startForeground fail after upgrade to Android 8.1

Java Solution (Android 9.0, API 28)

In your Service class, add this:

@Override
public void onCreate(){
    super.onCreate();
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O)
        startMyOwnForeground();
    else
        startForeground(1, new Notification());
}

private void startMyOwnForeground(){
    String NOTIFICATION_CHANNEL_ID = "com.example.simpleapp";
    String channelName = "My Background Service";
    NotificationChannel chan = new NotificationChannel(NOTIFICATION_CHANNEL_ID, channelName, NotificationManager.IMPORTANCE_NONE);
    chan.setLightColor(Color.BLUE);
    chan.setLockscreenVisibility(Notification.VISIBILITY_PRIVATE);
    NotificationManager manager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    assert manager != null;
    manager.createNotificationChannel(chan);

    NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(this, NOTIFICATION_CHANNEL_ID);
    Notification notification = notificationBuilder.setOngoing(true)
            .setSmallIcon(R.drawable.icon_1)
            .setContentTitle("App is running in background")
            .setPriority(NotificationManager.IMPORTANCE_MIN)
            .setCategory(Notification.CATEGORY_SERVICE)
            .build();
    startForeground(2, notification);
}

UPDATE: ANDROID 9.0 PIE (API 28)

Add this permission to your AndroidManifest.xml file:

<uses-permission android:name="android.permission.FOREGROUND_SERVICE" />

The first answer is great only for those people who know kotlin, for those who still using java here I translate the first answer

 public Notification getNotification() {
        String channel;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O)
            channel = createChannel();
        else {
            channel = "";
        }
        NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(this, channel).setSmallIcon(android.R.drawable.ic_menu_mylocation).setContentTitle("snap map fake location");
        Notification notification = mBuilder
                .setPriority(PRIORITY_LOW)
                .setCategory(Notification.CATEGORY_SERVICE)
                .build();


        return notification;
    }

    @NonNull
    @TargetApi(26)
    private synchronized String createChannel() {
        NotificationManager mNotificationManager = (NotificationManager) this.getSystemService(Context.NOTIFICATION_SERVICE);

        String name = "snap map fake location ";
        int importance = NotificationManager.IMPORTANCE_LOW;

        NotificationChannel mChannel = new NotificationChannel("snap map channel", name, importance);

        mChannel.enableLights(true);
        mChannel.setLightColor(Color.BLUE);
        if (mNotificationManager != null) {
            mNotificationManager.createNotificationChannel(mChannel);
        } else {
            stopSelf();
        }
        return "snap map channel";
    } 

For android, P don't forget to include this permission

<uses-permission android:name="android.permission.FOREGROUND_SERVICE" />

After some tinkering for a while with different solutions i found out that one must create a notification channel in Android 8.1 and above.

private fun startForeground() {
    val channelId =
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
                createNotificationChannel("my_service", "My Background Service")
            } else {
                // If earlier version channel ID is not used
                // https://developer.android.com/reference/android/support/v4/app/NotificationCompat.Builder.html#NotificationCompat.Builder(android.content.Context)
                ""
            }

    val notificationBuilder = NotificationCompat.Builder(this, channelId )
    val notification = notificationBuilder.setOngoing(true)
            .setSmallIcon(R.mipmap.ic_launcher)
            .setPriority(PRIORITY_MIN)
            .setCategory(Notification.CATEGORY_SERVICE)
            .build()
    startForeground(101, notification)
}

@RequiresApi(Build.VERSION_CODES.O)
private fun createNotificationChannel(channelId: String, channelName: String): String{
    val chan = NotificationChannel(channelId,
            channelName, NotificationManager.IMPORTANCE_NONE)
    chan.lightColor = Color.BLUE
    chan.lockscreenVisibility = Notification.VISIBILITY_PRIVATE
    val service = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
    service.createNotificationChannel(chan)
    return channelId
}

From my understanding background services are now displayed as normal notifications that the user then can select to not show by deselecting the notification channel.

Update: Also don't forget to add the foreground permission as required Android P:

<uses-permission android:name="android.permission.FOREGROUND_SERVICE" />