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POfile.createNewFile() creates files with last-modified time before actual creation time
 

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  1. POfile.createNewFile() creates files with last-modified time before actual creation time
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    <p>I'm using <a href="http://jpoller.sourceforge.net/" rel="nofollow noreferrer">JPoller</a> to detect changes to files in a specific directory, but it's missing files because they end up with a timestamp earlier than their actual creation time. Here's how I test:</p> <pre><code>public static void main(String [] files) { for (String file : files) { File f = new File(file); if (f.exists()) { System.err.println(file + " exists"); continue; } try { // find out the current time, I would hope to assume that the last-modified // time on the file will definitely be later than this System.out.println("-----------------------------------------"); long time = System.currentTimeMillis(); // create the file System.out.println("Creating " + file + " at " + time); f.createNewFile(); // let's see what the timestamp actually is (I've only seen it &lt;time) System.out.println(file + " was last modified at: " + f.lastModified()); // well, ok, what if I explicitly set it to time? f.setLastModified(time); System.out.println("Updated modified time on " + file + " to " + time + " with actual " + f.lastModified()); } catch (IOException e) { System.err.println("Unable to create file"); } } } </code></pre> <p>And here's what I get for output:</p> <pre><code>----------------------------------------- Creating test.7 at 1272324597956 test.7 was last modified at: 1272324597000 Updated modified time on test.7 to 1272324597956 with actual 1272324597000 ----------------------------------------- Creating test.8 at 1272324597957 test.8 was last modified at: 1272324597000 Updated modified time on test.8 to 1272324597957 with actual 1272324597000 ----------------------------------------- Creating test.9 at 1272324597957 test.9 was last modified at: 1272324597000 Updated modified time on test.9 to 1272324597957 with actual 1272324597000 </code></pre> <p>The result is a race condition:</p> <ol> <li>JPoller records time of last check as xyz...123</li> <li>File created at xyz...456</li> <li>File last-modified timestamp actually reads xyz...000</li> <li>JPoller looks for new/updated files with timestamp greater than xyz...123</li> <li>JPoller ignores newly added file because xyz...000 is less than xyz...123</li> <li>I pull my hair out for a while</li> </ol> <p>I tried digging into the code but both <a href="http://java.sun.com/javase/6/docs/api/java/io/File.html#lastModified()" rel="nofollow noreferrer"><code>lastModified()</code></a> and <a href="http://java.sun.com/javase/6/docs/api/java/io/File.html#createNewFile()" rel="nofollow noreferrer"><code>createNewFile()</code></a> eventually resolve to native calls so I'm left with little information.</p> <p>For <code>test.9</code>, <b>I lose 957 milliseconds</b>. What kind of accuracy can I expect? Are my results going to vary by operating system or file system? Suggested workarounds?</p> <p>NOTE: I'm currently running Linux with an XFS filesystem. I wrote a quick program in <code>C</code> and the <a href="http://linux.die.net/man/2/stat" rel="nofollow noreferrer">stat system call</a> shows <code>st_mtime</code> as <code>truncate(xyz...000/1000)</code>.</p> <p><strong>UPDATE</strong>: I ran the same program I have above on Windows 7 with NTFS and it <strong>does</strong> maintain full millisecond accuracy. The <a href="http://msdn.microsoft.com/en-us/library/ms724290%28VS.85%29.aspx" rel="nofollow noreferrer">MSDN link</a> @mdma provided further notes that FAT filesystems is accurate for creates with 10 ms resolution but for access is only accurate to 2 seconds. Thus, this is truly OS dependent.</p>
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