- **CrawlerContext** (`CrawlerContext.java`): Execution context - `sessionId`, `status`, `accessCount`, `numOfThread`, `maxDepth`, `maxAccessCount`
- **CrawlerThread** (`CrawlerThread.java`): Worker thread - Poll URL → Validate → Execute → Process → Queue children

### HTTP Client Architecture

```
SwitchableHttpClient (extends FaultTolerantClient)

     *
     * @param crawlerThread the thread to wait for; null threads are ignored
     */
    private void joinCrawlerThread(final Thread crawlerThread) {
        if (crawlerThread != null) {
            try {
                crawlerThread.join();
            } catch (final Exception e) {
                logger.info("Interrupted crawling process: name={}", crawlerThread.getName());
            }
        }