/**
 * Allow a previously configured network to be associated with. If
 * <code>disableOthers</code> is true, then all other configured
 * networks are disabled, and an attempt to connect to the selected
 * network is initiated. This may result in the asynchronous delivery
 * of state change events.
 * <p>
 * <b>Note:</b> If an application's target SDK version is
 * {@link android.os.Build.VERSION_CODES#LOLLIPOP} or newer, network
 * communication may not use Wi-Fi even if Wi-Fi is connected; traffic may
 * instead be sent through another network, such as cellular data,
 * Bluetooth tethering, or Ethernet. For example, traffic will never use a
 * Wi-Fi network that does not provide Internet access (e.g. a wireless
 * printer), if another network that does offer Internet access (e.g.
 * cellular data) is available. Applications that need to ensure that their
 * network traffic uses Wi-Fi should use APIs such as
 * {@link Network#bindSocket(java.net.Socket)},
 * {@link Network#openConnection(java.net.URL)}, or
 * {@link ConnectivityManager#bindProcessToNetwork} to do so.
 *
 * @param netId the ID of the network in the list of configured networks
 * @param disableOthers if true, disable all other networks. The way to
 * select a particular network to connect to is specify {@code true}
 * for this parameter.
 * @return {@code true} if the operation succeeded
 */

从第七行开始，大概意思就是，在应用目标版本大于或等于LOLLIPOP（5.0） 就算wifi是已连接的，网络通信也可能不用wifi，比喻说蜂窝数据。当 wifi 与蜂窝数据同时存在时，当wifi无法使用时，系统会自动切换到蜂窝数据。这不就是我们出现的问题吗，下面赶紧找解决办法。接着看，app确保使用wifi进行通信，应该使用下面三个方法 APIbindSocket、openConnection、bindProcessToNetwork。这个是不是说的有点抽象，就说三个方法，不告诉到底怎么样。额，我们只有迅速补脑了。

    ConnectivityManager mConnectivityManager = (ConnectivityManager)Context.getSystemService(Context.CONNECTIVITY_SERVICE);
    final NetworkRequest networkRequest = new NetworkRequest.Builder()
            .addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
            .addTransportType(NetworkCapabilities.TRANSPORT_WIFI)
            .build();
    mConnectivityManager.requestNetwork(networkRequest, new ConnectivityManager.NetworkCallback(){
        @Override
        public void onAvailable(Network network) {
            String url = "";
            try {
                HttpURLConnection conn = (HttpURLConnection) network.openConnection(new URL(url));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });