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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: WatchService race condition?
Date: Sat, 20 Jul 2013 15:09:15 -0400
Organization: A noiseless patient Spider
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     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