* This method is thread-safe and adds the session ID to be cleaned up later.
     *
     * @param sessionId the crawler session ID that has finished processing
     */
    public void addFinishedSessionId(final String sessionId) {
        synchronized (finishedSessionIdList) {
            finishedSessionIdList.add(sessionId);
        }
    }

    /**

                        crawlerStatusList.set(i, Constants.DONE);
                        final String sid = crawlerList.get(i).getCrawlerContext().getSessionId();
                        indexUpdater.addFinishedSessionId(sid);
                        activeCrawlerNum--;
                    }
                }
                ThreadUtil.sleep(crawlingExecutionInterval);
            }