Can I watch for single file change with WatchService (not the whole directory)?

Question

When I'm trying to register a file instead of a directory java.nio.file.NotDirectoryException is thrown. Can I listen for a single file change, not the whole directory?

Answer 1

Just filter the events for the file you want in the directory:

final Path path = FileSystems.getDefault().getPath(System.getProperty("user.home"), "Desktop"); System.out.println(path); try (final WatchService watchService = FileSystems.getDefault().newWatchService()) {     final WatchKey watchKey = path.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY);     while (true) {         final WatchKey wk = watchService.take();         for (WatchEvent<?> event : wk.pollEvents()) {             //we only register "ENTRY_MODIFY" so the context is always a Path.             final Path changed = (Path) event.context();             System.out.println(changed);             if (changed.endsWith("myFile.txt")) {                 System.out.println("My file has changed");             }         }         // reset the key         boolean valid = wk.reset();         if (!valid) {             System.out.println("Key has been unregisterede");         }     } } 

Here we check whether the changed file is "myFile.txt", if it is then do whatever.

Answer 2

No it isn't possible to register a file, the watch service doesn't work this way. But registering a directory actually watches changes on the directory children (the files and sub-directories), not the changes on the directory itself.

If you want to watch a file, then you register the containing directory with the watch service. Path.register() documentation says:

WatchKey java.nio.file.Path.register(WatchService watcher, Kind[] events, Modifier... modifiers) throws IOException

Registers the file located by this path with a watch service.

In this release, this path locates a directory that exists. The directory is registered with the watch service so that entries in the directory can be watched

Then you need to process events on entries, and detect those related to the file you are interested in, by checking the context value of the event. The context value represents the name of the entry (actually the path of the entry relatively to the path of its parent, which is exactly the child name). You have an example here.