mod rewrite, RequestDispatcher and intercepting requests to the server

[Michael Tickle <mi...@stud.umist.ac.uk>]

I am attempting to intercept all requests to a server so I can extract
information from the header.  I would then like to pass the request on to
the server for it to display the user the requested page.

I am led to believe that TomCat can not direct all requests to it on to a
servlet - is this correct?  Since I don't think this is possible I have
adapted a different strategy.
I have installed apache and using mod rewrite all requests are forwarded on
to my servlet.  The servlet the extracts the header information.  The
problem then is showing the user the requested page.  Ideally I would just
like to return the request to the server and let it deal with it. I am told
the closest to this is RequestDispatcher.  However RequestDispatcher seems
only to be able to use relative linking - this will not work for me as I
want to go from http://win2k:9090/servlet/myservlet?To=/index.html to
http://win2k/index.htm

Firstly is there a better way of doing what I am trying to do?

Secondly is there a similar feature to RequestDispatcher that can use full
URLs not relative URIs?

Thirdly has TomCat got any rewrite functionality (CF mod rewrite) since if I
can not pass the request back to the apache domain I will have to somehow
rewrite all the URLs in the tomcat domain.

Thanks for any help.

Mike

PS For future reference is this question more suited to the user or the
developer mailing list?

-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-

Michael Tickle
Computing Science
Dept. of Computation

ICQ 5252 8934
Mob: 0777 968 5548
Fax: 0870 705 8382

Re: mod rewrite, RequestDispatcher and intercepting requests to the server

[Mel Martinez <me...@yahoo.com>]

--- Michael Tickle <mi...@stud.umist.ac.uk>
wrote:
> I am attempting to intercept all requests to a
> server so I can extract
> information from the header.  I would then like to
> pass the request on to
> the server for it to display the user the requested
> page.

Sounds straightforward.

> 
> I am led to believe that TomCat can not direct all
> requests to it on to a
> servlet - is this correct?  

Uh? What exactly are you trying to say here?  If you
are saying that tomcat can not be configured to send
every single request all to the same servlet, then I'm
not sure that's true.

Couldn't you setup servlet pattern-mapping like so:

    <servlet-mapping>
        <servlet-name>
            DelagatorServlet
        </servlet-name>
        <url-pattern>
            /
        </url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>
            DelagatorServlet
        </servlet-name>
        <url-pattern>
            *
        </url-pattern>
    </servlet-mapping>

You may need a couple of other patterns like "/*.*" or
"/*/*" as I'm not sure of the exact behavior of the
wild-card algorithm used in TC.

You may also want to define your own context for
"webapps/" to intercept activity there from going to
webapps/ROOT instead of to your application context.

I won't claim that this is simpler or better than just
using Apache's rewrite module.  It is recommended you
use apache anyway for serving static content, so it
may be simplest to just settup some RewriteRules to do
this (funnel all requests to a single servlet).

> I have installed apache and using mod rewrite all
> requests are forwarded on
> to my servlet.  The servlet the extracts the header
> information.  The
> problem then is showing the user the requested page.
>  Ideally I would just
> like to return the request to the server and let it
> deal with it. I am told
> the closest to this is RequestDispatcher.  However
> RequestDispatcher seems
> only to be able to use relative linking - this will
> not work for me as I
> want to go from
> http://win2k:9090/servlet/myservlet?To=/index.html
> to
> http://win2k/index.htm
> 
> Firstly is there a better way of doing what I am
> trying to do?

You want to use the HttpServletResponse.sendRedirect()
method instead of RequestDispatcher.forward().  The
forward() method only accesses urls reachable in the
current servlet engine thus they must be relative URLs
or 'absolute' relative to the current servlet context
path.  The sendRedirect() method is an actual HTTP
redirect that tells the browser to go somewhere else.

Keep in mind that neither a forward() or a
sendRedirect() will work once the response has been
committed, which can happen if you modify request
headers or write more output to the response than the
buffer size (8k I believe).

> 
> Secondly is there a similar feature to
> RequestDispatcher that can use full
> URLs not relative URIs?
> 

HttpServletResponse.sendRedirect()


> Thirdly has TomCat got any rewrite functionality (CF
> mod rewrite) since if I
> can not pass the request back to the apache domain I
> will have to somehow
> rewrite all the URLs in the tomcat domain.
> 

TOmcat has pretty minimal rewrite capability
(basically the servlet-naming tag above).  This sounds
like a prime candidate for an Interceptor for a future
version (ideally it work consistently with apache
mod_rewrite rules in effect) but is not trivial and
probably won't happen real soon.

However, as indicated above, you should be able to
solve your problem by using sendRedirect() to send the
requests back to apache (or wherever).

Cheers,

Mel


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