FileWatcher.java

import model.IFileWatchListener;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

import java.io.IOException;
import java.nio.file.*;
import java.util.ArrayList;
import java.util.List;

public class FileWatcher extends Thread {
    private static final Logger LOG = LogManager.getLogger(FileWatcher.class.getName());
    private Path path;
    private List<IFileWatchListener> listeners = new ArrayList<>();
    private String fileToObserve;

    public FileWatcher(String path, String fileToObserve) {
        this.path = Paths.get(path);
        this.fileToObserve = fileToObserve;
    }

    public void addListeners(IFileWatchListener watchListener) {
        this.listeners.add(watchListener);
    }

    // print the events and the affected file
    private void printEvent(WatchEvent<?> event) {
        WatchEvent.Kind<?> kind = event.kind();
        if (kind.equals(StandardWatchEventKinds.ENTRY_MODIFY)) {
            Path pathModified = (Path) event.context();
            if (pathModified.getFileName().toString().equalsIgnoreCase(fileToObserve)) {
                LOG.info("Entry modified: {}", pathModified);
                for (IFileWatchListener listener : listeners) {
                    listener.fileChanged(event);
                }
            }
        }
    }

    @Override
    public void run() {
        try {
            WatchService watchService = path.getFileSystem().newWatchService();
            path.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY);

            // loop forever to watch directory
            while (true) {
                WatchKey watchKey;
                // Obtaining watch keys every 10 seconds
//                final WatchKey key = watchService.poll(10, TimeUnit.SECONDS);
                watchKey = watchService.take(); // this call is blocking until events are present

                // poll for file system events on the WatchKey
                watchKey.pollEvents().forEach(this::printEvent);

                // if the watched directed gets deleted, get out of run method
                if (!watchKey.reset()) {
                    LOG.info("No longer valid");
                    watchKey.cancel();
                    watchService.close();
                    break;
                }
            }

        } catch (InterruptedException ex) {
            LOG.error("interrupted. Goodbye");
            return;
        } catch (IOException ex) {
            LOG.error(ex.getMessage(), ex);
            return;
        }
    }
}

IFileWatchListener.java

import java.nio.file.WatchEvent;

public interface IFileWatchListener {
    void fileChanged(WatchEvent<?> event);
}