• A service is a component which runs in the background without interacting with the user.
• It has no interface and is not bound to the lifecycle of an activity.
• They are generally used for long running operations i.e. Internet downloads, checking new data, updating content providers and like.
• Even on closing or crashing of app, the services keep on running. They have a higher priority.
• By default, a service runs in the same process as the main thread of the application.
• Therefore, you need to use asynchronous processing for resource intensive tasks in the background.
• The Android platform has predefined system services and every Android application can use them via getSystemService() method.
• To create a service, you create a Java class that extends the Service base class or one of its existing subclasses.

1. Started A service can be started from an application component, such as an activity, by calling startService(). It can run in background indefinitely even if the component that started it is destroyed.
2. Bound A service is bound when an application component binds to it by calling bindService().

1. Create a Service class by extending the Service base class.

public class SampleService extends Service {

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

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Toast.makeText(getApplicationContext(), "Service Started", Toast.LENGTH_SHORT).show();
        return START_STICKY;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Toast.makeText(getApplicationContext(), "Service Destroyed", Toast.LENGTH_SHORT).show();
    }
}

Here, START_STICKY is the code which is relevant when the phone runs out of memory and kills the service.

1. START_STICKY: Tells the OS to recreate the service with a null intent when OS has regained sufficient memory.

2. START_NOT_STICKY: Tells OS to not bother recreating the service.

3. START_REDELIVER_INTENT: Tells OS to recreate the service and redeliver the same intent to onStartCommand().

4. In the MainActivity:

startService.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        // start the service from SampleService class
        startService(new Intent(getBaseContext(), SampleService.class));
    }
});

stopService.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        // stop the service from SampleService class
        stopService(new Intent(getBaseContext(), SampleService.class));
    }
});

3. Add Service to the AndroidManifest.xml file.

<service android:name=".java.SampleService"/>

4. Run the application.