public class MyReceiver extends BroadcastReceiver {
  @Override
  public void onReceive(Context context, Intent intent) {
    // TODO Auto-generated method stub
    //Toast.makeText(context, intent.getAction(), 1).show();
    ConnectivityManager manager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo mobileInfo = manager.getNetworkInfo(ConnectivityManager.TYPE_MOBILE);
    NetworkInfo wifiInfo = manager.getNetworkInfo(ConnectivityManager.TYPE_WIFI);
    NetworkInfo activeInfo = manager.getActiveNetworkInfo();
    Toast.makeText(context, "mobile:"+mobileInfo.isConnected()+"\n"+"wifi:"+wifiInfo.isConnected()
            +"\n"+"active:"+activeInfo.getTypeName(), 1).show();
  } //如果无网络连接activeInfo为null
}

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