How to watch a folder and subfolders for changes

Question

I´m trying to watch a specific folder for changes, and then if any addition/edition/removal happens inside of it, I need to get the change type of all files in that folder and its subfolders. I'm using WatchService for this but it only watches a single path, it doesn't handle subfolders.

Here's my approach:

try {
        WatchService watchService = pathToWatch.getFileSystem().newWatchService();
        pathToWatch.register(watchService, StandardWatchEventKinds.ENTRY_CREATE,
                StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_DELETE);

        // loop forever to watch directory
        while (true) {
            WatchKey watchKey;
            watchKey = watchService.take(); // This call is blocking until events are present

            // Create the list of path files
            ArrayList<String> filesLog = new ArrayList<String>();
            if(pathToWatch.toFile().exists()) {
                File fList[] = pathToWatch.toFile().listFiles();
                for (int i = 0; i < fList.length; i++) { 
                    filesLog.add(fList[i].getName());
                }
            }

            // Poll for file system events on the WatchKey
            for (final WatchEvent<?> event : watchKey.pollEvents()) {
                printEvent(event);
            }

            // Save the log
            saveLog(filesLog);

            if(!watchKey.reset()) {
                System.out.println("Path deleted");
                watchKey.cancel();
                watchService.close();
                break;
            }
        }

    } catch (InterruptedException ex) {
        System.out.println("Directory Watcher Thread interrupted");
        return;
    } catch (IOException ex) {
        ex.printStackTrace();  // Loggin framework
        return;
    }

Like I said before, I'm getting the log only for the files in the selected path, and I want to watch all folders and subfolders files, something like:

Example 1:

FileA (Created)
FileB
FileC
FolderA FileE
FolderA FolderB FileF

Example 2:

FileA
FileB (Modified)
FileC
FolderA FileE
FolderA FolderB FileF

Is there any better solution?

Answer 1

A WatchService only watches the Paths you register. It does not go through those paths recursively.

Given /Root as a registered path

/Root
    /Folder1
    /Folder2
        /Folder3

If there is a change in Folder3, it won't catch it.

You can register the directory paths recursively yourself with

private void registerRecursive(final Path root) throws IOException {
    // register all subfolders
    Files.walkFileTree(root, new SimpleFileVisitor<Path>() {
        @Override
        public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
            dir.register(watchService, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);
            return FileVisitResult.CONTINUE;
        }
    });
}

Now the WatchService will notify all changes in all subfolders of Path root, ie. the Path argument you pass.

Answer 2

Registering recursively will work as Sotirios has indicated. This effectively registers each directory/sub-directory that currently exists.

You can alternatively import and use *com.sun.nio.file.ExtendedWatchEventModifier.FILE_TREE* as in:

dir.register(watcher, standardEventsArray, ExtendedWatchEventModifier.FILE_TREE);

This will watch the entire sub-tree for change AND account for added directories and sub-directories.

Otherwise you will have to monitor for any new directories/sub-directories and register them also. There can also be an issue with deleting parts of the directory hierarchy since each registered directory has a handle watching it so the (lowest) sub-directories need to be removed first when deleting parts of the structure.

Answer 3

I have implemented something like this using Java 8 streams and lambdas.

The recursive folder discovery is implemented as a Consumer @FunctionalInterface:

    final Map<WatchKey, Path> keys = new HashMap<>();

    Consumer<Path> register = p -> {
        if (!p.toFile().exists() || !p.toFile().isDirectory()) {
            throw new RuntimeException("folder " + p + " does not exist or is not a directory");
        }
        try {
            Files.walkFileTree(p, new SimpleFileVisitor<Path>() {
                @Override
                public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
                    LOG.info("registering " + dir + " in watcher service");
                    WatchKey watchKey = dir.register(watcher, new WatchEvent.Kind[]{ENTRY_CREATE}, SensitivityWatchEventModifier.HIGH);
                    keys.put(watchKey, dir);
                    return FileVisitResult.CONTINUE;
                }
            });
        } catch (IOException e) {
            throw new RuntimeException("Error registering path " + p);
        }
    };

The above code is called every time a new folder is created to dynamically add folders at later stages. Full solution and more details here.