> Hi All,
>
> Every day I get hundreds of hits on my JSP pages from proxy servers who are
>
> trying to determine if the links are still good. This is great for my
> static pages, but on pages with forms and processing logic it causes havoc.
>
> For example, if I have 3 pages of forms and the final page adds something
> to my database, hitting just the second page throws errors.
>
> I know that there is a pragma directive I need to add to each page, but
> isn't there also something that can be added to the HTTP header each time.
> And if so, what is the easiest way to add this to every outgoing header?
You can send out HTTP headers saying not to cache (PRAGMA: no-cache, etc),
but I think you way want to implement some other scheme. If your program
cannot handle random calls to ANY page then you may be in trouble.
I would suggest checking the REFERER header and redirecting clients to the
main page if the referer is NOT the page it is supposed to be. Note that
this is NOT a security measure (referer is easily forged), but it can help
with this situation, or people who have bookmarked pages.
You can also use a session ( either servlet sessions or one you devise
yourself) and embed that in urls (could even be as simple as the current
time in millis). Then if that sessesion has expired (or too much time has
passed in the case of the time thing) then you send the client to the main
page. Note that adding the current time to your requests pretty much makes
them non-cacheable anyway. Also note that most proxy servers will not
cache POST requests or GEt requests with the "?" char in them (i.e. with a
query string). By adding ?t=<timeinmillis>" you make it less likely to be
cached. Also, the browser itself will not cache the page between calls
because the time will be different each time it is generated.
Note that two clients COULD receive the same current time in millis due to
the threading issues of servlets. This is not a problem unless you make it
one.
Joe Laffey
LAFFEY Computer Imaging
St. Louis, MO
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