写一个文件下载的controller,service

下面是一个简单的文件下载的controller和service的示例代码，使用Spring框架和Java语言编写：

Controller:

@RestController
public class FileDownloadController {

  @Autowired
  private FileDownloadService fileDownloadService;

  @GetMapping("/download")
  public ResponseEntity<Resource> downloadFile(@RequestParam("filename") String filename) {
    Resource file = fileDownloadService.downloadFile(filename);
    return ResponseEntity.ok()
        .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + file.getFilename() + "\"")
        .body(file);
  }
}

Service:

@Service
public class FileDownloadServiceImpl implements FileDownloadService {

  @Value("${file.upload-dir}")
  private String uploadDir;

  @Override
  public Resource downloadFile(String filename) {
    Path file = Paths.get(uploadDir).resolve(filename);
    Resource resource;
    try {
      resource = new UrlResource(file.toUri());
    } catch (MalformedURLException e) {
      throw new RuntimeException("Error: " + e.getMessage());
    }
    if (resource.exists() || resource.isReadable()) {
      return resource;
    } else {
      throw new RuntimeException("File not found: " + filename);
    }
  }
}

其中，`FileDownloadController`处理HTTP请求，并调用`FileDownloadService`提供的下载文件功能。`FileDownloadService`实现了下载文件的具体逻辑，包括查找文件，创建`Resource`对象并返回。在示例代码中，上传文件存储在`uploadDir`指定的目录下，具体目录路径在配置文件中进行配置。