Android: Force data to be sent over cellular network vs WiFi

(I answered this same question here)

You can't explicitly force the communications channel on a per-app basis (you can request to use a preferred mode via ConnectivityManager.setNetworkPreference(...), but that's not "forcing").

Though it's probably terrible UX, you can inform the user that your app disallows use of WiFi, then disable their WiFi if they want to continue. To do so, you need the ACCESS_WIFI_STATE and CHANGE_WIFI_STATE permissions. The code would look something like this:

manager = (WifiManager)this.getSystemService(Context.WIFI_SERVICE);

if(manager.isWifiEnabled()) {
    manager.setWifiEnabled(false);
}
// and to be sure:
ConnectivityManager.setNetworkPreference(ConnectivityManager.TYPE_MOBILE);