On Thu, Nov 11, 2004 at 11:28:21AM +1100, Derek Clarkson wrote:
> This sounds like something I have encountered. The basic question is
> that how do you redirect to an error page if youa re writing to the
> output stream rather than going to another JSP, and have an
> exception ?
I thought I posted a reply to this earlier, but I don't see it in
my inbox, so maybe I should repost.
In a nutshell, this topic makes a lot more sense if you
understand what an HTTP request and an HTTP response really look like.
My number one recommendation to folks doing web applications is to get
a packet sniffer, or a logging proxy, and take a look at what the
browser and server are actually sending back and forth.
Each HTTP request and response look a lot like an ordinary email:
a series of header lines, a blank line, and a message body. The
header lines are all name: value. In an HTTP response the body is the
HTML of the page, or the binary data of an image. In some cases there
is no body at all (a client-side redirect, for example).
Any logical operation is handled with a header - setting a
cookie, redirecting the browser, etc. Once the blank line separating
the header from the body has been sent, you can't go back and send
another header line. So you _must_ do any sort of redirect before the
server starts to flush output back to the client. If you try to set a
header after the server has already sent the blank lnie separating the
headers from the body, you'll get a java.lang.IllegalStateException.
One way to make this easier to do is to set your buffer larger,
so the server will wait longer before flushing output to the browser.
Another way is to use an MVC architecture. Have the submit from
the browser go to a servlet that does any logical processing you need,
but doesn't send any output back to the user. Instead, the controller
servlet sets request attributes and user session attributes containing
the results of the logic, then redirects the request off to the right
view page. The view page looks for the request attributes and user
session attributes and generates HTML tags for the display. The
controller doesn't muck with display stuff, the view page doesn't muck
with the headers.
--
Steven J. Owens
[EMAIL PROTECTED]
"I'm going to make broad, sweeping generalizations and strong,
declarative statements, because otherwise I'll be here all night and
this document will be four times longer and much less fun to read.
Take it all with a grain of salt." - http://darksleep.com/notablog
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