Android7.0前，Android系统前网络切换时，会发广播，业务只要监听广播即可。

public class NetChangeReceiver extends BroadcastReceiver {

    private static final String ANDROID_NET_CHANGE_ACTION = "android.net.conn.CONNECTIVITY_CHANGE";

    @Override
    public void onReceive(Context context, Intent intent) {
        if(intent.getAction().equalsIgnoreCase(ANDROID_NET_CHANGE_ACTION)){
            Toast.makeText(context, "Net Changed", Toast.LENGTH_SHORT).show();
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.outman.example.androidtest">

    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
    <uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"/>
    <uses-permission android:name="android.permission.CHANGE_WIFI_STATE"/>

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true">

        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <receiver android:name=".NetChangeReceiver">
            <intent-filter >
                <action android:name="android.net.conn.CONNECTIVITY_CHANGE"/>
            </intent-filter>
        </receiver>
    </application>

</manifest>

上面代码，在Android7.0设备前，当网络切换时，能收到消息。而在Android7.0及以上设备，则无法收到。

Android7.0 行为变更上明确说明

Android 7.0 移除了三项隐式广播，因为隐式广播会在后台频繁启动已注册侦听这些广播的应用。删除这些广播可以显著提升设备性能和用户体验。

为缓解这些问题，Android 7.0 应用了以下优化措施：

面向 Android 7.0 开发的应用不会收到 CONNECTIVITY_ACTION 广播，即使它们已有清单条目来请求接受这些事件的通知。在前台运行的应用如果使用 BroadcastReceiver 请求接收通知，则仍可以在主线程中侦听 CONNECTIVITY_CHANGE。
应用无法发送或接收 ACTION_NEW_PICTURE 或 ACTION_NEW_VIDEO 广播。此项优化会影响所有应用，而不仅仅是面向 Android 7.0 的应用。

解决办法

1. 大家都知道，注册广播有两种方式，一种是在AndroidManifest.xml中，另一种通过register方法。

Android文档中描述，通过在AndroidManifest.xml中注册方式，App在前后台都无法接收到广播。而通过register的方式，当App在运行时，是可以接收到广播的。

Note: A BroadcastReceiver registered with Context.registerReceiver() continues to receive these broadcasts while the app is running.

public class MainActivity extends Activity {

    private NetChangeReceiver netChangeReceiver;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        netChangeReceiver = new NetChangeReceiver();
        registerReceiver(netChangeReceiver, new IntentFilter("android.net.conn.CONNECTIVITY_CHANGE"));
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        unregisterReceiver(netChangeReceiver);
    }
}

2. 提供了一种更稳定的方式

Monitoring Network Connectivity While the App is Running
Apps that are running can still listen for CONNECTIVITY_CHANGE with a registered BroadcastReceiver. However, the ConnectivityManager API provides a more robust method to request a callback only when specified network conditions are met.

NetworkRequest objects define the parameters of the network callback in terms of NetworkCapabilities. You create NetworkRequest objects with the NetworkRequest.Builder class. registerNetworkCallback() then passes the NetworkRequest object to the system. When the network conditions are met, the app receives a callback to execute the onAvailable() method defined in its ConnectivityManager.NetworkCallback class.

The app continues to receive callbacks until either the app exits or it calls unregisterNetworkCallback().

public class MainActivity extends Activity {

    private ConnectivityManager.NetworkCallback networkCallback;
    private ConnectivityManager connectivityManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        networkCallback = new NetworkCallbackImpl();
        NetworkRequest.Builder builder = new NetworkRequest.Builder();
        NetworkRequest request = builder.build();
        connectivityManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
        connectivityManager.registerNetworkCallback(request, networkCallback);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        connectivityManager.unregisterNetworkCallback(networkCallback);
    }

    private class NetworkCallbackImpl extends ConnectivityManager.NetworkCallback {
        @Override
        public void onAvailable(Network network) {
            super.onAvailable(network);
            Toast.makeText(getBaseContext(), "onAvailable", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onLosing(Network network, int maxMsToLive) {
            super.onLosing(network, maxMsToLive);
            Toast.makeText(getBaseContext(), "onLosing", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onLost(Network network) {
            super.onLost(network);
            Toast.makeText(getBaseContext(), "onLost", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onCapabilitiesChanged(Network network, NetworkCapabilities networkCapabilities) {
            super.onCapabilitiesChanged(network, networkCapabilities);
            Toast.makeText(getBaseContext(), "onCapabilitiesChanged", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onLinkPropertiesChanged(Network network, LinkProperties linkProperties) {
            super.onLinkPropertiesChanged(network, linkProperties);
            Toast.makeText(getBaseContext(), "onLinkPropertiesChanged", Toast.LENGTH_SHORT).show();
        }
    }
}

另外补充下：

Android8.0开始，Android将不支持大部分隐式广播，包括自定义的广播和系统的。

需要通过context.regeister的方式注册。