WatchService

Uses polling on MacOS may miss rapid event changes.

GPT response

The WatchService in Java uses different mechanisms depending on the operating system. On macOS, the WatchService implementation generally relies on polling rather than true event-based notifications.

Why does WatchService use polling on macOS?

  1. Native Limitations: The Java NIO WatchService was designed with Windows' ReadDirectoryChangesW and Linux's inotify in mind, both of which provide efficient event-based file change notifications. macOS does not have a direct equivalent in its POSIX-compliant API.
  2. Polling Implementation: As a result, the implementation on macOS often falls back to a polling mechanism, where it periodically checks for changes in the directory's metadata (like timestamps or file lists).

Implications of Polling

  • Performance: Polling is less efficient than event-based mechanisms because it consumes more CPU resources, especially for directories with many files.
  • Latency: Polling introduces a delay in detecting changes, which depends on the polling interval.

Alternatives for Event-Based File Watching on macOS

If you need a more efficient and event-driven approach, consider using native tools or libraries that wrap macOS's FSEvents or kqueue APIs:

  1. Third-Party Libraries:
    • JNotify: Provides a cross-platform wrapper for native file system events.
    • Watchdog: Cross-platform file watching tool in Go with better macOS support.
  2. Native Integration: Use FSEvents or kqueue directly through JNI or with a library like JNA.

For robust file watching on macOS, leveraging native tools or libraries tailored to the platform can avoid the inefficiencies of Java's default WatchService implementation.

In practice when attempting to use kfswatch (which used WatchService for MacOS), when batch of 1000 events was tested only 602 were caught.

While when testing fswatch we could reliably catch 5000 events.