How to Monitor Changes to a Single File Using WatchService in Java

Question

Is it possible to monitor a single file change using WatchService in Java, without watching the entire directory?

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

public class WatchFileExample {
    public static void main(String[] args) throws Exception {
        Path file = Paths.get("path/to/your/file.txt");
        Path dir = file.getParent();
        WatchService watchService = FileSystems.getDefault().newWatchService();
        dir.register(watchService, ENTRY_MODIFY);

        System.out.println("Watching for changes to: " + file);
        while (true) {
            WatchKey key = watchService.take();
            for (WatchEvent<?> event : key.pollEvents()) {
                if (event.kind() == ENTRY_MODIFY) {
                    System.out.println("File modified: " + event.context());
                }
            }
            key.reset();
        }
    }
}

Answer

While the Java NIO WatchService does not allow direct registration of a single file for change notifications, you can monitor the parent directory for modifications to that file. This approach effectively allows you to respond to changes in a specific file without monitoring the entire directory actively.

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

public class WatchFileExample {
    public static void main(String[] args) throws Exception {
        Path file = Paths.get("path/to/your/file.txt");
        Path dir = file.getParent();
        WatchService watchService = FileSystems.getDefault().newWatchService();
        dir.register(watchService, ENTRY_MODIFY);

        System.out.println("Watching for changes to: " + file);
        while (true) {
            WatchKey key = watchService.take();
            for (WatchEvent<?> event : key.pollEvents()) {
                if (event.kind() == ENTRY_MODIFY && event.context().equals(file.getFileName())) {
                    System.out.println("File modified: " + file);
                }
            }
            key.reset();
        }
    }
}

Causes

The NotDirectoryException occurs because the WatchService only registers directories, not individual files.

Solutions

Register the parent directory of the file you want to monitor with the WatchService.
Filter the events received to check if they pertain to the specific file you are interested in.

Common Mistakes

Mistake: Not checking for the specific file in events, leading to catching changes in other files.

Solution: Always compare the event's context with the target file name to ensure you're reacting to the correct file.

Mistake: Forgetting to reset the WatchKey, which can lead to missed events.

Solution: Always call key.reset() at the end of your event loop.

Helpers

Java WatchService
Monitor single file changes Java
NotDirectoryException
Java file change notifications
NIO WatchService example