Android Service Tutorial

Lars Vogel

Version 2.5

Copyright © 2011, 2012, 2013 Lars Vogel

05.01.2013

Revision History
Revision 0.1	07.03.2011	Lars Vogel	created
Revision 0.2 - 2.5	08.03.2011 - 05.01.2013	Lars Vogel	bug fixed and enhancements

Developing own services and using system services in Android

This tutorial describes how to create and consume Android services. It is based on Eclipse 4.2, Java 1.6 and Android 4.2.

Table of Contents

1. Android Services
1.1. Service1.2. Android Platform Service1.3. Defining new services

2. Defining services
2.1. Declaring own services2.2. Running a Services in its own process2.3. When to run a service in a separate process?2.4. Intent Services
3. Broadcast receiver4. Starting services
4.1. Start a service4.2. Stopping a service4.3. Binding between activities and local services4.4. Starting services regulary via AlarmManager
5. Communicating with Services
5.1. Activity binding to local service5.2. Using receiver5.3. AIDL for services in a different process5.4. Sending Intent data and bundle to the services
5.5. Handler and Messenger
6. Tutorial: Using IntentService
7. Tutorial: Define and consume local service8. Thank you
9. Questions and Discussion10. Links and Literature
10.1. Source Code10.2. Android Resources10.3. vogella Resources

1. Android Services

1.1. Service

A service is a component which runs in the background, without direct interaction with the user. The Android platform provides and runs predefined system services and every Android application can use them, given the right permissions.

An Android application can, in addition to consuming the existing Android platform services, define and use new
services.

1.2. Android Platform Service

The Android platform provides pre-defined services, usually exposed via a specific Manager class. Access to them can be gained via the

getSystemService()

method.

1.3. Defining new services

Every Android application can define and start new services

If you use asynchronous processing in activities or
fragments the corresponding threads are still connected to the life-cycle of the corresponding
activity. The Android system may decide to terminate them at any point in time.
Services run with a higher priority than inactive or invisible
activities and therefore it is less likely that the Android system terminates them.

Defining your own services allows you to design very responsive applications. You can fetch the application via a
service and once the application is started by the user, it can present fresh data to the user.

2. Defining services

2.1. Declaring own services

A service needs to be declared in the

AndroidManifest.xml

and the implementing class must extend the

Service

class or one of its subclasses. The following code shows an example for a
service declaration and its implementation.

<service
android:name="MyService"
android:icon="@drawable/icon"
android:label="@string/service_name"
>
</service>

public class MyService extends Service {

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
//TODO do something useful
return Service.START_NOT_STICKY;
}

@Override
public IBinder onBind(Intent intent) {
//TODO for communication return IBinder implementation
return null;
}
}

A service runs by default in the same process as the application. in its own thread.

Therefore you need to use asynchronous processing in the
service to to perform resource intensive tasks in the background.

Services

which run in the process of the application are sometimes called local

services

.

2.2. Running a Services in its own process

You can also specify that your

Service

runs in a separate process via the

android:process=":process_description"

attribute.

<service
android:name="WordService"
android:process=":my_process"
android:icon="@drawable/icon"
android:label="@string/service_name"
>
</service>

The colon prefix before the name tells Android that the

Service

is private to its declaring application. If the colon is not used the

Service

would be a global process and can be used by other Android applications.

Running a service in its own process will not block the application in case the service performs long running operations in its main thread. But as the services runs in its own process you need to use some interprocess communication (IPC) to communicate
to your service from other parts.

Even if the service runs in its own process you need to use asynchronous processing to perform network access because Android does not allow network access in the main thread of a process.

2.3. When to run a service in a separate process?

Running a service in its own process gives it its own memory address space and a garbage collector of the virtual machine in this process does not affect the application process.

Application rarely need to run a service in its own process. Running a services in its own process make the communication of the other Android components and the
service harder to implement.

If you want to make a service to other Android application available, they must run in their own process.

2.4. Intent Services

You can also extend the

IntentService

class for your service implementation.

