Watch for changes on the filesystem.

• HelloWorld.java
• Main.java
• Recursive.java
• Overflow.java

make run

On another terminal:

cd d
mkdir d
touch a b d/a
rm a b d/a
rmdir d

The first terminal shows:

ENTRY_CREATE: d
ENTRY_CREATE: a
ENTRY_MODIFY: a
ENTRY_CREATE: b
ENTRY_MODIFY: b
ENTRY_DELETE: a
ENTRY_DELETE: b
ENTRY_DELETE: d

Note how events under d/d were not considered: operation is non-recursive by default.

• register directories to the watcher service

• each directory registration returns a key corresponding to that directory

• watchService.take returns one of the previously registered keys which has an event. poll is the non-blocking version which may return null.

• each key returns events only for the directory on which it was registered.

  You cannot get the directory from the key because key.watchable() returns a Watchable, so you have to keep a map of key -> directory

  The only JDK implementation of Watchable however is Path.

  TODO confirm: So registering the same path multiple times should return equal keys:

  final WatchService watchService = FileSystems.getDefault().newWatchService();
  final WatchKey key = Paths.get(WATCH_DIR).register(
          watchService,
          StandardWatchEventKinds.ENTRY_MODIFY);
  final WatchKey key2 = Paths.get(WATCH_DIR).register(
          watchService,
          StandardWatchEventKinds.ENTRY_CREATE,
          StandardWatchEventKinds.ENTRY_DELETE,
          StandardWatchEventKinds.ENTRY_MODIFY);
  assert key.equals(key2);
  

Not possible: the best you can do is filter the events.

http://stackoverflow.com/questions/16251273/can-i-watch-for-single-file-change-with-watchservice-not-the-whole-directory

TODO count