为什么我在Tp中实例化了Upload后调用不了upload方法

public function insertDoit(){
$model=M('product');
$up=new Upload();

    $up->savePath='images/';
    $info=$up->upload();

    if($info){
        $_POST['product_img']=$info['product_img']['savepath'].$info['product_img']['savename'];
    }
    $_POST['product_time']=date('Y-m-d H:i:s',time(true));
    echo "<pre>";
    print_r($up);
    echo "</pre>";
    exit;
    if($model->add($_POST)){
        echo 1;
    }else{
        echo 0;
    }
}



    $up->upload()这个里面没有东西，返回的false;之前还好使，突然就不好使了，求大神解答？？？新手

https://www.oschina.net/question/2484599_2136991