Java WebAPI可以通過使用Servlet來實現文件上傳下載。以下是一個簡單的示例：

1. 文件上傳

@WebServlet("/upload")
@MultipartConfig
public class FileUploadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Part filePart = request.getPart("file");
        String fileName = filePart.getSubmittedFileName();
        InputStream fileContent = filePart.getInputStream();
        
        // 保存文件到指定目錄
        File file = new File("C:/upload/" + fileName);
        try (OutputStream out = new FileOutputStream(file)) {
            IOUtils.copy(fileContent, out);
        } catch (IOException e) {
            e.printStackTrace();
        }
        
        response.getWriter().println("文件上傳成功");
    }
}

2. 文件下載

@WebServlet("/download")
public class FileDownloadServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String fileName = "example.txt";
        File file = new File("C:/upload/" + fileName);
        
        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition", "attachment; filename=\"" + fileName + "\"");
        
        try (InputStream in = new FileInputStream(file);
             OutputStream out = response.getOutputStream()) {
            IOUtils.copy(in, out);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

在以上示例中，通過MultipartConfig注解標記文件上傳Servlet，并使用Part對象獲取上傳的文件，然后將文件保存到指定目錄。在文件下載Servlet中，設置響應的Content-Type和Content-Disposition頭，然后將文件內容寫入到輸出流中實現文件下載。