* @return The session ID of the crawling process.
     */
    public String execute() {
        parentThread = new Thread(this, "Crawler-" + crawlerContext.sessionId);
        parentThread.setDaemon(daemon);
        parentThread.start();
        if (!background) {
            awaitTermination();
        }
        return crawlerContext.sessionId;
    }

    /**

                    exceptionSet.add(e);
                } finally {
                    done.set(true);
                }
            }, Thread.currentThread().getName() + "-pdf");
            task.setDaemon(isDaemonThread);
            task.start();
            task.join(timeout);
            if (!done.get()) {
                for (int i = 0; i < 100 && !done.get(); i++) {
                    task.interrupt();