Craig,
now I understand: according to your reply, prior to API 2.2,
providing a response buffer and when to commit this buffer was
something left to the servlet engine. Getting the session from
a request before accessing a response Writer is guaranteed to
work with all servlet engines regardless of their API level.
Regards,
Heinz Wehner
(Karlsruhe, Germany)
> -----Original Message-----
>
> Heinz Wehner wrote:
>
> > Nic,
> >
> > thanks for your answer that explains why getSession must be called
> > before writing any output to the response. However, it's still not
> > clear why you must even do this before sending any output in case
> > you respond using a writer. My guess would be that by accessing a
> > Writer the servlet container already writes the response header.
>
> That is not correct. Neither response.getWriter() or
> response.getOutputStream() necessarily triggers the response headers being
> sent. What matters is how buffering of the response data works in your
> servlet container. To improve efficiency, most servlet containers buffer
> the response output. Prior to the 2.2 spec the buffer configuration was
> in the hands of the servlet container; with 2.2 you now get to control
> it, as well as being able to reset the output buffer (if nothing has yet
> been sent), and tell whether any output has been sent yet.
>
> As long as the output has not yet been committed, you can still add HTTP
> headers (which is how the cookie that is used for session state
> maintenenace is actually sent to the browser).
>
> > Is that correct? I'm also uncertain as to wether this behaviour
> > conforms with the Servlet API specification.
>
> If I'm using sessions, I always make it a habit to call
> request.getSession() at the very beginning of my doGet() or doPost()
> method. This protects me from any cases where the buffer might be
> smaller than I expect, or against me moving code around and putting
> output-generating code earlier than it used to be (suddenly causing
> sessions not to work).
>
> In JSP pages, this happens for you automatically, because the generated
> code does a request.getSession() call at the beginning of the service
> method -- unless you've explicitly indicated that this page does not
> participate in sessions.
>
>
> Craig McClanahan
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