ExtractException (checked)          # Extraction failures
  └─ UnsupportedExtractException
```

### Thread-Local Storage

```java
// Set (in CrawlerThread)
CrawlingParameterUtil.setCrawlerContext(context);
CrawlingParameterUtil.setUrlQueue(urlQueue);

// Get (anywhere in same thread)
CrawlerContext ctx = CrawlingParameterUtil.getCrawlerContext();

// Clear (ALWAYS in finally)