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Detecting if an FTP file is being written to before attempting download

Posted on 2008-10-02
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Last Modified: 2013-12-29
Hi,
I am polling an IIS FTP Server for files on a regular basis using Java.

Sometimes i am receiving an IOException, because the file is being written to while i am reading it.

Using a "ready" or trigger file is not an option at the moment with the vendor.

I am using Java SDK 1.4.2 and Apache Commons Net 1.4.1.

I am dealing with large files, in the code snippet i copy the stream to a ByteArrayOutputStream. The error occurs in the copyStream method while reading in the file.

Does anyone have any suggestions? i guess i could retry a few times...

Thanks

Mike


try {
 
            ftp = connect();
 
            ftp.setFileType(FTP.BINARY_FILE_TYPE);
 
            inputStream = new BufferedInputStream(ftp.retrieveFileStream(srcfilename));
 
            byteOutputStream = new ByteArrayOutputStream();
 
            /** Thread safe byte copy * */
            FTPUtil ecfu = new FTPUtil();
            ecfu.copyStream(inputStream, byteOutputStream);
 
            ftp.completePendingCommand();
            logger.debug("FTP: Byte transfer from server sucessful");
 
        } catch (IOException e) {
            throw new FTPServiceException("FTP: IOException downloading " + srcfilename, e);
        }

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Question by:kwikksilva
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Accepted Solution

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mbodewes earned 500 total points
ID: 22626479
Not really, afaik there is no option in the FTP protocol that tells you it is done. I've had the same problem, but then with detecting it after delivery by FTP.

You could try and see if the server understands this command:


MDTM

Syntax: MDTM remote-filename

Returns the last-modified time of the given file on the remote host in the format "YYYYMMDDhhmmss": YYYY is the four-digit year, MM is the month from 01 to 12, DD is the day of the month from 01 to 31, hh is the hour from 00 to 23, mm is the minute from 00 to 59, and ss is the second from 00 to 59.


And wait during polling until it is hasn't been changed for some time.
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LVL 23

Expert Comment

by:Ajay-Singh
ID: 22630742
Whats the error you are getting? If the file is huge, keeping into memory is not a good idea.
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LVL 9

Expert Comment

by:mbodewes
ID: 22652633
kwikksilva, have you solved the problem? It might be interesting to post the error message so we can exclude other errors.
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Author Closing Comment

by:kwikksilva
ID: 31502314
This is the closest to the correct answer, i used the getTimestamp() method of FTPFile, and then set a parameter called timestamp-offset (in seconds) is my settings file. I don't touch the file unless it is older than the offset. This works for me in a production environment.
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Author Comment

by:kwikksilva
ID: 22653382
Thanks guys, sorry i did not want to post the Exception, as it was only a side effect of the file being in transit. I used a play on mbodewes solution to avoid the problem altogether.
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