DevTut

# Service

A Service runs in background to perform long-running operations or to perform work for remote processes. A service does not provide any user interface it runs only in background with User’s input. For example a service can play music in the background while the user is in a different App, or it might download data from the internet without blocking user’s interaction with the Android device.

# Lifecycle of a Service

The services lifecycle has the following callbacks

  • onCreate() :

Executed when the service is first created in order to set up the initial configurations you might need. This method is executed only if the service is not already running.

  • onStartCommand() :

Executed every time startService() is invoked by another component, like an Activity or a BroadcastReceiver. When you use this method, the Service will run until you call stopSelf() or stopService(). Note that regardless of how many times you call onStartCommand(), the methods stopSelf() and stopService() must be invoked only once in order to stop the service.

  • onBind() :

Executed when a component calls bindService() and returns an instance of IBInder, providing a communication channel to the Service. A call to bindService() will keep the service running as long as there are clients bound to it.

  • onDestroy() :

Executed when the service is no longer in use and allows for disposal of resources that have been allocated.

It is important to note that during the lifecycle of a service other callbacks might be invoked such as onConfigurationChanged() and onLowMemory()

https://developer.android.com/guide/components/services.html (opens new window)

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# Defining the process of a service

The android:process field defines the name of the process where the service is to run. Normally, all components of an application run in the default process created for the application. However, a component can override the default with its own process attribute, allowing you to spread your application across multiple processes.

If the name assigned to this attribute begins with a colon ('😂, the service will run in its own separate process.

<service
  android:name="com.example.appName"
  android:process=":externalProcess" />

If the process name begins with a lowercase character, the service will run in a global process of that name, provided that it has permission to do so. This allows components in different applications to share a process, reducing resource usage.

# Creating an unbound service

The first thing to do is to add the service to AndroidManifest.xml, inside the <application> tag:

<application ...>

    ...        

    <service
        android:name=".RecordingService"
        <!--"enabled" tag specifies Whether or not the service can be instantiated by the system — "true" -->
        <!--if it can be, and "false" if not. The default value is "true".-->
        android:enabled="true"
        <!--exported tag specifies Whether or not components of other applications can invoke the -->
        <!--service or interact with it — "true" if they can, and "false" if not. When the value-->
        <!--is "false", only components of the same application or applications with the same user -->
        <!--ID can start the service or bind to it.-->
        android:exported="false" />

</application>

If your intend to manage your service class in a separate package (eg: .AllServices.RecordingService) then you will need to specify where your service is located. So, in above case we will modify:

android:name=".RecordingService"

to

android:name=".AllServices.RecordingService"

or the easiest way of doing so is to specify the full package name.

Then we create the actual service class:

public class RecordingService extends Service {
    private int NOTIFICATION = 1; // Unique identifier for our notification

    public static boolean isRunning = false;
    public static RecordingService instance = null;


    private NotificationManager notificationManager = null;


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

    @Override
    public void onCreate(){
        instance = this;
        isRunning = true;

        notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);

        super.onCreate();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId){
        // The PendingIntent to launch our activity if the user selects this notification
        PendingIntent contentIntent = PendingIntent.getActivity(this, 0, new Intent(this, MainActivity.class), 0);

        // Set the info for the views that show in the notification panel.
        Notification notification = new NotificationCompat.Builder(this)
                .setSmallIcon(R.mipmap.ic_launcher)        // the status icon
                .setTicker("Service running...")           // the status text
                .setWhen(System.currentTimeMillis())       // the time stamp
                .setContentTitle("My App")                 // the label of the entry
                .setContentText("Service running...")      // the content of the entry
                .setContentIntent(contentIntent)           // the intent to send when the entry is clicked
                .setOngoing(true)                          // make persistent (disable swipe-away)
                .build();

        // Start service in foreground mode
        startForeground(NOTIFICATION, notification);

        return START_STICKY;
    }


    @Override
    public void onDestroy(){
        isRunning = false;
        instance = null;

        notificationManager.cancel(NOTIFICATION); // Remove notification

        super.onDestroy();
    }


    public void doSomething(){
        Toast.makeText(getApplicationContext(), "Doing stuff from service...", Toast.LENGTH_SHORT).show();
    }

}

All this service does is show a notification when it's running, and it can display toasts when its doSomething() method is called.

