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Help needed to compare timestamp

arch123
Greenhorn

Joined: Oct 16, 2002
Posts: 3
Hi All,
I needed some help on how to compare if a file on the server is modified. I wud like 2 explain what I am looking for. Hope this explains.
I have created a server socket n a client socket. Whenever the client connects to the server it downloads some files for the first time. Now if later the clinet again connects to the server, and if there has been an update on these files on the server, then the client must download them. Else the client shud not download those files.
My problem is I am unable to acces the file on the server to read its timestamp . Is there any way to read the file information on the server thru the socket.
Well, I am able to download the file without any hassles. But am unable to find if the server files has been modfied or not.
Can anyone help plz.
Thanks in advance for ur time
Dave Vick
Ranch Hand

Joined: May 10, 2001
Posts: 3244
java_wizard
Welcome to the Java Ranch, we hope you�ll enjoy visiting as a regular however,
your name is not in keeping with our naming policy here at the ranch. Please change your display name to an appropriate name as shown in the policy.
Thanks again and we hope to see you around the ranch!!


Dave
William Barnes
Ranch Hand

Joined: Mar 16, 2001
Posts: 986

Don't know how check that a file has changed the way you are asking.
Maybe you could download the file and check it against the one you got last time. Or have a daemon on the server which would keep track of the files and if they change make a notation in a different file - which you could download first to determine which of the files had changed.


Please ignore post, I have no idea what I am talking about.
arch123
Greenhorn

Joined: Oct 16, 2002
Posts: 3
I dont want 2 download the file if there are no changes. Only if there are any wud I download. Isnt there any way for me to check atleast for the size of the file if not the timestamp.
Coz my main objective is to only download those files for updation on client only if there was any modfication on server.
William Barnes
Ranch Hand

Joined: Mar 16, 2001
Posts: 986

Example 17.7 MkIndex.java in Java Cookbook creates a web page that is a directory listing. Maybe that would point you in the right direction. You can download the all examples from the book here: Java Cookbook
Dave Vick
Ranch Hand

Joined: May 10, 2001
Posts: 3244
arch123
Thanks for changing your name, however, it still does not meet the naming requirments. Your display name should be a first and last name separated by a space. You can use the link in my post above to change it.
thanks
arch123
Greenhorn

Joined: Oct 16, 2002
Posts: 3
This link seems to be quite in depth. A good link though but couldnt find exactly want i needed.
I am working on a java applet now. This applet is however residing on my server. I wud want 2 invoke this applet locally . For that , I create a serevr socket n a client socket for downloading the jar files n other related files from the server.
Now I am able to do the downlaoding part. So at present I am having my applet's jar file on my hard disk on my local machine. Now whenever there is any changes made to the applet on the server , I want the clinet machine"(my local machine)to detect the changes in the jar file n download the jar again to my local machine.
For this I was wondering if I cud check for the timestamp of my local machine's jar file and that on the server. Thereby , if they dont match then it indiactes that server files have been modified recently.
The problem is how to check for the timestamp or say how do i check if the file on the server n that on my local machine are similar or not.
Hope this explains
Eddy Chang
Greenhorn

Joined: Apr 05, 2001
Posts: 27
Comparing timestamps is not completely reliable in detecting changes to a file. You can compare checksums of the file, using a md5 or crc32 checksum to determine if the file has changed.
You could do something like this:
1. The client calculates the checksum of it's files.
2. The client sends the checksum to the server.
3. The server calculates the checksum of it's file and compares it against the checksum that the client sent over (the client's checksum).
4. If the checksums do not match, then the server must have a newer file, send the file to the client.
As for the java code to calculate the checksum, you can probably find something using a search engine since I haven't done this before in java.
java.util.zip.CRC32 in the JDK and http://www.ietf.org/rfc/rfc1321.txt might get you started.
Eddy
Dave Landers
Ranch Hand

Joined: Jul 24, 2002
Posts: 401
How about looking at URLConnection.getLastModified(), which will give you the last-modified header value from the web server. Your web server should support filling out this field based on the file's modified timestamp.
 
It is sorta covered in the JavaRanch Style Guide.
 
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