WatchService：ファイル変更の監視

WatchService とは

import java.nio.file.*;

WatchService watcher = FileSystems.getDefault().newWatchService();
Path dir = Path.of("/home/user/watched");

dir.register(watcher,
    StandardWatchEventKinds.ENTRY_CREATE,
    StandardWatchEventKinds.ENTRY_MODIFY,
    StandardWatchEventKinds.ENTRY_DELETE
);
System.out.println("Watching: " + dir);

イベントのポーリング

while (true) {
    WatchKey key;
    try {
        key = watcher.take(); // blocks until events arrive
    } catch (InterruptedException e) {
        break;
    }
    
    for (WatchEvent<?> event : key.pollEvents()) {
        WatchEvent.Kind<?> kind = event.kind();
        Path changed = ((WatchEvent<Path>) event).context();
        System.out.println(kind.name() + ": " + changed);
    }
    
    key.reset(); // must reset to receive further events
}

ノンブロッキングポーリング

// Non-blocking — returns null if no events
WatchKey key = watcher.poll();
if (key != null) {
    // process events
    key.reset();
}

// Wait up to 5 seconds:
WatchKey key2 = watcher.poll(5, java.util.concurrent.TimeUnit.SECONDS);
if (key2 != null) {
    // process events
    key2.reset();
}

OVERFLOWイベント

for (WatchEvent<?> event : key.pollEvents()) {
    if (event.kind() == StandardWatchEventKinds.OVERFLOW) {
        System.out.println("OVERFLOW: some events missed, rescanning directory");
        // rescan dir to find current state
        continue;
    }
    // process normal events
}

完全な動作例

class FileWatcher implements Runnable {
    private final WatchService watcher;
    
    FileWatcher(Path dir) throws IOException {
        watcher = FileSystems.getDefault().newWatchService();
        dir.register(watcher,
            StandardWatchEventKinds.ENTRY_CREATE,
            StandardWatchEventKinds.ENTRY_MODIFY,
            StandardWatchEventKinds.ENTRY_DELETE);
    }
    
    @Override
    public void run() {
        while (!Thread.currentThread().isInterrupted()) {
            try {
                WatchKey key = watcher.take();
                for (WatchEvent<?> e : key.pollEvents())
                    System.out.println(e.kind() + ": " + e.context());
                key.reset();
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }
    }
}

複数のディレクトリの監視

Map<WatchKey, Path> keyMap = new HashMap<>();

for (Path dir : List.of(Path.of("/logs"), Path.of("/config"))) {
    WatchKey key = dir.register(watcher,
        StandardWatchEventKinds.ENTRY_MODIFY);
    keyMap.put(key, dir);
}

WatchKey key = watcher.take();
Path dir = keyMap.get(key);
System.out.println("Changed in: " + dir);

完全なパスの解決

WatchEvent<Path> pathEvent = (WatchEvent<Path>) event;
Path filename = pathEvent.context();
Path fullPath = watchedDir.resolve(filename);
System.out.println("Full path: " + fullPath);

ディレクトリの再帰的な監視

WatchService watcher = FileSystems.getDefault().newWatchService();
Path root = Path.of("/project");

try (var stream = Files.walk(root)) {
    stream.filter(Files::isDirectory)
          .forEach(dir -> {
              try { dir.register(watcher, StandardWatchEventKinds.ENTRY_CREATE,
                  StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_DELETE);
              } catch (IOException e) { throw new RuntimeException(e); }
          });
}

リソースのクリーンアップ

try (WatchService watcher = FileSystems.getDefault().newWatchService()) {
    Path dir = Path.of("/tmp/watch");
    dir.register(watcher, StandardWatchEventKinds.ENTRY_CREATE);
    
    WatchKey key = watcher.poll(10, java.util.concurrent.TimeUnit.SECONDS);
    if (key != null) {
        key.pollEvents().forEach(e -> System.out.println(e.context()));
        key.reset();
    }
} // watcher.close() called automatically

実践例：設定のホットリロード

class ConfigWatcher {
    private volatile Map<String, String> config = new HashMap<>();
    
    void loadConfig(Path file) throws IOException {
        config = Files.readAllLines(file).stream()
            .filter(l -> l.contains("="))
            .collect(Collectors.toMap(
                l -> l.split("=")[0].trim(),
                l -> l.split("=")[1].trim(),
                (a,b) -> b
            ));
    }
    
    // Call loadConfig again on ENTRY_MODIFY event
}

WatchServiceの制限

確認問題

復習：WatchService