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Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

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© 2006-2013 Andrew Cooke (site) / post authors (content).

Servlet Filters

From: "andrew cooke" <andrew@...>

Date: Wed, 11 Jan 2006 21:48:18 -0300 (CLST)

"The Java Servlet specification version 2.3 introduces a new component
type, called a filter. A filter dynamically intercepts requests and
responses to transform or use the information contained in the requests or
responses.
[...]
In this paper we'll first discuss how to program filters to perform the
following types of tasks:
    * Querying the request and acting accordingly
    * Blocking the request and response pair from passing any further.
    * Modifying the request headers and data. You do this by providing a
customized version of the request.
    * Modifying the response headers and data. You do this by providing a
customized version of the response."

http://java.sun.com/products/servlet/Filters.html

This is great.  It lets you completely rewrite the request, so you can
define your own protocol layered over HTTP.  Obviously you still want URLs
to be exposed to users, but this makes it easy to have, in parallel, a
second system that uses form data to select pages.  And why do that? 
Because then you can have a single form in a page that goes to the
dispatcher filter, which (for example) data before jumping to another web
page.  This example is particularly relevant if you're using a lot of
javascript to make a nice dynamic page, but want to avoid the whole ajax
back channel.  Which lets you do moderately cool stuff in a way that
degrades savely if the client isn't that cooperative/standards compliant.

Oh, and doesn't redirect do this too?  Well, for POST data redirect is
kinda weird - http://ppewww.ph.gla.ac.uk/~flavell/www/post-redirect.html

One final goodie - this layers on top of spring-based mvc perfectly (it
has to, right, if you are exposing the individual URLs too...)

Sweet!
Andrew

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