The

IntentService

is used to perform a certain task in the background. Once done, the instance of

IntentService

terminate itself automatically. Examples for its usage would be to download a certain resources from the Internet.

The

IntentService

class offers the

onHandleIntent()

method which will be asynchronously called by the Android system.

3. Broadcast receiver

For an introduction into

BroadcastReceiver

please see

Android Broadcast Receiver tutorial .

4. Starting services

4.1. Start a service

An Android component (service, receiver, activity) can start and trigger a
service via the

startService(intent)

method. This method call starts the
service if it is not running.

Intent service = new Intent(context, MyService.class);
context.startService(service);

If the service started the

onCreate()

method is called.

Once the service is started the method call to start the
service triggers

startService(intent)

method in the
service. It passes in the

Intent

for the

startService(intent)

call.

If

startService(intent)

is called while the
service is running, its

onStartCommand()

is also called. Therefore your
service needs to be prepared that

onStartCommand()

can be called several times. This method is called in the main user interface thread therefore it cannot be called simultaneously from two different threads.

Alternatively to

startService(intent)

you can also start a
service via the

bindService()

method call. This allows you to communicate directly with the
service.

4.2. Stopping a service

You stop a service via the

stopService()

method. No matter how frequently you started the
service with

startService(intent)

a call to

stopService()

stops it.

A service can stop itself by calling the

stopSelf()

method.

4.3. Binding between activities and local services

If the activity wants to interact with the
service it can use the

bindService()

method to start the
service.

This method requires as parameter a

ServiceConnection

object which allows to connect to the
service. In the service the

onBind()

method is called. This method returns a

IBinder

object to the

ServiceConnection

.

This

IBinder

object can be used by the activity to communicate with the
service.

Afterwards the binding was done the

onStartCommand()

method is called with the

Intent

data provided by the activity.

startService()

also allows you to provide a flag which determines the restart behavior of the services.

Service.START_STICKY

is used for services which are explicit started or stopped. If these services are terminated by the Android system, they are restarted if sufficient resource are available again.
Services started with

Service.START_NOT_STICKY

are not automatically restarted if terminated by the Android system.

4.4. Starting services regulary via AlarmManager

As with activities the Android system may terminate the process of a service at any time to save resources. For this reason you cannot simple use a

TimerTask

in the service to ensure that it is executed on a regular basis.

For correct scheduling of the

Service

use the

AlarmManager

class. The following code demonstrates how to do this.

Intent intent = new Intent(this, MyService.class);
PendingIntent pintent = PendingIntent.getService(this, 0, intent, 0);
AlarmManager alarm = (AlarmManager)getSystemService(Context.ALARM_SERVICE);
// Start every 30 seconds
alarm.setRepeating(AlarmManager.RTC_WAKEUP, cal.getTimeInMillis(), 30*1000, pintent); 

5. Communicating with Services

There are several way for an activity to communicate with an
service and vice versa. This section dicusses the different ways and gives recommendation which to use.

5.1. Activity binding to local service

If the

Service

is started in the same process as the

Activity

, the

Activity

can directly bind to the service. This is a relatively simple and efficient way to communication.

5.2. Using receiver

You can also use dynamically registered receivers for the communication. For example your
activity can dynamically register a
receiver and the service sends outs corresponding events.

5.3. AIDL for services in a different process

To bind to a service which runs in a different process you need to use Inter Process Communication (IPC) as the data needs to be send between different processes. For this you need to create a AIDL file which looks similar to
an Java interface but ends with the

.aidl

file extension and is only allowed to extend other AIDL files.

This approach is required if your service should be provided to other applications, otherwise you should prefer a local service.

5.4. Sending Intent data and bundle to the services

The service receives data from the starting Android component and can use this data.

5.5. Handler and Messenger

If the service should be communicating back to the activity it can receive an object of type

Messenger

via the

Intent

data it receives from the

Activity

. If the

Messenger

is bound to a

Handler

in the activity the

service

can send objects of type

Message

to the
activity.

