CRAWLING_EXCEPTION,
    /** Indicates no URL is available in the queue. */
    NO_URL_IN_QUEUE,
    /** Indicates the start of a crawler thread. */
    START_THREAD,
    /** Indicates the finish of a crawler thread. */
    FINISHED_THREAD,
    /** Indicates no response processor is available. */
    NO_RESPONSE_PROCESSOR,
    /** Indicates no rule is available for processing. */
    NO_RULE,

                    task.interrupt();
                    ThreadUtil.sleep(100L);
                }
                throw new ExtractException("PDFBox process cannot finish in " + timeout + " sec.");
            }
            if (!exceptionSet.isEmpty()) {
                throw exceptionSet.iterator().next();
            }
            writer.flush();