCache();      // 手动清空缓存
        Emp emp2 = mapper1.getEmpByEid(1);
        System.out.println(emp2);
    }