java-native-access/jna · error · Error

FileMonitor not implemented for " + os

Error message

FileMonitor not implemented for " + os

What it means

FileMonitor's Holder static initializer throws an Error when the os.name system property does not start with "Windows", because only a W32FileMonitor implementation exists in this code path. Loading FileMonitor.getInstance() on any non-Windows platform aborts class initialization.

Source

Thrown at contrib/platform/src/com/sun/jna/platform/FileMonitor.java:138

    protected void finalize() {
        for (File watchedFile : watched.keySet()) {
            removeWatch(watchedFile);
        }

        dispose();
    }

    /** Canonical lazy loading of a singleton. */
    private static class Holder {
        public static final FileMonitor INSTANCE;
        static {
            String os = System.getProperty("os.name");
            if (os.startsWith("Windows")) {
                INSTANCE = new W32FileMonitor();
            }
            else {
                throw new Error("FileMonitor not implemented for " + os);
            }
        }
    }

    public static FileMonitor getInstance() {
        return Holder.INSTANCE;
    }
}

View on GitHub (pinned to d036ad9781)

Solutions

1. Only call FileMonitor.getInstance() after verifying Platform.isWindows()
2. Use a cross-platform file watcher (java.nio.file.WatchService) on non-Windows platforms
3. Check os.name at startup and disable file-monitor features on unsupported OSes

Example fix

// before
FileMonitor fm = FileMonitor.getInstance();
// after
if (Platform.isWindows()) {
    FileMonitor fm = FileMonitor.getInstance();
} else {
    WatchService ws = FileSystems.getDefault().newWatchService();
}

if (!System.getProperty("os.name", "").startsWith("Windows")) {
    throw new IllegalStateException("FileMonitor requires Windows");
}

boolean fileMonitorSupported() {
    return Platform.isWindows();
}

try {
    monitor = FileMonitor.getInstance();
} catch (Throwable t) {
    monitor = null; // fall back to WatchService
}

When it happens

Trigger: Calling FileMonitor.getInstance() (or otherwise triggering Holder class init) on macOS, Linux, or any OS whose name does not start with "Windows".

Common situations: Running directory/file watching code developed for Windows on a Linux CI server or a Mac developer machine; os.name overridden via -Dos.name=... causing even Windows to fail.

Understand the failure class

Background: "unsupported platform" / "not supported on this platform" errors: what they mean and how to fix them — this error's family across 47 libraries.

Related errors

• No support (yet) for " + System.getProperty("os.name")
• No trash location found (define fileutils.trash to be the pa
• The following files could not be trashed: " + failed
• KeyboardUtils requires a keyboard
• Can't open X Display

AI-assisted analysis of java-native-access/jna@d036ad9781 (2026-09-12). Data as JSON: /api/errors/4ff00dff46f4f698. Report an issue: GitHub.