How can I create an android service that dalvik will not kill?

Before getting into example, we should know what service is in android. Service is going to do back ground operation without interact with UI and it works even after activity destroy.
This example demonstrate about How can I create an android service that dalvik will not kill.
Step 1 − Create a new project in Android Studio, go to File ⇒ New Project and fill all required details to create a new project.
Step 2 − Add the following code to res/layout/activity_main.xml.
<?xml version = "1.0" encoding = "utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android = "http://schemas.android.com/apk/res/android"
   xmlns:app = "http://schemas.android.com/apk/res-auto"
   xmlns:tools = "http://schemas.android.com/tools"
   android:layout_width = "match_parent"
   android:layout_height = "match_parent"
   tools:context = ".MainActivity">
   <TextView
      android:id = "@+id/text"
      android:layout_width = "wrap_content"
      android:layout_height = "wrap_content"
      android:text = "Start Service"
      android:textSize = "25sp"
      app:layout_constraintBottom_toBottomOf = "parent"
      app:layout_constraintLeft_toLeftOf = "parent"
      app:layout_constraintRight_toRightOf = "parent"
      app:layout_constraintTop_toTopOf = "parent" />
</android.support.constraint.ConstraintLayout>
In the above code, we have taken text view, when user click on text view, it will start service and stop service.
Step 3 − Add the following code to src/MainActivity.java
package com.example.andy.myapplication;
import android.app.ActivityManager;
import android.content.Context;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      final TextView text = findViewById(R.id.text);
      text.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View v) {
            if (isMyServiceRunning(service.class)) {
               text.setText("Stoped");
               stopService(new Intent(MainActivity.this, service.class));
            } else {
               text.setText("Started");
               startService(new Intent(MainActivity.this, service.class));
            }
         }
      });
   }
   private boolean isMyServiceRunning(Class<?> serviceClass) {
      ActivityManager manager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
      for (ActivityManager.RunningServiceInfo service : manager.getRunningServices(Integer.MAX_VALUE)) {
         if (serviceClass.getName().equals(service.service.getClassName())) {
            return true;
         }
      }
      return false;
   }
}
In the above code to start and stop service. We have used intent and passed context and service class. Now create a service class in package folder as service.class and add the following code –
package com.example.andy.myapplication;
import android.annotation.SuppressLint;
import android.annotation.TargetApi;
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.graphics.Color;
import android.media.MediaPlayer;
import android.os.Build;
import android.os.IBinder;
import android.os.PowerManager;
import android.support.annotation.RequiresApi;
import android.support.v4.app.NotificationCompat;
import android.util.Log;
import android.widget.Toast;
public class service extends Service {
   PowerManager pm;
   @SuppressLint("InvalidWakeLockTag")
   PowerManager.WakeLock wl;
   @Override
   public IBinder onBind(Intent intent) {
      return null;
   }
   @SuppressLint("InvalidWakeLockTag")
   @Override
   public void onCreate() {
      super.onCreate();
      pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
      wl = pm.newWakeLock(PowerManager.SCREEN_DIM_WAKE_LOCK, "My Tag");
   }
   @TargetApi(Build.VERSION_CODES.O)
   @RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN)
   @Override
   public int onStartCommand(Intent intent, int flags, int startId) {
      wl.acquire();
      //Do some task
      Toast.makeText(this, "Notification Service started by user.", Toast.LENGTH_LONG).show();
      return START_STICKY;
   }
   @RequiresApi(api = Build.VERSION_CODES.N)
   @Override
   public void onDestroy() {
      super.onDestroy();
      wl.release();
      Toast.makeText(this, "Notification Service destroyed by user.", Toast.LENGTH_LONG).show();
   }
}
In the above code we have called power manager to stop screen lock so android thread not going to stop the service thread as shown below –
@SuppressLint("InvalidWakeLockTag")
@Override
public void onCreate() {
   super.onCreate();
   pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
   wl = pm.newWakeLock(PowerManager.SCREEN_DIM_WAKE_LOCK, "My Tag");
}
@TargetApi(Build.VERSION_CODES.O)
@RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN)
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
   wl.acquire();
   //Do some task
   Toast.makeText(this, "Notification Service started by user.", Toast.LENGTH_LONG).show();
   return START_STICKY;
}
@RequiresApi(api = Build.VERSION_CODES.N)
@Override
public void onDestroy() {
   super.onDestroy();
   wl.release();
   Toast.makeText(this, "Notification Service destroyed by user.", Toast.LENGTH_LONG).show();
}
Step 4 − Add the following code to manifest.xml
<?xml version = "1.0" encoding = "utf-8"?>
<manifest xmlns:android = "http://schemas.android.com/apk/res/android"
   package = "com.example.andy.myapplication">
   <uses-permission android:name = "android.permission.WAKE_LOCK"/>
   <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/AppTheme">
      <activity android:name = ".MainActivity">
         <intent-filter>
            <action android:name = "android.intent.action.MAIN" />
            <category android:name = "android.intent.category.LAUNCHER" />
         </intent-filter>
      </activity>
      <service android:name = ".service"/>
   </application>
</manifest>
Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from android studio, open one of your project's activity files and click Run  icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen –
In the above result is an initial screen, Click on Text view, it will start Notification service as shown below –
In the above result, service is startd now click on text view, it will stop notification service as shown below -
Click here to download the project code