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

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

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