java 49 lines · 8 steps

Watching a directory with Java NIO

A small AutoCloseable wrapper around WatchService that reports file creates, modifies, and deletes as they happen.

Explained by highlit
Intermediate file-io event-loop resource-management nio blocking-queue
1package com.example.fs;
2 
3import java.io.IOException;
4import java.nio.file.FileSystems;
5import java.nio.file.Path;
6import java.nio.file.StandardWatchEventKinds;
7import java.nio.file.WatchEvent;
8import java.nio.file.WatchKey;
9import java.nio.file.WatchService;
10 
11public class DirectoryWatcher implements AutoCloseable {
12 
13 private final Path directory;
14 private final WatchService watchService;
15 
16 public DirectoryWatcher(Path directory) throws IOException {
17 this.directory = directory;
18 this.watchService = FileSystems.getDefault().newWatchService();
19 directory.register(
20 watchService,
21 StandardWatchEventKinds.ENTRY_CREATE,
22 StandardWatchEventKinds.ENTRY_MODIFY,
23 StandardWatchEventKinds.ENTRY_DELETE);
24 }
25 
26 public void watch() throws InterruptedException {
27 while (true) {
28 WatchKey key = watchService.take();
29 for (WatchEvent<?> event : key.pollEvents()) {
30 WatchEvent.Kind<?> kind = event.kind();
31 if (kind == StandardWatchEventKinds.OVERFLOW) {
32 continue;
33 }
34 @SuppressWarnings("unchecked")
35 WatchEvent<Path> ev = (WatchEvent<Path>) event;
36 Path changed = directory.resolve(ev.context());
37 System.out.printf("%s -> %s%n", kind.name(), changed);
38 }
39 if (!key.reset()) {
40 break;
41 }
42 }
43 }
44 
45 @Override
46 public void close() throws IOException {
47 watchService.close();
48 }
49}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1WatchService turns filesystem changes into a blocking event stream you consume in a loop.
  2. 2Calling reset() after draining a WatchKey is mandatory — skip it and you stop receiving events.
  3. 3Implementing AutoCloseable lets try-with-resources guarantee the native watch handle is released.

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