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From "Geoff Soutter" <>
Subject Tomcat 4.0 m4 filter impl.
Date Thu, 14 Dec 2000 08:27:36 GMT
Hi there,

I've got some questions about Servlet API 2.3 Filters and the Tomcat 4.0 m4
implementation of them.

I've been thinking about implementing one of those new-fangled Filter
things. I've been reading the spec, and I've found a few things which are
unclear. I've also read the source for Tomcat 4.0 m4 where appropriate to
try and figure out what is currently implemented, and I'd like to know how
the implementation relates to the unclear areas that I've found in the spec.

The areas I'm confused about follow:

1) Filters and UnavailableException

Filters can throw UnavailableException, but the 2.3 PFD doesn't seem to
specify what to do with them if it does. It looks like Tomcat 4.0 m4
disables the Servlet that the request referenced. This doesn't seem like a
very good idea to me, primarily because there is not necessarily a 1:1
mapping between a Filter and the Servlet it is filtering.

For example, imagine if you had an authentication filter which was
protecting a few static files in a subdir of your context. If the filter
died and thew UnavailableException, that would mean that the DefaultServlet
which is serving all the static content for your webapp would be disabled.

Perhaps in this case the Filter itself ought to be disabled? It would be
more work, but it would seem more consistent.

2) Filters and RequestDispatcher

Filters are apparently allowed to call the include() and forward() methods
of RequestDispatcher. However, the 2.3 PFD doesn't seem to mention if those
calls should invoke the filter chain again or if they are supposed to
reference the target servlet directly. It looks like Tomcat 4.0 m4
references them directly. I'm not sure which is the best way, but I can see
some potential problems with the way Tomcat is doing it.

For example, imagine if you had a nice site you'd written in XML and which
used an XSLT filter transform the content into HTML. Then, you wrote an
authentication filter which is inserted in the filter chain before the XSLT
filter. It forward()s to "login-error.xml" when the user enters the wrong
password. What happens? the login-error.xml is sent to the client directly,
without the XSLT transformation. Not nice.

Would anyone care to comment on what they think is the right behaviour in
these cases, or to clarify my confusion?



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