$(function () {
             alert($("#div_mian").height());//数值是1440
             $("#div_mian").height($("#div_mian").height());
             $(window.document.body).css("overflow-y", "hidden");
            alert($("#div_mian").height());//数值是1440但是层没出滚动条。（样式中这样设置：overflow-y:auto;）
         });