HTTP request to 127.0.0.1:80 fails - dart

Question

Environment
Android Studio latest release
Android Emulator running Android Nougat
MAMP to serve as HTTP server
Problem
My simple method
Future<String> _getSimpleReply( String command, callback, errorCallback ) async {
try {
HttpClientRequest request = await _myClient.get( '127.0.0.1', 80 '/' );
HttpClientResponse response = await request.close();
await callback( response.toString() );
} on SocketException catch( e ) {
errorCallback( e.toString() );
}
}
fails with an error:
cbLoginFailed :: SocketException: OS Error:
Connection refused, errno = 111, address = 127.0.0.1, port = 46414
Currently using the Android emulator, which has been configured for internet access:
<uses-permission android:name="android.permission.INTERNET"/>
For test purpose, the HTTP server is MAMP, which executes the index.php script and returns some simple string, if I enter this link in a browser:
http://127.0.0.1/index.php
The situation does not change, if I replace 127.0.0.1 with localhost.
But if I do change 127.0.0.1 with a valid public FQDN, the method works just fine:
doLoginSuccess :: Instance of '_HttpClientResponse'
Question
Why does the android emulator / flutter / dart handles 127.0.0.1 different than a
valid FQDN?
And how to I change this behaviour?

Answer 1

As pointed out by the first comment, localhost or 127.0.0.1 is - when running an app on Android emulator - in fact the local machine on which the Android app is executing. With respect of the Android app, the local machine is the emulated Android system.
Therefore, the http request from the flutter code doesn't leave the emulator at all. Since the Flutter app doesn't provide a HTTP Server, the emulator refuses the http request.
Need to change to the address of the host machine, on which the emulator is executing.
The Android emulator uses special IP addresses to communicate with its host or the host gateway.
Habe a look in the Android documentation.
E.g., the host‘s loopback Interface is identified by this IP: 10.0.2.2
So, if normally one would use 127.0.0.1:80, one can substitute it with 10.0.2.2:80.

Answer 2

You are trying to access localhost which is your phone/emulator not your PC which I assume is running your test server.
You need to create a network between your PC and phone and use the local IP address instead