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Java WatchService API Tutorial

In this precedent, we will figure out how to watch an index alongside all sub-registries and records inside it, utilizing java 8 WatchService API.

How to enroll Java 8 WatchService

To Register WatchService, get the registry way and use path.register() method.

Path way = Paths.get(".");
WatchService watchService =  path.getFileSystem().newWatchService();
path.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY);

Watching for change events

To get the progressions occured on registry and documents inside it, use watchKey.pollEvents() technique which return the accumulation of all change occasions in type of stream.

WatchKey watchKey = null;
while (true) {
    watchKey = watchService.poll(10, TimeUnit.MINUTES);
    if(watchKey != null) {
        watchKey.pollEvents().stream().forEach(event - > System.out.println(event.context()));

The key stays legitimate until:

  • It is dropped, expressly, by conjuring its drop technique, or
  • Cancelled certainly, in light of the fact that the item is never again available, or
  • By shutting the watch service.
  • Watch Directory, Sub-registries and Files for Changes Example

    In this model, we will see a case of viewing a catalog with all sub-registries and documents inside it. We will keep up a guide of watch keys and registries Map<WatchKey, Path> keys to effectively distinguish which index has been modified.

    Below strategy will enroll a solitary catalog to watcher and after that store the registry and key inside a map.

    private void registerDirectory(Path dir) throws IOException
        WatchKey key = dir.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);
        keys.put(key, dir);
    We will call this strategy recursively while strolling down a registry structure and calling this for every catalog we experience.
    private void walkAndRegisterDirectories(final Path begin) throws IOException {
        //register registry and sub-directories
        Files.walkFileTree(start, new SimpleFileVisitor<Path>() {
            public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
                return FileVisitResult.CONTINUE;

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