How Can I Efficiently Monitor File Changes in Java?

Monitoring File Changes with Java

Detecting file changes in a system is an important task for various applications. However, simply polling the lastModified property of a file is not an optimal solution due to its performance implications.

Fortunately, Java 7 introduced the WatchService API as part of NIO.2, specifically designed to notify applications about file change events. This API offers a more efficient and reliable way to monitor files.

By using WatchService, you can register a directory with the WatchService instance and specify the types of events you want to be notified about, such as file creation, deletion, or modification. When a qualifying event occurs, WatchService wakes up the thread that is listening for these events and provides information about the file that has changed.

Here's an example of how to use WatchService:

    Path dir = Paths.get("/path/to/directory");
    WatchService watchService = FileSystems.getDefault().newWatchService();

    WatchKey key = dir.register(watchService, StandardWatchEventKinds.ENTRY_CREATE, StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_DELETE);

    // Start a thread to listen for events
    ExecutorService executorService = Executors.newSingleThreadExecutor();
    executorService.submit(() -> {
        try {
            for (;;) {
                WatchKey keyFromPoll = watchService.take();
                for (WatchEvent<?> event : keyFromPoll.pollEvents()) {
                    System.out.println("Event occurred: " + event.kind());
                    System.out.println("Context (file affected): " + event.context());
                }
                keyFromPoll.reset();
            }
        } catch (InterruptedException e) {
            System.err.println("Error: " + e.getMessage());
        }
    });

This code registers a directory with the WatchService and starts a thread to continuously listen for file change events. When an event occurs, the thread wakes up, retrieves information about the affected file, and prints it to the console.

By leveraging WatchService, you can effectively monitor file changes in a responsive and efficient manner, without the performance overhead associated with polling.

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