Posted by Rich Hyndman, Android Developer Advocate

With the growth of the Internet of Things, connecting Android applications to Wi-Fi enabled devices is becoming more and more common. Whether you’re building an app for a remote viewfinder, to set up a connected light bulb, or to control a quadcopter, if it’s Wi-Fi based you will need to connect to a hotspot that may not have Internet connectivity.

From Lollipop onwards the OS became a little more intelligent, allowing multiple network connections and not routing data to networks that don’t have Internet connectivity. That’s very useful for users as they don’t lose connectivity when they’re near Wi-Fis with captive portals. Data routing APIs were added for developers, so you can ensure that only the appropriate app traffic is routed over the Wi-Fi connection to the external device.

To make the APIs easier to understand, it is good to know that there are 3 sets of networks available to developers:

WiFiManager#startScan returns a list of available Wi-Fi networks. They are primarily identified by SSID.

WiFiManager#getConfiguredNetworks returns a list of the Wi-Fi networks configured on the device, also indexed by SSID, but they are not necessarily currently available.

ConnectivityManager#getAllNetworks returns a list of networks that are being interacted with by the phone. This is necessary as from Lollipop onwards a device may be connected to multiple networks at once, Wi-Fi, LTE, Bluetooth, etc… The current state of each is available by calling ConnectivityManager#getNetworkInfo and is identified by a network ID.

In all versions of Android you start by scanning for available Wi-Fi networks with WiFiManager#startScan, iterate through the ScanResults looking for the SSID of your external Wi-Fi device. Once you’ve found it you can check if it is already a configured network using WifiManager#getConfiguredNetworks and iterating through the WifiConfigurations returned, matching on SSID. It’s worth noting that the SSIDs of the configured networks are enclosed in double quotes, whilst the SSIDs returned in ScanResults are not.

If your network is configured you can obtain the network ID from the WifiConfiguration object. Otherwise you can configure it using WifiManager#addNetwork and keep track of the network id that is returned.

To connect to the Wi-Fi network, register a BroadcastReceiver that listens for WifiManager.NETWORK_STATE_CHANGED_ACTION and then call WifiManager.enableNetwork (int netId, boolean disableOthers), passing in your network ID. The enableNetwork call disables all the other Wi-Fi access points for the next scan, locates the one you’ve requested and connects to it. When you receive the network broadcasts you can check with WifiManager#getConnectionInfo that you’re successfully connected to the correct network. But, on Lollipop and above, if that network doesn’t have internet connectivity network, requests will not be routed to it.

Routing network requests

To direct all the network requests from your app to an external Wi-Fi device, call ConnectivityManager#setProcessDefaultNetwork on Lollipop devices, and on Marshmallow call ConnectivityManager#bindProcessToNetwork instead, which is a direct API replacement. Note that these calls require android.permission.INTERNET; otherwise they will just return false.

Alternatively, if you’d like to route some of your app traffic to the Wi-Fi device and some to the Internet over the mobile network:

For HTTP requests you can use Network#openConnection(java.net.URL), directly routing your request to this network.

For low-level socket communication, open a socket and call Network#bindSocket(java.net.Socket), or alternatively use Network#getSocketFactory.

Now you can keep your users connected whilst they benefit from your innovative Wi-Fi enabled products.