Makes sense...
"Craig R. McClanahan" wrote:
> Gabriel Wong wrote:
>
> > Hi,
> >
> > I have some API questions, hopefully someone can shed some light.
> >
> > RequestDistpatcher:
> > Why does forward throw an IllegalStateException while include throw IOException if
>the
> > ServletOutputStream or a writer had already been obtained from the response object.
> > Shouldn't they both throw the same exception (IllegalStateException)?
> >
>
> The two exceptions are thrown for different reasons.
>
> RequestDispatcher.forward() throws IllegalStateException if you have already grabbed
>the
> output stream (or writer). The whole idea of forwarding is that you are delegating
> responsibility for generating this output response to the resource you are
>forwarding to
> (such as a JSP page). The purpose of the servlet that performs this call is to do
> whatever business logic is required to store beans in the request, session, or
>context
> and then forward to the destination resource for output generation.
>
> RequestDispatcher.include() throws IOException (as does forward()) if the destination
> resource throws an I/O error. I believe that the description of the "throws
>IOException"
> in the API documentation that was included with the 2.1-EA servlet classes was a
>botched
> copy-and-paste job in the Javadoc comments. Note that the include() call does *not*
> throw IllegalStateException, because it is legal for the called resource to generate
> output on the same stream as the calling servlet -- that is the whole point of an
>include
> operation.
>
> Both of these calls can also throw ServletException -- again, this would be because
>the
> called resource threw it.
>
> >
> > ServletResponse:
> > Why is it that an exception is not thrown if getWriter/getOutputStream are called
>more
> > than once?
> >
>
> On the same response, you can call getWriter more than once, or getOutputStream more
>than
> once. You just cannot call *both* of them on the same response, because the choice
>you
> make implicitly selects whether you are generating binary or character output. If
>you
> call getOutputStream more than once, you should get a reference to the same stream
>every
> time. Correspondingly, calling getWriter more than once should give you back the
>same
> PrintWriter.
>
> Craig McClanahan
>
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