Cách nhận thông báo Android khi ứng dụng đã đóng hoàn toàn

Giới thiệu

Để nhận thông báo khi app bị kill (swipe khỏi recent apps), cần dùng **Foreground Service** kết hợp với **Firebase Cloud Messaging (FCM)** hoặc **WorkManager/AlarmManager** cho local notification. Bài này demo cách dùng Service chạy nền tạo thông báo định kỳ.

Kiến trúc

• MainActivity: UI chính, nút "Đóng app để test".
• NotificationService (extends Service): Chạy nền, dùng Timer/Handler tạo thông báo mỗi 5 giây.
• START_STICKY: System restart service nếu bị kill do bộ nhớ thấp.

Code hoàn chỉnh

1. activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="https://schemas.android.com/apk/res/android"
    xmlns:tools="https://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp"
    tools:context=".MainActivity">

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:onClick="closeApp"
        android:text="Đóng app để test thông báo nền" />

</RelativeLayout>

2. MainActivity.java

package com.example.backgroundnotification;

import android.content.Intent;
import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import android.view.View;

public class MainActivity extends AppCompatActivity {

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

    @Override
    protected void onStop() {
        super.onStop();
        // Khởi động service khi app không còn foreground
        startService(new Intent(this, NotificationService.class));
    }

    public void closeApp(View view) {
        finishAffinity(); // Đóng hoàn toàn app
    }
}

3. NotificationService.java

package com.example.backgroundnotification;

import android.app.Notification;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.os.Build;
import android.os.Handler;
import android.os.IBinder;
import android.os.Looper;
import androidx.annotation.Nullable;
import androidx.core.app.NotificationCompat;
import java.util.Timer;
import java.util.TimerTask;

public class NotificationService extends Service {

    private static final String CHANNEL_ID = "bg_notif_channel";
    private static final String CHANNEL_NAME = "Background Notifications";
    private Timer timer;
    private final Handler handler = new Handler(Looper.getMainLooper());

    @Override
    public void onCreate() {
        super.onCreate();
        createNotificationChannel();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        startTimer();
        return START_STICKY; // Restart nếu bị kill
    }

    private void createNotificationChannel() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel channel = new NotificationChannel(
                    CHANNEL_ID, CHANNEL_NAME, NotificationManager.IMPORTANCE_DEFAULT);
            NotificationManager manager = getSystemService(NotificationManager.class);
            manager.createNotificationChannel(channel);
        }
    }

    private void startTimer() {
        timer = new Timer();
        timer.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                handler.post(this::showNotification);
            }
        }, 5000, 5000); // Delay 5s, repeat 5s
    }

    private void showNotification() {
        Intent intent = new Intent(this, MainActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);
        PendingIntent pendingIntent = PendingIntent.getActivity(
                this, 0, intent, PendingIntent.FLAG_IMMUTABLE);

        Notification notification = new NotificationCompat.Builder(this, CHANNEL_ID)
                .setSmallIcon(R.drawable.ic_launcher_foreground)
                .setContentTitle("App đang chạy nền")
                .setContentText("Thông báo từ Service - " + System.currentTimeMillis())
                .setContentIntent(pendingIntent)
                .setAutoCancel(true)
                .build();

        NotificationManager manager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        manager.notify((int) System.currentTimeMillis(), notification);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        if (timer != null) {
            timer.cancel();
            timer = null;
        }
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

4. AndroidManifest.xml

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

    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" /> <!-- Android 13+ -->

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

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

        <service
            android:name=".NotificationService"
            android:exported="false"
            android:foregroundServiceType="dataSync" />

    </application>
</manifest>

Lưu ý quan trọng (Android 8.0+)

• Foreground Service bắt buộc: Service chạy nền lâu dài phải gọi startForeground() với notification ongoing. Code trên chỉ demo đơn giản, thực tế cần upgrade thành Foreground Service.
• Doze Mode / App Standby: Timer có thể bị delay. Dùng WorkManager hoặc AlarmManager.setExactAndAllowWhileIdle() cho timing chính xác.
• FCM (Firebase Cloud Messaging): Cách chuẩn cho push notification từ server khi app đóng. Data message vẫn xử lý được khi app kill.
• Android 13+ (API 33): Cần quyền POST_NOTIFICATIONS runtime permission.

Cải tiến cho Production

// Trong onStartCommand - bắt buộc cho Foreground Service
Notification ongoingNotif = new NotificationCompat.Builder(this, CHANNEL_ID)
        .setContentTitle("Đang chạy nền")
        .setContentText("Nhận thông báo...")
        .setSmallIcon(R.drawable.ic_launcher)
        .setOngoing(true)
        .build();
startForeground(1, ongoingNotif); // ID = 1