The Watch Service API in Java (from java.nio.file) allows a program to monitor a directory for file system events such as creation, deletion, or modification of files. It provides a non-blocking, event-driven way to react to changes in the file system, making it ideal for tasks like auto-reload, logging, and file synchronization.

Common Use Cases:

Sample Usage: Watch for File Events 

import java.io.IOException;
import java.nio.file.*;

public class WatchServiceExample {
    public static void main(String[] args) throws IOException, InterruptedException {
        Path pathToWatch = Paths.get("your/directory");
        WatchService watchService = FileSystems.getDefault().newWatchService();

        pathToWatch.register(
            watchService,
            StandardWatchEventKinds.ENTRY_CREATE,
            StandardWatchEventKinds.ENTRY_DELETE,
            StandardWatchEventKinds.ENTRY_MODIFY
        );

        System.out.println("Watching directory: " + pathToWatch);

        while (true) {
            WatchKey key = watchService.take(); // blocks until an event occurs

            for (WatchEvent<?> event : key.pollEvents()) {
                WatchEvent.Kind<?> kind = event.kind();
                Path changed = (Path) event.context();
                System.out.println("Event kind: " + kind + " - File affected: " + changed);
            }

            boolean valid = key.reset();
            if (!valid) {
                break; // Exit if directory is inaccessible
            }
        }
    }
}

Key Concepts:

Component Purpose
WatchService Core service to monitor file system events
WatchKey Represents a registration and tracks events
WatchEvent.Kind<?> Type of event (create, modify, delete)
StandardWatchEventKinds Constants for supported event types

Limitations:


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