*/
    @PreDestroy
    public void destroy() {
        if (!finishCrawling) {
            if (logger.isInfoEnabled()) {
                logger.info("Stopping all crawlers.");
            }
            forceStop();
        }
    }

    /**
     * Adds a finished session ID to the cleanup list.
     * This method is thread-safe and adds the session ID to be cleaned up later.
     *

     */
    public boolean isForceStop() {
        return forceStop.get();
    }

    /**
     * Sets the force-stop state of the system.
     *
     * @param b true to force-stop the system.
     */
    public void setForceStop(final boolean b) {
        forceStop.set(b);
    }

    /**
     * Generates a document ID.
     *