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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