A

Messenger

is parcelable, which means it can be passed to another process and you can use this object to send

Messages

to the

Handler

in the
activity.

Messenger

provides also the method

getBinder()

which allows to pass a

Messenger

to the
activity. The

Activity

can therefore send

Messages

to the service.

6. Tutorial: Using IntentService

The following example demonstrates how to use the

IntentService

class to download a file from the Internet. Once done the

IntentService

will use an instance of the

Messenger

class to inform the

Activity

which started the service about the location of the downloaded file.

Create a new project called de.vogella.android.intentservice.download with a

Activity

called MainActivity.

Create a service "DownloadService" by creating the following class and the entry in

AndroidManifest.xml

. Also add the permission to write to external storage and to access the Internet to the file.

package de.vogella.android.intentservice.download;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;

import android.app.Activity;
import android.app.IntentService;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.os.Message;
import android.os.Messenger;
import android.util.Log;

public class DownloadService extends IntentService {

private int result = Activity.RESULT_CANCELED;

public DownloadService() {
super("DownloadService");
}

// Will be called asynchronously be Android
@Override
protected void onHandleIntent(Intent intent) {
Uri data = intent.getData();
String urlPath = intent.getStringExtra("urlpath");
String fileName = data.getLastPathSegment();
File output = new File(Environment.getExternalStorageDirectory(),
fileName);
if (output.exists()) {
output.delete();
}

InputStream stream = null;
FileOutputStream fos = null;
try {

URL url = new URL(urlPath);
stream = url.openConnection().getInputStream();
InputStreamReader reader = new InputStreamReader(stream);
fos = new FileOutputStream(output.getPath());
int next = -1;
while ((next = reader.read()) != -1) {
fos.write(next);
}
// Sucessful finished
result = Activity.RESULT_OK;

} catch (Exception e) {
e.printStackTrace();
} finally {
if (stream != null) {
try {
stream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (fos != null) {
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}

Bundle extras = intent.getExtras();
if (extras != null) {Messenger messenger = (Messenger) extras.get("MESSENGER");
Message msg = Message.obtain();
msg.arg1 = result;
msg.obj = output.getAbsolutePath();
try {
messenger.send(msg);
} catch (android.os.RemoteException e1) {
Log.w(getClass().getName(), "Exception sending message", e1);
}

}
}
} 

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="de.vogella.android.intentservice.download"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk android:minSdkVersion="15" />
<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

<application
android:icon="@drawable/ic_launcher"
android:label="@string/app_name" >
<activity
android:name=".MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />

<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>

<service android:name="DownloadService" >
</service>
</application>

</manifest>

Change the

main.xml

layout to the following.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:onClick="onClick"
android:text="Button" />

</LinearLayout>

Change

MainActivity

to the following.

package de.vogella.android.intentservice.download;

import android.app.Activity;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.os.Messenger;
import android.view.View;
import android.widget.Toast;

public class MainActivity extends Activity {
private Handler handler = new Handler() {
public void handleMessage(Message message) {
Object path = message.obj;
if (message.arg1 == RESULT_OK && path != null) {
Toast.makeText(MainActivity.this,
"Downloaded" + path.toString(), Toast.LENGTH_LONG)
.show();
} else {
Toast.makeText(MainActivity.this, "Download failed.",
Toast.LENGTH_LONG).show();
}

};
};

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

}

public void onClick(View view) {
Intent intent = new Intent(this, DownloadService.class);
// Create a new Messenger for the communication backMessenger messenger = new Messenger(handler);
intent.putExtra("MESSENGER", messenger);
intent.setData(Uri.parse("http://www.vogella.com/index.html"));
intent.putExtra("urlpath", "http://www.vogella.com/index.html");
startService(intent);
}
} 

If you run your example and press the button, the download should be performed by the

Service

and once done the

Activity

should show a Toast with the file name.

7. Tutorial: Define and consume local service

The following chapter demonstrates how to create and consume a service from an
activity. The service startes at boot time and periodically fetch data. The

activity

binds itself to the service and use connection to communicate with the service.

