OpenSearchUrlQueue urlQueue = waitingQueue.poll();
        if (urlQueue != null) {
            if (crawlingQueue.size() > maxCrawlingQueueSize) {
                crawlingQueue.poll();
            }
            crawlingQueue.add(urlQueue);
            return urlQueue;
        }

        synchronized (queueHolder) {
            urlQueue = waitingQueue.poll();
            if (urlQueue == null) {

     *
     * @see org.codelibs.fess.crawler.service.UrlQueueService#poll(java.lang.String)
     */
    @Override
    public UrlQueueImpl<Long> poll(final String sessionId) {
        final Queue<UrlQueueImpl<Long>> urlQueueList = dataHelper.getUrlQueueList(sessionId);
        synchronized (urlQueueList) {
            return urlQueueList.poll();
        }
    }

    /*
     * (non-Javadoc)
     *

     * @param newUrlQueueList The list of new URL queues.
     */
    void offerAll(String sessionId, List<QUEUE> newUrlQueueList);

    /**
     * Polls a URL queue.
     *
     * @param sessionId The session ID.
     * @return The URL queue.
     */
    QUEUE poll(String sessionId);

    /**
     * Saves the session.
     *
     * @param sessionId The session ID.
     */
    void saveSession(String sessionId);

    }

    @Override
    public synchronized Map<String, Object> read() {
        while (!isFinished.get() && queue.isEmpty()) {
            addDocumentToQueue();
        }

        return queue.poll();
    }

    @Override
    public void close() {
        isFinished.set(true);
        queue.clear();
    }

    /**
     * Sets the scroll size.
     * @param scrollSize The scroll size.

        convertedTexts.add(text);

        for (final ReadingConverter converter : converters) {
            String s;
            while ((s = queue.poll()) != null && convertedTexts.size() <= getMaxReadingNum()) {
                final List<String> results = converter.convert(s, field, lang);
                convertedTexts.addAll(results);
            }

            UrlQueue urlQueue;
            while ((urlQueue = urlQueueService.poll(sessionId1)) != null) {
                assertTrue(urlQueue.getUrl() + "=>" + url1, urlQueue.getUrl().startsWith(url1));
            }
            while ((urlQueue = urlQueueService.poll(sessionId2)) != null) {
                assertTrue(urlQueue.getUrl() + "=>" + url2, urlQueue.getUrl().startsWith(url2));
            }

                return;
            }
            response = r;

            executeCallbacks = new ArrayList<>(doneCallbacks.size());
            Consumer<RESPONSE> callback;
            while ((callback = doneCallbacks.poll()) != null) {
                executeCallbacks.add(callback);
            }
        }
        if (executeCallbacks.size() > 0) {
            try {

            UrlQueue urlQueue;
            while ((urlQueue = urlQueueService.poll(sessionId1)) != null) {
                assertTrue(urlQueue.getUrl().startsWith(url1));
            }
            while ((urlQueue = urlQueueService.poll(sessionId2)) != null) {
                assertTrue(urlQueue.getUrl().startsWith(url2));
            }

            UrlQueue urlQueue;
            while ((urlQueue = urlQueueService.poll(sessionId1)) != null) {
                assertTrue(urlQueue.getUrl().startsWith(url1));
            }
            while ((urlQueue = urlQueueService.poll(sessionId2)) != null) {
                assertTrue(urlQueue.getUrl().startsWith(url2));
            }

 * to perform its tasks.
 * </p>
 *
 * <p>
 * The crawling process involves the following steps:
 * </p>
 * <ol>
 *   <li>Fetching a URL from the queue using {@link UrlQueueService#poll(String)}.</li>
 *   <li>Checking if the URL is valid using {@link #isValid(UrlQueue)}.</li>
 *   <li>Accessing the content using a {@link CrawlerClient} obtained from {@link CrawlerClientFactory}.</li>