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

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

I'm using JPoller 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:

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 <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");         }     } } 

And here's what I get for output:

----------------------------------------- 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 

The result is a race condition:

  1. JPoller records time of last check as xyz...123
  2. File created at xyz...456
  3. File last-modified timestamp actually reads xyz...000
  4. JPoller looks for new/updated files with timestamp greater than xyz...123
  5. JPoller ignores newly added file because xyz...000 is less than xyz...123
  6. I pull my hair out for a while

I tried digging into the code but both lastModified() and createNewFile() eventually resolve to native calls so I'm left with little information.

For test.9, I lose 957 milliseconds. What kind of accuracy can I expect? Are my results going to vary by operating system or file system? Suggested workarounds?

NOTE: I'm currently running Linux with an XFS filesystem. I wrote a quick program in C and the stat system call shows st_mtime as truncate(xyz...000/1000).

UPDATE: I ran the same program I have above on Windows 7 with NTFS and it does maintain full millisecond accuracy. The MSDN link @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.

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FAT has a 2-second resolution for creation time, and NTFS can delay updating the last access time by up to an hour.

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(details on MSDN.) Although not in your case, in general, there is also the problem of synchronizing clocks if the file is created on another computer..
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It seems this might be an issue for the JPoller folks, since this is where the time handling logic is.

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The last-modified timestamp is apparently stored in seconds, not in milliseconds.

That might be a file system limitation.

I'd suggest to compare it against seconds rather than milliseconds..

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