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

Space play ←→ step click any line

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.

Related explainers

java

@RestController
@RequestMapping("/api/files")
public class FileUploadController {

Handling secure file uploads in Spring

file-upload input-validation path-traversal

Intermediate 10 steps

java

public final class QueryParams {
 
    private final Map<String, List<String>> params;

Parsing and building URL query strings in Java

parsing multimap url-encoding

Intermediate 9 steps

java

public final class ShippingAddress {
    private final String recipient;
    private final String line1;
    private final String line2;

Building immutable objects with the Builder pattern

builder-pattern immutability validation

Intermediate 7 steps

java

import java.time.Instant;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;

Converting timestamps across time zones in Java

date-time time-zones instant

Intermediate 6 steps

java

package com.example.payments.config;
 
import com.stripe.StripeClient;
import org.springframework.beans.factory.annotation.Value;

Swapping payment gateways by profile in Spring

dependency-injection profiles configuration

Intermediate 6 steps

java

public class EventBus {
 
    private final Map<Class<?>, List<Subscriber>> subscribers = new ConcurrentHashMap<>();
    private final Executor executor;