/** Value for HC5 client selection. */
    public static final String HC5_CLIENT = "hc5";

    /** The HC4 HTTP client instance. */
    protected CrawlerClient hc4Client;

    /** The HC5 HTTP client instance. */
    protected CrawlerClient hc5Client;

    /** Flag indicating which client is selected (true = hc5, false = hc4). */
    private boolean useHc5 = true;

    /**

                    config.initializeClientFactory(() -> ComponentUtil.getComponent(CrawlerClientFactory.class));
            final CrawlerClient client = crawlerClientFactory.getClient(url);
            if (client == null) {
                throw new ThumbnailGenerationException("No CrawlerClient: " + configId + ", url: " + url);
            }
            String u = url;
            for (int i = 0; i < maxRedirectCount; i++) {

import org.codelibs.core.misc.Pair;
import org.codelibs.fess.app.service.FailureUrlService;
import org.codelibs.fess.crawler.builder.RequestDataBuilder;
import org.codelibs.fess.crawler.client.CrawlerClient;
import org.codelibs.fess.crawler.entity.RequestData;
import org.codelibs.fess.crawler.entity.ResponseData;
import org.codelibs.fess.crawler.entity.UrlQueue;
import org.codelibs.fess.crawler.log.LogType;

import org.codelibs.core.io.CloseableUtil;
import org.codelibs.core.lang.StringUtil;
import org.codelibs.core.lang.SystemUtil;
import org.codelibs.fess.crawler.builder.RequestDataBuilder;
import org.codelibs.fess.crawler.client.CrawlerClient;
import org.codelibs.fess.crawler.client.CrawlerClientFactory;
import org.codelibs.fess.crawler.container.CrawlerContainer;
import org.codelibs.fess.crawler.entity.AccessResult;

        final CrawlerClientFactory crawlerClientFactory = crawlingConfig.initializeClientFactory(ComponentUtil::getCrawlerClientFactory);
        final CrawlerClient client = crawlerClientFactory.getClient(url);
        if (client == null) {
            throw new CrawlingAccessException(
                    "CrawlerClient is null for URL: " + url + ". Unable to access the document without a crawler client.");
        }

                return;
            }

            final String url = dataMap.get(fessConfig.getIndexFieldUrl()).toString();
            final CrawlerClient client = crawlerClientFactory.getClient(url);
            if (client == null) {
                logger.warn("CrawlerClient not available for url='{}'. Protocol may not be supported.", url);
                return;
            }

└── fess-crawler-opensearch/   # OpenSearch backend
```

### Key Design Patterns

- **Factory**: `CrawlerClientFactory`, `ExtractorFactory` - protocol/format-specific component selection
- **Strategy**: `CrawlerClient`, `Extractor`, `Transformer` - pluggable implementations
- **Builder**: `RequestDataBuilder`, `ExtractorBuilder` - fluent construction
- **Template Method**: `AbstractCrawlerClient`, `AbstractExtractor` - common logic with overrides

                config.initializeClientFactory(() -> ComponentUtil.getComponent(CrawlerClientFactory.class));
        final CrawlerClient client = crawlerClientFactory.getClient(url);
        if (client == null) {
            throw new FessSystemException("No CrawlerClient: " + configId + ", url: " + url);
        }
        return writeContent(configId, url, client);
    }

    /**

     */
    public void setCharset(final String charset) {
        this.charset = charset;
    }

    /*
     * (non-Javadoc)
     *
     * @see org.codelibs.fess.crawler.client.CrawlerClient#doGet(java.lang.String)
     */
    @Override
    public ResponseData doGet(final String uri) {
        return processRequest(uri, true);
    }

    /**
     * Executes a HEAD request for the given URL.