<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<intent-filter> 
  <action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
</intent-filter>

@Override
protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  checkNetwork();
  if (!checkNetwork()) {
    Toast.makeText(this, "没有网络", Toast.LENGTH_LONG).show();
    Intent intent = new Intent("android.settings.WIRELESS_SETTINGS");
    startActivity(intent);
    return;
  }
}
private boolean checkNetwork() {
  ConnectivityManager conn = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
  NetworkInfo net = conn.getActiveNetworkInfo();
  if (net != null && net.isConnected()) {
    return true;
  }
  return false;
}