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【摘 要】
Update 1/8/2004
At the top of the page:
<%
pStr = "private, no-cache, must-revalidate"
Response.ExpiresAbsolute = #2000-01-01#
Response.AddHeader "pragma", "no-cache"
Response.AddHeader "cache-control", pStr
%>
-------------------------------------------------------------------
Don't allow your page to be cached. Why? You might want to prevent
users from seeing old content. Insert the following code in the
<HEAD> tag of your document.
<meta http-equiv="Expires" content="Tue, 04-Dec-1993
21:29:02 GMT">
Cache No More (ASP)
by Phil Paxton
(Phil@matchw.com)
Here are the things dealing with the issue of caching in ASP:
1. Response.Expires = 0
2. Response.ExpiresAbsolute = Now() - 1
3. Response.AddHeader "cache-control", "private"
4. Response.AddHeader "pragma", "no-cache"
5. Adding a "cachebuster" by creating a unique URL.
Notes:
#1 is said to expire at 60 seconds, not 0. Also, Khin Zaw (from
ActiveServerPages@ and ASPAdvanced@) has posted research from
time spent with some IIS internals experts revealing this can
be a very touchy parameter to rely upon and usually requires a
rather "large" negative number or pedantically, that
would be a very small number).
#2 (my own creation) says "expire this page 24 hours ago",
allowing for time differences, rather than specify a static date.
#3, #4 from an MS KB article. The code is correct but there are
some incorrect statements in the article itself.
n.b. some related KB articles include:
(INFO: Controlling the Caching of Web Pages with IIS 4.0)
(PRB: Browser Doesn't Show Most Recent Versions of htm/asp Files)
(How to Use Pragma: No-cache with IIS and IE)
#5 my term, but not my technique. IE 5.0 can defeat #1-#4 used
in conjunction but adding #5 will break it. I usually use something
like "?NoCache=Rnd" after a statement. Bill Wilkinson
(of Chili!Soft) has proposed an alternate of ?NoCache=Server.URLEncode(Now())".
--------------------------------------------------------------------------------
Another thing to remember: Netscape will continue to cache, even
if you turn all caching off. This behavior persisted through 4.5
PR1, PR2, and now in the released version of 4.5.
If you fear you might have to deal with caching later, you might
want to build contingencies into your app as you go. Retrofitting
#5 throughout even a medium-sized app would take a rather sizeable
effort. You could retrofit #1-#4 (inclusive) rather quickly with
a single pass through the application, but #5 takes a lot of extra
effort. And to that end, I don't ever Response.Redirect anywhere
in my code except sample code I post to the lists (then again,
the only time I use Response.Write is to the list because I rely
on my Utilities-Form.inc library for Display() and HTML()). Everything
is Redirect(NewURL) where the Redirect function looks like this:
Function Redirect( NewURL )
'
If Not IsEmpty( NewURL & "" ) Then
Dim QuestionMark
'
QuestionMark = Instr( NewURL, "?" )
'
If QuestionMark = 0 Then
Response.Redirect NewURL & "?" & NoCacheURL()
Response.End
Else
Response.Redirect NEWURL & "&" & NoCacheURL()
Response.End
End If
End If
'
End Function
and NoCacheURL looks like this:
Function NoCacheURL()
'
On Error Resume Next
'
Randomize
' Randomize not needed if you use Now()
'
NoCacheURL = "NoCache=" & Server.URLEncode(rnd)
'
' or NoCacheURL = "NoCache=" & Server.URLEncode(Now())
' per Bill
'
End Function
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