如何解决Flutter上传图片到Node.js后端的Connection refused错误

错误信息 #

终端错误日志 #

> Performing hot restart... Syncing files to device sdk gphone64 x86
> 64... Restarted application in 1,691ms. D/CompatibilityChangeReporter( 9724): Compat change id reported: 78294732; UID 10162; state: DISABLED
> D/MediaScannerConnection( 9724): Scanned
> /data/user/0/com.example.mongo_backend/cache/2fdd677e-2569-4d9c-8349-b461215be87f8625165904540994379.jpg
> to null
> 
> ======== Exception caught by image resource service ================================================ The following SocketException was thrown resolving an image codec: Connection
> refused (OS Error: Connection refused, errno = 111), address =
> localhost, port = 46640
> 
> When the exception was thrown, this was the stack: 
> #0      _NativeSocket.startConnect (dart:io-patch/socket_patch.dart:682:35)
> #1      _RawSocket.startConnect (dart:io-patch/socket_patch.dart:1817:26)
> #2      RawSocket.startConnect (dart:io-patch/socket_patch.dart:27:23)
> #3      Socket._startConnect (dart:io-patch/socket_patch.dart:2038:22)
> #4      Socket.startConnect (dart:io/socket.dart:759:21)
> #5      _ConnectionTarget.connect (dart:_http/http_impl.dart:2453:20)

应用端错误截图 #

已完成的排查操作 #

• 确认Node.js后端服务器正在运行并监听指定端口（本次为46640）
• 验证无防火墙规则阻止连接

问题 #

如何排查该「Connection refused」错误，成功实现Flutter应用到Node.js后端的图片上传？

排查与解决步骤 #

1. 替换localhost为宿主机IP地址
   模拟器中的localhost指向模拟器自身，而非你的电脑。Android模拟器需用10.0.2.2替代localhost；iOS模拟器若无法用localhost，则换成电脑的局域网IP。例如将请求地址从http://localhost:46640/upload改为http://10.0.2.2:46640/upload（Android）或http://你的电脑IP:46640/upload。

2. 调整Node.js服务器监听地址
   确保后端服务器监听所有网络地址而非仅localhost，修改启动代码：

   // 错误写法：仅监听本地
   app.listen(46640, 'localhost', () => { console.log('Server running') })
   // 正确写法：监听所有外部请求
   app.listen(46640, '0.0.0.0', () => { console.log('Server running') })
   // 或省略地址参数，默认监听0.0.0.0
   app.listen(46640, () => { console.log('Server running') })
   

3. 验证模拟器与后端的连通性
   在模拟器内置浏览器中访问http://10.0.2.2:46640（Android）或电脑IP+端口，若无法打开，说明后端配置仍有问题；若能正常访问，检查Flutter代码中的请求地址是否拼写错误。

4. 核对请求路径与方法
   确认Flutter上传代码的接口路径、请求方法（需为POST）与后端路由完全一致，比如后端路由是/upload，Flutter中不能写成/uploads。

5. 检查Android网络权限
   确保Flutter项目的android/app/src/main/AndroidManifest.xml中添加了网络权限：

   <uses-permission android:name="android.permission.INTERNET" />
   

6. 关闭代理或VPN
   若当前开启了代理或VPN，暂时关闭，避免干扰模拟器与电脑之间的网络连接。

内容的提问来源于stack exchange，提问作者pomoworko.com