How to find “last modified time” of a folder depending on changes in its files and subfolders in java

Question

I want to continuously monitor a folder (say C:/Users/me/rootFolder) for any change in its content -

1. New files added,
2. Sub-folder changes
3. Existing file modified

I have used watch services from java.nio package and am able to obtain result for the root folder. But how can we use watch services for the whole directory tree?

Also, I am searching a way to do this operation using timestamp instead of watchservice as it is the clients requirement.

Here is the code which I have written for root directory.

 public void watchServices() throws IOException,
        InterruptedException
{
    destFolder = LoginController.destFolder;
    logger.info("Started Watching Folder");
    Path watchFolder = Paths.get(sourceFolder);
    WatchService watchService = FileSystems.getDefault().newWatchService();
    watchFolder.register(watchService, StandardWatchEventKinds.ENTRY_CREATE);
    boolean valid = true;
    do {
        WatchKey watchKey = watchService.take();
        for (WatchEvent event : watchKey.pollEvents()) {
            WatchEvent.Kind kind = event.kind();
            if (StandardWatchEventKinds.ENTRY_CREATE.equals(kind))
            {
                String fileName = event.context().toString();
                File file = new File(watchFolder + "\\" + fileName);
                    if (file.isFile())
                    {
                        logger.info("uploading file to cdi-dms server: " + 
                     file.getAbsolutePath());
                        uploadFile(file,destFolder);
                    }
                    else if(file.isDirectory())
                    {
                        logger.info("creating directory in cdi-dms server: " 
                      + file.getAbsolutePath());
                        createDirectory(file,destFolder);
                        destFolder = destFolder+file.getName()+"/";
                        sync(file);
                    }
                    }
                   }
                  valid = watchKey.reset();
               }while (valid);

                }