在Java中可以使用Servlet來返回文件流給前端。以下是一個簡單的示例代碼：

@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();
    }
}

在上面的代碼中，我們首先指定了要下載的文件路徑，然后設置了響應的Content-Type和Content-Disposition頭信息，將文件流寫入到響應的輸出流中，最后關閉輸入輸出流。當瀏覽器請求/download路徑時，就會彈出下載文件的對話框，用戶可以選擇保存文件或直接打開文件。