/** The system property name for user dictionary path. */
    public static final String USER_DICT_PATH = "fess.user.dict.path";

    /** The text separator. */
    public static final String TEXT_SEPARATOR = " ";

    /** The default role for guest users. */
    public static final String DEFAULT_ROLE = "_guest_";

    /** The default document type. */

 * including adding URLs to the queue, filtering URLs, managing crawler threads,
 * and handling the overall crawling lifecycle.
 *
 * <p>It implements the Runnable interface to be executed in a separate thread,
 * and the AutoCloseable interface to ensure resources are properly released after use.
 *
 * <p>The crawler uses several services and components, such as UrlQueueService, DataService,

 * processing the response, and extracting child URLs.
 *
 * <p>
 * This class implements the {@link Runnable} interface, allowing it to be executed in a separate thread.
 * It uses various services and components, such as {@link UrlQueueService}, {@link DataService},
 * {@link CrawlerContainer}, {@link LogHelper}, {@link CrawlerClientFactory}, and {@link CrawlerContext},

     * The date of last modification of the file. This date should be in W3C
     * Datetime format. This format allows you to omit the time portion, if
     * desired, and use YYYY-MM-DD.
     *
     * Note that this tag is separate from the If-Modified-Since (304) header
     * the server can return, and search engines may use the information from
     * both sources differently.
     */
    private String lastmod;

    /**

/**
 * PdfExtractor extracts text content from PDF files using Apache PDFBox.
 * It supports password-protected PDFs and can extract embedded documents and annotations.
 *
 * <p>The extractor runs text extraction in a separate thread with a configurable timeout
 * to prevent hanging on problematic PDF files. It also extracts metadata from the PDF
 * document and includes it in the extraction result.
 *
 * <p>Features:
 * <ul>