Send data through wifi (no internet) when mobile data on

Question

I'm developing and application that connects to a hardware device through wifi (generated by the device) and send data to it through a socket connection. The problem is that when mobile data (3G/4G) is activated android tries to send the data through it instead of sending it through the wifi generated by the device, because because the wifi has no internet connection. I was thinking of using ConnectivityManager#setNetworkPreference() but it has been deprecated in api 21.

How can I set it to send data using the wifi generated by the device instead of the mobile data interface?

Answer 1

After a lot of research and time trying to understand it all I found out that this isn't possible in versions previous to Android 5.0 (Lollipop), the OS only keeps one network interface up at a time and the apps don't have control over this.

So to do this on versions greater or equal to the Lollipop I did the following:

1. Before doing your socket connection check if android greater or equal to Lollipop, in case it isn't you just do whatever you have to do normally;

2. In case the version is equal or greater to Lollipop you need to do something like this:

   if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.LOLLIPOP) {
       final ConnectivityManager manager = (ConnectivityManager) context
               .getSystemService(Context.CONNECTIVITY_SERVICE);
       NetworkRequest.Builder builder;
       builder = new NetworkRequest.Builder();
       //set the transport type do WIFI
       builder.addTransportType(NetworkCapabilities.TRANSPORT_WIFI);
       manager.requestNetwork(builder.build(), new ConnectivityManager.NetworkCallback() {
           @Override
           public void onAvailable(Network network) {
               if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                   manager.bindProcessToNetwork(network);
               } else {
                   //This method was deprecated in API level 23
                   ConnectivityManager.setProcessDefaultNetwork(network);
               }
               try {
                   //do a callback or something else to alert your code that it's ok to send the message through socket now
               } catch (Exception e) {
                   e.printStackTrace();
               }
               manager.unregisterNetworkCallback(this);
           }
       });
   }
   

3. After you finish you should stop binding the process with this:

   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
       ConnectivityManager manager = (ConnectivityManager) InovePlugApplication.getContext()
               .getSystemService(Context.CONNECTIVITY_SERVICE);
       manager.bindProcessToNetwork(null);
   } else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
       ConnectivityManager.setProcessDefaultNetwork(null);
   }
   

This answer helped me unsderstand how to do it https://stackoverflow.com/a/29837637/2550932.

Answer 2

Network will switch back to mobile after setProcessDefaultNetwork to wifi, you can't send large data by this way, because all sockets created in this way will cease to work. I don't find a perfect way now, here is for reference only.

1.If you can get root access, do this before connect to wifi:

Process process = runtime.exec("su");
DataOutputStream dataOutputStream = new DataOutputStream(process.getOutputStream());
dataOutputStream.writeBytes("settings put global captive_portal_server 127.0.0.1\n");
dataOutputStream.writeBytes("exit\n");
dataOutputStream.flush();
int status = process.waitFor();

And delete it after you send data:

dataOutputStream.writeBytes("settings delete global captive_portal_server\n");

2.If you can get WRITE_SECURE_SETTINGS permission:

Settings.Global.putString(getContentResolver(),"captive_portal_server","127.0.0.1");

Or:

Settings.Global.putInt(getContentResolver(),"captive_portal_detection_enabled",0);