Basic Watcher class in JAVA for reference.

Watcher.java

/*
This class is a mere template and can be modified for better methods of access of lists and can be written in a more elegant manner.
It is to demonstrate the basics of writing your own watcher, since the JAVA API was causing issues regarding locking of 
files in a directory when multiple events are generated.
*/


import java.io.IOException;
import java.nio.file.DirectoryStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;

public class Watcher {

    private String dir;
    // Contains a list of files which are now old/already present.
    public static List<String> oldFiles;
    // Queue to access list of new files in the directory.
    public static ConcurrentLinkedQueue<String> newFiles;


    // Set the directory to watch
    public Watcher(String dir) {
        this.dir = dir;
        oldFiles = new ArrayList<>();
        newFiles = new ConcurrentLinkedQueue<>();
    }


    // Spawn a background thread to keep track of files being added in the directory.
    public void start() {
        new Thread(new Runnable() {

            // Helper function to list files in a directory.
            private List<String> fileList(String directory) {
                List<String> fileNames = new ArrayList<>();
                try (DirectoryStream<Path> directoryStream = Files.newDirectoryStream(Paths.get(directory))) {
                    for (Path path : directoryStream) {
                        fileNames.add(path.toAbsolutePath().toString());
                    }
                } catch (IOException ex) {
                    ex.printStackTrace();
                }
                return fileNames;
            }

            @Override
            public void run() {
                while (true) {
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    if (!dir.equals(null) && dir.length() != 0) {
                        List<String> files = fileList(dir);
                        // Handle cases: 1st scan and subsequent scans.
                        if (oldFiles.isEmpty()) {
                            newFiles.addAll(files);
                        } else {
                            if (files.removeAll(oldFiles))
                                newFiles.addAll(files);
                        }
                    }
                }
            }
        }).start();
    }
}