private JimfsPath checkWatchable(Watchable watchable) {
    if (!(watchable instanceof JimfsPath) || !isSameFileSystem((Path) watchable)) {
      throw new IllegalArgumentException(
          "watchable ("
              + watchable
              + ") must be a Path "
              + "associated with the same file system as this watch service");
    }

    return (JimfsPath) watchable;
  }
Map of keys to the most recent directory snapshot for each key.
jimfs/src/main/java/com/google/common/jimfs/PollingWatchService.java