----- Original Message -----
From: "Johnny Kewl" <[EMAIL PROTECTED]>
To: "Tomcat Users List" <users@tomcat.apache.org>
Sent: Sunday, July 01, 2007 11:22 AM
Subject: Re: Filters and getLastModified
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.
LIGHTBULB.... before I confuse the hell out of you.... in the above
paragraph, meant the say HTML page.... not JSP page.
JSP.... is naturally also a servlet, once compiled, so same applies to
that... Default Servlet does static pages, and thus the file attributes come
into play.
Sorry rushing before shops closed.... also just the way I think about it in
the MVC model, JSP pages are just used for display of results, they tend to
become micky mouse things, with all the work done in the servlet.... I
forget that some people put all their logic in the things.
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" <[EMAIL PROTECTED]>
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.
--
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