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How to know if a web page's last update time

Posted on 2011-02-17
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Last Modified: 2012-05-11
Hi,

As noted above, I want to use Java to detect a web page's last update time via httpresponse, etc. I don't want to save the web page's contents and verify through the contents.

Thanks
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Question by:wsyy
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LVL 92

Expert Comment

by:objects
ID: 34922538
check the Last-Modified value in the http response header
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Assisted Solution

by:objects
objects earned 41 total points
ID: 34922550
theres some code here that shows how to access the headers

http://helpdesk.objects.com.au/java/how-do-i-get-the-cookies-from-a-http-connection

If you just want the one value you can use

long lastModified = urlconnection.getLastModified();
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Author Comment

by:wsyy
ID: 34923452
what if the header doesn't include the info? are there any other ways?
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LVL 92

Expert Comment

by:objects
ID: 34923472
not unless they including it some other custom header
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calboronster earned 42 total points
ID: 34924969
As a caution comment, not a very good idea to detect browser time and take action because :

1. Browser time can be tampered by the end user.
2. There could be a real time difference between server and the client.

The best way is for the server to set a session token in response which can then be traced back in next responses and compared each time by the server for last access time (for that session token).
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Assisted Solution

by:CEHJ
CEHJ earned 42 total points
ID: 34924978
You can generally only do that if the web page is a static file unless the site has some other means of announcing it.

It's worth checking if the If-Modified-Since header is set though - some sites implement this
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Author Comment

by:wsyy
ID: 34925943
calboronster:

My crawling application wants to detect the changes of a web page ever since last crawl. The solution you provided seems that a good check can be done only on the server side which may not help my case.

Thanks
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LVL 86

Expert Comment

by:CEHJ
ID: 34971137
>>The solution you provided seems that a good check can be done only on the server side which may not help my case.

I wonder why you accepted it then?
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