* </p>
 *
 * <pre>
 * {@code
 * FtpClient ftpClient = new FtpClient();
 * ftpClient.init(); // Initialize with configured parameters
 * ResponseData responseData = ftpClient.doGet("ftp://example.com/path/to/file.txt");
 * // Process the responseData
 * ftpClient.close(); // Close and release resources
 * }
 * </pre>
 *
 * @author shinsuke
 */
public class FtpClient extends AbstractCrawlerClient {

                .singleton("smbClient", SmbClient.class)//
                .singleton("smb1Client", org.codelibs.fess.crawler.client.smb1.SmbClient.class)//
                .singleton("ftpClient", FtpClient.class)//
                .singleton("storageClient", StorageClient.class)//
                .singleton("clientFactory", CrawlerClientFactory.class);
        clientFactory = container.getComponent("clientFactory");

Pattern-based client selection using `LinkedHashMap<Pattern, CrawlerClient>`.

**Standard Patterns**:
```java
"^https?://.*"     → httpClient
"^file:.*"         → fileSystemClient
"^ftp://.*"        → ftpClient
"^smb://.*"        → smbClient
"^storage://.*"    → storageClient
```

### Services

**UrlQueueService**: URL queue management (FIFO), duplicate detection via `visited()`

- **CrawlerThread**: Individual crawler thread implementation

#### Client Architecture
- **HcHttpClient**: HTTP/HTTPS client using Apache HttpComponents
- **FileSystemClient**: File system access
- **FtpClient**: FTP protocol support
- **SmbClient**: SMB/CIFS network shares
- **StorageClient**: Cloud storage integration

#### Content Processing Pipeline
- **Extractors**: Content extraction from various formats