在Java中可以使用Servlet来返回文件流给前端。以下是一个简单的示例代码：

@WebServlet("/download")public class DownloadServlet extends HttpServlet {protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {String filePath = "/path/to/your/file.txt";File file = new File(filePath);response.setContentType("application/octet-stream");response.setHeader("Content-Disposition", "attachment; filename=" + file.getName());FileInputStream fileInputStream = new FileInputStream(file);OutputStream outputStream = response.getOutputStream();byte[] buffer = new byte[4096];int bytesRead = -1;while ((bytesRead = fileInputStream.read(buffer)) != -1) {outputStream.write(buffer, 0, bytesRead);}fileInputStream.close();outputStream.close();}}

在上面的代码中，我们首先指定了要下载的文件路径，然后设置了响应的Content-Type和Content-Disposition头信息，将文件流写入到响应的输出流中，最后关闭输入输出流。当浏览器请求/download路径时，就会弹出下载文件的对话框，用户可以选择保存文件或直接打开文件。