private void flush() {
        fileUrlWriter.flush();
        fileCursorWriter.flush();
    }
Store urls and cursor in files so that a Spider can resume the status when shutdown.<br>

@author code4crafter@gmail.com <br>
@since 0.2.0
webmagic-extension/src/main/java/us/codecraft/webmagic/scheduler/FileCacheQueueScheduler.java