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WatchService race condition?

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  WatchService race condition? Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-07-20 15:09 -0400
    Re: WatchService race condition? Arne Vajhøj <arne@vajhoej.dk> - 2013-07-20 16:46 -0400

#24779 — WatchService race condition?

     I had occasion recently to use java.nio.file.WatchService
for the first time, and have run across a problem I hope more
experienced practitioners can solve.  How do I iterate over the
current contents of a directory and then get notified of new
arrivals?  If I do

	iterate_over_the_files();
	register_with_watchservice();

... then there's a race: Any file created after iteration (or
after the directory gets read) but before registration gets
missed.  But if I do

	register_with_watchservice();
	iterate_over_the_files();

... then there's a different race: Any file created after
registration but before the iteration slurps the directory
is processed twice.

     I wound up keeping a Set of the files already processed,
and using the WatchService only as a "This would be a good
time to run another iteration" notice.  But that's clumsy: It
feels like there ought to be a sleeker way.  Any ideas?

     (Application: One program periodically fetches data from a
URL and dumps it into a time-stamped file in a known directory.
Another program reads the time-stamped files and extracts data
into a JFreeChart, which it would like to update whenever the
first program deposits a new file.  It's working, but ... ick.)

-- 
Eric Sosman
esosman@comcast-dot-net.invalid

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#24781

On 7/20/2013 3:09 PM, Eric Sosman wrote:
>      I had occasion recently to use java.nio.file.WatchService
> for the first time, and have run across a problem I hope more
> experienced practitioners can solve.  How do I iterate over the
> current contents of a directory and then get notified of new
> arrivals?  If I do
>
>      iterate_over_the_files();
>      register_with_watchservice();
>
> ... then there's a race: Any file created after iteration (or
> after the directory gets read) but before registration gets
> missed.  But if I do
>
>      register_with_watchservice();
>      iterate_over_the_files();
>
> ... then there's a different race: Any file created after
> registration but before the iteration slurps the directory
> is processed twice.
>
>      I wound up keeping a Set of the files already processed,
> and using the WatchService only as a "This would be a good
> time to run another iteration" notice.  But that's clumsy: It
> feels like there ought to be a sleeker way.  Any ideas?
>
>      (Application: One program periodically fetches data from a
> URL and dumps it into a time-stamped file in a known directory.
> Another program reads the time-stamped files and extracts data
> into a JFreeChart, which it would like to update whenever the
> first program deposits a new file.  It's working, but ... ick.)

Either do as you do now or use a between processes locking
mechanism (like FileChannel lock).

Arne

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