try {
        if (typeof top == "undefined")
            canUseCache = false;
        else {
            top.t_m_p = "tmp";
            if (top.t_m_p != "tmp") /*chrome里，抓不住这个异常，只好这么判断了*/
                canUseCache = false;
            else {
                if (typeof top.__cache == "undefined")
                    top.__cache = {};
                
                canUseCache = true;
            }
            
        }    
    } catch (e) {
        /* 出异常，不允许访问，不缓存了。*/
        canUseCache = false;
    }