file.createNewFile() creates files with last-modified time before actual creation time
- by Kaleb Pederson
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:
JPoller records time of last check as xyz...123
File created at xyz...456
File last-modified timestamp actually reads xyz...000
JPoller looks for new/updated files with timestamp greater than xyz...123
JPoller ignores newly added file because xyz...000 is less than xyz...123
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).
