/**
     * Waits for the crawling process to terminate for a specified duration.
     * @param millis The maximum time to wait in milliseconds. A value of 0 means wait indefinitely.
     */
    public void awaitTermination(final long millis) {
        if (parentThread != null) {
            try {
                parentThread.join(millis);
            } catch (final InterruptedException e) {