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From "Johnny Kewl" <j...@kewlstuff.co.za>
Subject Re: Filters and getLastModified
Date Sun, 01 Jul 2007 09:22:46 GMT

You know LightBulb, been seeing your questions for a while now, you clearly 
learning, but you have got one serious conceptual mind on you, not enough 
just to use TC, hey ;).... I think its great, a guru in the making.

To try answer your question, if its HTTP, the getLastModified() that you 
provide in your servlet will be used. Naturally you could just set the 
headers directly yourself as well. When you provide that overridden method, 
its called by the servlet container, and all that really does is set the 
headers for you.

Theres a thing in TC called a Default-Servlet.... and this typically serves 
up JSP pages.... it can already do this, and in fact does it for you.
So typically in a MVC model.... a request comes into your servlet, and if 
you just using that servlet to "dispatch" it off to the correct JSP page, 
then you dont need to worry, because the JSP will be served by the default 
servlet, and the Last-Modified headers will be done automatically for you.

Filters should not affect the outcome unless off course its a CACHING filter 
in which case you need to undersstand what its doing and just how its 
playing with the Last-Modified and cache headers that are coming in. So for 
example if the filter looks at a If-Modified-Since header from the browser, 
and decides its not going to bother your servlet, and just return it from a 
cache, your servlet will never see that request.

In general if you have got a servlet that always serves new data, then you 
dont bother with this stuff, and you just need to return the data, ie its 
always fresh.
Where it starts to get interesting is when people store images in a dB.... 
then they lose those attributes that one normally has associated with a file 
system, and if it is part of a feed, they got to do extra stuff, like add 
another field to the dB to monitor if the image has changed, and then that 
getLastModified() facility is going to come in very handy, so yes you need 
to think about this stuff because just thinking that a dB is a cool place to 
stick images, also opens up a whole efficiency issue.

I'm no expert at this, but conceptually ;) there are two main area's of 
caching.... the old HTTP spec which just deals with a browser cache, ie 
typically it says, if the image is no older than say 30 mins, dont get it 
again from the server.... thus helping servers cope, then there is the newer 
stuff that allows for things like RSS feeds.
These things depend very much on these If-Modified-Since headers, and 
basically they poll the server, and for the server not to fall over, it 
relys very much on these headers and the proper implementation of 
getLastModified() to function correctly.

Anyway, best advice I can give you is to search for HTTP CACHE HEADERS, on 
google, and those articles will shed new light on why TC provides these 
facilities, and just when and how to use them.

In any normal browser you will see traditional caching at work, but if you 
have IE 7, you can also play with RSS feeds, and get a feel for how this all 
fits together.

Great Question.... again ;)

----- Original Message ----- 
From: "lightbulb432" <veerukrishnan@hotmail.com>
To: <users@tomcat.apache.org>
Sent: Sunday, July 01, 2007 3:19 AM
Subject: Filters and getLastModified


>
> Could somebody explain when in the request process getLastModified is 
> called
> on a servlet? Is it before everything - all servlets, filters, listeners,
> anything else?
>
> If you have multiple filters, how does this fit into the process - are 
> they
> all guaranteed to see the process through before the getLastModified call
> occurs on the servlet? (i.e. Can you count on the filtering to have 
> occurred
> before the code in your getLastModified runs?)
>
> I ask in the case that your getLastModified is more than a simple file
> system last modified timestamp check, and where you're using some kind of
> controller servlet rather than servlet-per-path - in which case your one
> getLastModified method in the controller servlet could become complex,
> having to deal with multiple getLastModified semantics of multiple pages
> that map to one servlet?
>
> With what request methods is it called (GET, POST, etc...)? The servlet 
> spec
> gives only a slight indication that it's for GETs, but doesn't really
> elaborate.
>
> Thanks.
> -- 
> View this message in context: 
> http://www.nabble.com/Filters-and-getLastModified-tf4006572.html#a11378442
> Sent from the Tomcat - User mailing list archive at Nabble.com.
>
>
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