nio.Buffer

        0 <= mark <= position <= limit <= capacity
        [mark,limit]为活跃元素。IO读写关心的。
        init()  {mark = -1;position = 0;}
        
        
        clear() {position = 0; limit = capacity; mark = -1; return this;}
        flip()  {limit = position; position = 0; mark = -1; return this;}
        reset() {position = mark;return this;}
        rewind(){position = 0;mark = -1;return this;}