How to get a file's creation date or file size, for example this Hello.jpg at http://www.mywebsite.com/now/Hello.jpg(note: This URL does not exist)? The purpose of this question is to make my application re-download the files from the any website when it has detected that the website has an updated version of the files and the files in my local folder are out of date. Any ideas?
Thanks in advance!
If you use the HEAD request it will send the headers for the resource, there you can check the cache control headers which will tell you if the resource has been modified, last modification time, size (content-length) and date.
$ telnet www.google.com 80
Trying 216.239.59.103...
Connected to www.l.google.com.
Escape character is '^]'.
HEAD /intl/en_ALL/images/logo.gif HTTP/1.0
HTTP/1.0 200 OK
Content-Type: image/gif
Last-Modified: Wed, 07 Jun 2006 19:38:24 GMT
Expires: Sun, 17 Jan 2038 19:14:07 GMT
Cache-Control: public
Date: Tue, 16 Sep 2008 09:45:42 GMT
Server: gws
Content-Length: 8558
Connection: Close
Connection closed by foreign host.
Note that you'll probably have to decorate this basic and easy approach with many heuristics depending on the craziness of each webserver's admin, as each can send whatever headers they like. If they do not provide caching headers (Last-Modified, Expires, Cache-Control) nor Content-Length nor etag, you'd be stuck with redownloading it to test.
The webserver might send a last-modified and/or etag header for that purpose. And you might send an if-modified-since header in your request.
see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html sections 14.19, 14.25 and 14.29