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

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?

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.

Other answers are right that you must watch a directory and filter for your particular file. However, you probably want a thread running in the background. The accepted answer can block indefinitely on watchService.take(); and doesn't close the WatchService. A solution suitable for a separate thread might look like:

public class FileWatcher extends Thread {
    private final File file;
    private AtomicBoolean stop = new AtomicBoolean(false);

    public FileWatcher(File file) {
        this.file = file;
    }

    public boolean isStopped() { return stop.get(); }
    public void stopThread() { stop.set(true); }

    public void doOnChange() {
        // Do whatever action you want here
    }

    @Override
    public void run() {
        try (WatchService watcher = FileSystems.getDefault().newWatchService()) {
            Path path = file.toPath().getParent();
            path.register(watcher, StandardWatchEventKinds.ENTRY_MODIFY);
            while (!isStopped()) {
                WatchKey key;
                try { key = watcher.poll(25, TimeUnit.MILLISECONDS); }
                catch (InterruptedException e) { return; }
                if (key == null) { Thread.yield(); continue; }

                for (WatchEvent<?> event : key.pollEvents()) {
                    WatchEvent.Kind<?> kind = event.kind();

                    @SuppressWarnings("unchecked")
                    WatchEvent<Path> ev = (WatchEvent<Path>) event;
                    Path filename = ev.context();

                    if (kind == StandardWatchEventKinds.OVERFLOW) {
                        Thread.yield();
                        continue;
                    } else if (kind == java.nio.file.StandardWatchEventKinds.ENTRY_MODIFY
                            && filename.toString().equals(file.getName())) {
                        doOnChange();
                    }
                    boolean valid = key.reset();
                    if (!valid) { break; }
                }
                Thread.yield();
            }
        } catch (Throwable e) {
            // Log or rethrow the error
        }
    }
}

I tried working from the accepted answer and this article. You should be able to use this thread with new FileWatcher(new File("/home/me/myfile")).start() and stop it by calling stopThread() on the thread.

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.

Apache offers a FileWatchDog class with a doOnChange method.

private class SomeWatchFile extends FileWatchdog {

	protected SomeWatchFile(String filename) {
		super(filename);
	}

	@Override
	protected void doOnChange() {
		fileChanged= true;
	}

}

And where ever you want you can start this thread:

SomeWatchFile someWatchFile = new SomeWatchFile (path);
someWatchFile.start();

The FileWatchDog class polls a file's lastModified() timestamp. The native WatchService from Java NIO is more efficient, since notifications are immediate.

You cannot watch an individual file directly but you can filter out what you don't need.

Here is my FileWatcher class implementation:

import java.io.File;
import java.nio.file.*;
import java.nio.file.WatchEvent.Kind;

import static java.nio.file.StandardWatchEventKinds.*;

public abstract class FileWatcher
{
	private Path folderPath;
	private String watchFile;

	public FileWatcher(String watchFile)
	{
		Path filePath = Paths.get(watchFile);

		boolean isRegularFile = Files.isRegularFile(filePath);

		if (!isRegularFile)
		{
			// Do not allow this to be a folder since we want to watch files
			throw new IllegalArgumentException(watchFile + " is not a regular file");
		}

		// This is always a folder
		folderPath = filePath.getParent();

		// Keep this relative to the watched folder
		this.watchFile = watchFile.replace(folderPath.toString() + File.separator, "");
	}

	public void watchFile() throws Exception
	{
		// We obtain the file system of the Path
		FileSystem fileSystem = folderPath.getFileSystem();

		// We create the new WatchService using the try-with-resources block
		try (WatchService service = fileSystem.newWatchService())
		{
			// We watch for modification events
			folderPath.register(service, ENTRY_MODIFY);

			// Start the infinite polling loop
			while (true)
			{
				// Wait for the next event
				WatchKey watchKey = service.take();

				for (WatchEvent<?> watchEvent : watchKey.pollEvents())
				{
					// Get the type of the event
					Kind<?> kind = watchEvent.kind();

					if (kind == ENTRY_MODIFY)
					{
						Path watchEventPath = (Path) watchEvent.context();

						// Call this if the right file is involved
						if (watchEventPath.toString().equals(watchFile))
						{
							onModified();
						}
					}
				}

				if (!watchKey.reset())
				{
					// Exit if no longer valid
					break;
				}
			}
		}
	}

	public abstract void onModified();
}

To use this, you just have to extend and implement the onModified() method like so:

import java.io.File;

public class MyFileWatcher extends FileWatcher
{
	public MyFileWatcher(String watchFile)
	{
		super(watchFile);
	}

	@Override
	public void onModified()
	{
		System.out.println("Modified!");
	}
}

Finally, start watching the file:

String watchFile = System.getProperty("user.home") + File.separator + "Desktop" + File.separator + "Test.txt";
FileWatcher fileWatcher = new MyFileWatcher(watchFile);
fileWatcher.watchFile();

Not sure about others, but I groan at the amount of code needed to watch a single file for changes using the basic WatchService API. It has to be simpler!

Here are a couple of alternatives using third party libraries:

• Using Apache Commons Configuration

• Using spring-loaded package from the Spring Framework (didn't find an example implementation for this off-hand, but it looks straight-forward to use)

I have created a wrapper around Java 1.7's WatchService that allows registering a directory and any number of glob patterns. This class will take care of the filtering and only emit events you are interested in.

try {
    DirectoryWatchService watchService = new SimpleDirectoryWatchService(); // May throw
    watchService.register( // May throw
            new DirectoryWatchService.OnFileChangeListener() {
                @Override
                public void onFileCreate(String filePath) {
                    // File created
                }
        
                @Override
                public void onFileModify(String filePath) {
                    // File modified
                }
                
                @Override
                public void onFileDelete(String filePath) {
                    // File deleted
                }
            },
            <directory>, // Directory to watch
            <file-glob-pattern-1>, // E.g. "*.log"
            <file-glob-pattern-2>, // E.g. "input-?.txt"
            <file-glob-pattern-3>, // E.g. "config.ini"
            ... // As many patterns as you like
    );
    
    watchService.start(); // The actual watcher runs on a new thread
} catch (IOException e) {
    LOGGER.error("Unable to register file change listener for " + fileName);
}

Complete code is in this repo.