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    <h1>UPDATE - Set a timeout for the data connection</h1> <p>Since the last file is only partially downloaded, and given the source of <a href="http://commons.apache.org/net/api-3.1/src-html/org/apache/commons/net/ftp/FTPClient.html#line.1666" rel="nofollow noreferrer"><code>FTPClient.retrieveFile()</code></a>, I think it may be a problem on the server side (something that make it hang, or even die - <em>who knows</em>). Obviously one can't repair the server or even know what's going on there, anyway I suggest to add a timeout with <a href="http://commons.apache.org/net/api-3.1/org/apache/commons/net/ftp/FTPClient.html#setDataTimeout%28int%29" rel="nofollow noreferrer"><code>setDataTimeout(int)</code></a> and catch the possible <code>SocketTimeoutException</code> separately to be logged in a different place and maybe sent to the FTP server admins (along with the time information when it happened) so they can merge the logs and see what's the issue.</p> <p><strong>OLD ANSWER</strong></p> <p>I didn't notice that you connect and login for each and every file, so the following is just an <em>optimization</em> not to close the control connection and succesfully logout, but it should <strong>not</strong> address the problem.</p> <p>You could start the JVM in debug mode and attach a debugger when it hangs, anyway according to <a href="https://stackoverflow.com/a/10530366/315306">this answer</a> and <a href="http://apache-commons.680414.n4.nabble.com/FTPClient-hangs-when-downloading-large-files-td3307715.html" rel="nofollow noreferrer">this thread</a> it can be a timeout problem on the network equipment devices (routers). From the <a href="http://commons.apache.org/net/api-3.1/org/apache/commons/net/ftp/FTPClient.html" rel="nofollow noreferrer">FTPClient Javadoc</a></p> <blockquote> <p>During file transfers, the data connection is busy, but the control connection is idle. FTP servers know that the control connection is in use, so won't close it through lack of activity, but it's a lot harder for network routers to know that the control and data connections are associated with each other. Some routers may treat the control connection as idle, and disconnect it if the transfer over the data connection takes longer than the allowable idle time for the router.</p> </blockquote> <p>One solution to this is to send a safe command (i.e. NOOP) over the control connection to reset the router's idle timer. This is enabled as follows:</p> <pre><code>ftpClient.setControlKeepAliveTimeout(300); // set timeout to 5 minutes </code></pre>
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    1. COGood point about the size of the files, but also the connection setup is pretty critical if your in a firewalled or mainframe environment, as they will kill any inactive connections in a pretty short timeframe.
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    2. CO@Mike nice addition :) Anyway, even in such environments, I think all the program can do is keeping sending NOOP. Or is there some *special* connection configuration?
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    3. COThe net commons doesn't always throw exceptions. In your case of a large file, the setControlKeepAliveTimeout(300) should stop it from dissapearing. This shouldn't be the issue here though, since he establishes a new connection each time through the loop (the method is self contained). He also doesn't switch to passive mode enterLocalPassiveMode(), so he's sending the file over the control connection. I think it's running so fast the server is probably having an issue and is rejecting the connection. The code had a few critical spots where he wasn't checking the return value.
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