如何样才能使err1异常提示和提交表单在一个页面显示

怎么样才能使err1错误提示和提交表单在一个页面显示
下面的代码文件名称是adminlogin.asp



<%
dim names
dim pass
dim err1
'if Request.ServerVariables( "REQUEST_METHOD ") = "POST " then
names = trim(Cstr(Request.Form( "username ")))
pass = trim(Cstr(Request.Form( "password ")))
if names = Empty or pass = empty then
err1 = "错误:用户名和密码不能为空! "
response.write err1
end if
if dx(names) = "true " or dx(pass) = "true " then
err1 = "错误:用户名和密码只能在5-16个字符之间! "
response.write err1
end if
if nochar(names) = "false " or nochar(pass) = "false " then
err1 = "错误:用户名和密码只能在A-Z,a-z,0-9之间! "
response.write err1
end if

if len(err1) = 0 then
dim sql1
Sql1 = "SELECT * FROM administrator WHERE username= ' "&names& " ' and password= ' "&pass& " ' "
rs.open sql1,con,1,1
if not rs.eof then
Session( "username ") = names
Session( "password ") = pass
response.Redirect "manage.asp "
rs.close()
set rs = Nothing
con.close()
set con = Nothing

response.Flush()
response.End()
else
response.write " <font color=red> <strong> 错误: </strong> </font> 用户名或密码错误! "
response.end
end if
end if
'end if
%>
<html>
<head>
<meta http-equiv= "Content-Type " content= "text/html; charset=gb2312 ">
<title> 登录 </title>
<style>
<!--
.INPUT.buttonface{BACKGROUND: #DDDDFF; border:1 solid #183ead; COLOR: #000000; FONT-SIZE: 9pt; FONT-STYLE: normal; FONT-VARIANT: normal; FONT-WEIGHT: normal; HEIGHT: 18px; LINE-HEIGHT: normal}
.snow1{ font-size:12px; font-family: "宋体 "; color:#000000}

.snow2{ font-size:24px; font-family: "经典圆体简 "; color:#ffffff}
-->
</style>
</head>
<body background= "../image/userlogin/background.gif ">
<form action= "adminlogin.asp " method= "post " name= "f1 ">
<table align= "center " cellspacing=0 border=0 cellpadding=2 width=350>
<tr bgcolor= "#316AC5 ">
<td align= "center " colspan=2 height=35 class= "snow2 "> 管理员登录 </td>
</tr>
<tr bgcolor= "silver ">
<td align= "right " height=40 class= "snow1 " width=100 bgcolor= "#B5CBFF "> 管理员用户名: </td>
<td align= "left " bgcolor= "#ECF1FF "> <input type= "text " name= "username " style= "width:120px; "> </td>
</tr>
<tr bgcolor= "silver ">
<td align= "right " height=40 class= "snow1 " bgcolor= "#B5CBFF "> 管理员密码: </td>
<td align= "left " bgcolor= "#ECF1FF "> <input type= "password " name= "password " style= "width:120px; "> </td>
</tr>
<tr bgcolor= "#316AC5 ">
<td align= "center " colspan=2 height=35>
<input type= "submit " value= "提交 " class= "buttonface ">  
<input type= "reset " name= "r1 " value= "重置 " class= "buttonface ">  
</td>
</tr>
</table>
</form>
</body>
</html>

我把用户登陆表单和处理这个表单的程序放在了一起，我是想在本页面显示错误提示，但是一开始表单为空就提示错误，我怎么才能避免为空的错误？
请帮我改下，谢谢了

[解决办法]
加判断
if Session( "username ") = " "
登陆页面代码
elseif request( "username ")= " " ...
判断登陆和错误提示代码
else
主页面代码
end if

[解决办法]
写个js验证
function checkinfo(form){
if(form.username.value== ' '){
window.alert( '用户名不能为空 ');
form.username.focus();
return false;}
else if……
}
用onclick= "return checkinfo(this.value) "验证。这样不用刷新页面还能返回定位到出处的位置。
最好不要提交同一个页面处理。不方便修改管理。

如何样才能使err1异常提示和提交表单在

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