As you'll notice, it's implemented as a singleton (opens new window), keeping track of its own instance - but without the usual static singleton factory method because services are naturally singletons and are created by intents. The instance is useful to the outside to get a "handle" to the service when it's running.

Last, we need to start and stop the service from an activity:

public void startOrStopService(){
    if( RecordingService.isRunning ){
        // Stop service
        Intent intent = new Intent(this, RecordingService.class);
        stopService(intent);
    }
    else {
        // Start service
        Intent intent = new Intent(this, RecordingService.class);
        startService(intent);
    }
}

In this example, the service is started and stopped by the same method, depending on it's current state.

We can also invoke the doSomething() method from our activity:

public void makeServiceDoSomething(){
    if( RecordingService.isRunning )
        RecordingService.instance.doSomething();
}

# Starting a Service

Starting a service is very easy, just call startService with an intent, from within an Activity:

Intent intent = new Intent(this, MyService.class);  //substitute MyService with the name of your service
intent.putExtra(Intent.EXTRA_TEXT, "Some text"); //add any extra data to pass to the service

startService(intent); //Call startService to start the service.

# Creating Bound Service with help of Binder

Create a class which extends Service class and in overridden method onBind return your local binder instance:

public class LocalService extends Service {
    // Binder given to clients
    private final IBinder mBinder = new LocalBinder();

    /**
     * Class used for the client Binder.  Because we know this service always
     * runs in the same process as its clients, we don't need to deal with IPC.
     */
    public class LocalBinder extends Binder {
        LocalService getService() {
            // Return this instance of LocalService so clients can call public methods
            return LocalService.this;
        }
    }

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

Then in your activity bind to service in onStart callback, using ServiceConnection instance and unbind from it in onStop:

public class BindingActivity extends Activity {
    LocalService mService;
    boolean mBound = false;

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

    @Override
    protected void onStart() {
        super.onStart();
        // Bind to LocalService
        Intent intent = new Intent(this, LocalService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }

    /** Defines callbacks for service binding, passed to bindService() */
    private ServiceConnection mConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // We've bound to LocalService, cast the IBinder and get LocalService instance
            LocalBinder binder = (LocalBinder) service;
            mService = binder.getService();
            mBound = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName arg0) {
            mBound = false;
        }
    };
}

# Creating Remote Service (via AIDL)

Describe your service access interface through .aidl file:

// IRemoteService.aidl
package com.example.android;

// Declare any non-default types here with import statements

/** Example service interface */
interface IRemoteService {
    /** Request the process ID of this service, to do evil things with it. */
    int getPid();
}

Now after build application, sdk tools will generate appropriate .java file. This file will contain Stub class which implements our aidl interface, and which we need to extend:

public class RemoteService extends Service {

    private final IRemoteService.Stub binder = new IRemoteService.Stub() {
        @Override
        public int getPid() throws RemoteException {
            return Process.myPid();
        }
    };

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

Then in activity:

public class MainActivity extends AppCompatActivity {
    private final ServiceConnection connection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
            IRemoteService service = IRemoteService.Stub.asInterface(iBinder);
            Toast.makeText(this, "Activity process: " + Process.myPid + ", Service process: " + getRemotePid(service), LENGTH_SHORT).show();
        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {}
    };

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

    @Override
    protected void onStart() {
        super.onStart();
        Intent intent = new Intent(this, RemoteService.class);
        bindService(intent, connection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        unbindService(connection);
    }

    private int getRemotePid(IRemoteService service) {
        int result = -1;

        try {
            result = service.getPid();
        } catch (RemoteException e) {
            e.printStackTrace();
        }

        return result;
    }
}

# Remarks

If you have not defined your service in your AndroidManifest.xml, you will receive a ServiceNotFoundException when attempting to start it.

Note:

For info on IntentService, see here: IntentService Example (opens new window)

← The Manifest File →