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Checking file size with FTPClient

Posted on 1998-07-30
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Last Modified: 2010-04-16
Hi Everybody,

I was wandering if there is a way of checking a file size using File Transfer Protocol prior to the file's download.
I am developing an application that handles very large files
(15 - 20M), and one of the requirements is not to download a file if it doesn't conform to a certain size requirements.
Please help.
Thank you in advance.
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Question by:apetrovic
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by:gadio
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When you are using the FTPClient (if you are using the one that is part of the java library) you can issue a ls command which returns a list of the files with sizes, similar to the unix ls -l command. Take the list and extract the file sizes.
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by:apetrovic
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Hi gadio,
Thanks for your answer.
I was thinking about the same solution, but resulted list would be dependent upon server's operating system, which is unacceptable in the specifications I have.
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by:gadio
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Ok I agree. What you actually need to do is use the SIZE ftp command. This command returns to you the size of a given file. I assume that you are using the FTPClient in the java library. I'm afraid that this option is closed for you since the interface is protected. There are few options for you to take:
1. write the whole communication layer yourself and then you can issue the SIZE command to the ftp server.
2. Decompile the FTPClient so that you can insert your own commands (I did that this require a lot of work!).
3. Search the web for ftp clients implementations. www.gamelan.com would be a good place to start.

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by:apetrovic
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It seems that the first option you proposed is inevitable (I already searched Gamelan and a few other sites, without any success).
Thanks for your help gadio.
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by:msmolyak
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If you leave me your e-mail address I will send you a Java FTP client I found on the Internet (do not remeber where) which looks quite reasonable. I has an FTP client implementation as well as the GUI client which you do not have to use.
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by:gadio
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You are welcome. By the way, I can also email you the decompiled FTPClient and you can change it to work for you.
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by:apetrovic
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You can email me at apetrovic@imagictv.com
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by:gadio
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Done.
If this solves your problem, tell me and I'll "answer" this question.
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gadio earned 100 total points
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Hope that this solved your problem.
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by:apetrovic
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Hi gadio,

Sorry I did not reply sooner.

The SIZE command is not a part of the FTP standard, and is not supported by many FTP servers, so it can't be used as the ultimate solution. 

The best solution to the problem is to use FTP's LIST command, and extract the file size from the returned directory list (the solution you proposed at the begining).
It is true that the returned list would depend on FTP server's OS. However, vast majority of FTP servers returns directory listings whose layout complies to standard UNIX listing (the only exception is Win95). So, if you build your FTPClient knowing these facts, you can end up with relativly safe solution.
Or, you can ask your boss to change the specs, as I did.

Once again thanks for your help,

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by:gadio
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You are welcome. Note however that there are still operations that are not supported by the default FTPClient. The most common operation that I needed thats not implemented is MKDIR. If you want to implement one of those - you will have no choice but to use the FTPClient source I sent you.
Good luck,
G.
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