public class Main {

	public static void main(String[] args) throws Exception {
		new DirWatcher(ClassManager.DIR).watch(new WatcherHandler() {

			@Override
			public void handler() {
        //todo

			}
		});
	}

}


import java.io.IOException;
import java.nio.file.FileSystems;
import java.nio.file.Paths;
import java.nio.file.StandardWatchEventKinds;
import java.nio.file.WatchEvent;
import java.nio.file.WatchEvent.Kind;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;

public class DirWatcher {

	private WatchService service;

	public DirWatcher(String filePath) throws IOException {
		service = FileSystems.getDefault().newWatchService();
		// register create event
		Paths.get(filePath).register(service, StandardWatchEventKinds.ENTRY_CREATE);

	}

	public void watch(WatcherHandler handler) throws InterruptedException {
		while (true) {
			WatchKey key = service.take();
			for (WatchEvent<?> event : key.pollEvents()) {
				Kind<?> kind = event.kind();
				if (kind == StandardWatchEventKinds.ENTRY_CREATE) {
					handler.handler();
				}
			}
			key.reset();
		}
	}

}

public interface WatcherHandler {
	public void handler();
}