Watcher.java

import java.io.IOException;
import java.nio.file.FileSystems;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;
import java.util.List;
import java.util.function.Consumer;
import java.util.stream.Stream;

import static java.util.stream.Collectors.toList;
import static java.nio.file.StandardWatchEventKinds.*;

public final class Watcher {
    private static final WatchEvent.Kind<?>[] WATCH_EVENTS = {
        ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY};

    private final Path mWatchDirectory;

    public Watcher(Path watchDirectory) {
        mWatchDirectory = watchDirectory;
    }

    public void watch(final Consumer<WatchEvent<?>> consumer) throws IOException {
        try (WatchService watchService = FileSystems.getDefault().newWatchService()) {
            mWatchDirectory.register(watchService, WATCH_EVENTS);
            WatchKey key;
            do {
                key = watchService.take();
                key.pollEvents().forEach(consumer);
            } while (key.reset());
        } catch (InterruptedException e) {
            throw new IOException(e);
        }
    }

    public static void main(String... args) throws IOException {
        if (args.length != 1) throw new IllegalArgumentException("usage: java Watcher DIR");
        new Watcher(Paths.get(args[0])).watch(event -> System.out.format(
                 "%s %s\n", event.kind(), event.context()));
    }
}