Create a new project called de.vogella.android.ownservice.local with an

Activity

called MainActivity.

Create the

LocalWordService

class.

package de.vogella.android.ownservice.local;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

import android.app.Service;
import android.content.Intent;
import android.os.Binder;
import android.os.IBinder;

public class LocalWordService extends Service {
private final IBinder mBinder = new MyBinder();
private ArrayList<String> list = new ArrayList<String>();

@Override
public int onStartCommand(Intent intent, int flags, int startId) {

Random random = new Random();
if (random.nextBoolean()) {
list.add("Linux");
}
if (random.nextBoolean()) {
list.add("Android");
}
if (random.nextBoolean()) {
list.add("iPhone");
}
if (random.nextBoolean()) {
list.add("Windows7");
}
if (list.size() >= 20) {
list.remove(0);
}
return Service.START_NOT_STICKY;
}

@Override
public IBinder onBind(Intent arg0) {
return mBinder;
}

public class MyBinder extends Binder {
LocalWordService getService() {
return LocalWordService.this;
}
}

public List<String> getWordList() {
return list;
}

}

Create the following two classes, which will be registered as

BroadcastReceivers

.

package de.vogella.android.ownservice.local;

import java.util.Calendar;

import android.app.AlarmManager;
import android.app.PendingIntent;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

public class MyScheduleReceiver extends BroadcastReceiver {

// Restart service every 30 seconds
private static final long REPEAT_TIME = 1000 * 30;

@Override
public void onReceive(Context context, Intent intent) {AlarmManager service = (AlarmManager) context
.getSystemService(Context.ALARM_SERVICE);
Intent i = new Intent(context, MyStartServiceReceiver.class);
PendingIntent pending = PendingIntent.getBroadcast(context, 0, i,
PendingIntent.FLAG_CANCEL_CURRENT);
Calendar cal = Calendar.getInstance();
// Start 30 seconds after boot completed
cal.add(Calendar.SECOND, 30);
//
// Fetch every 30 seconds
// InexactRepeating allows Android to optimize the energy consumption
service.setInexactRepeating(AlarmManager.RTC_WAKEUP,
cal.getTimeInMillis(), REPEAT_TIME, pending);

// service.setRepeating(AlarmManager.RTC_WAKEUP, cal.getTimeInMillis(),
// REPEAT_TIME, pending);

}
} 

package de.vogella.android.ownservice.local;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

public class MyStartServiceReceiver extends BroadcastReceiver {

@Override
public void onReceive(Context context, Intent intent) {
Intent service = new Intent(context, LocalWordService.class);
context.startService(service);
}
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="de.vogella.android.ownservice.local"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk android:minSdkVersion="10" />

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

<application
android:icon="@drawable/icon"
android:label="@string/app_name" >
<activity
android:name=".MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />

<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>

<service
android:name=".LocalWordService"
android:icon="@drawable/icon"
android:label="@string/service_name" >
</service>

<receiver android:name="MyScheduleReceiver" >
<intent-filter>
<action android:name="android.intent.action.BOOT_COMPLETED" />
</intent-filter>
</receiver>
<receiver android:name="MyStartServiceReceiver" >
</receiver>
</application>

</manifest>

Change the layout file of the activity similar to the following example.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="@string/hello" />

<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:onClick="showServiceData"
android:text="Button" >
</Button>

<ListView
android:id="@id/android:list"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
</ListView>

</LinearLayout>

Change your

Activity

class to the following code.

package de.vogella.android.ownservice.local;

import java.util.ArrayList;
import java.util.List;

import android.app.ListActivity;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.IBinder;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Toast;

public class MainActivity extends ListActivity {
private LocalWordService s;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
wordList = new ArrayList<String>();
adapter = new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_1, android.R.id.text1,
wordList);
setListAdapter(adapter);
doBindService();
}

private ServiceConnection mConnection = new ServiceConnection() {

public void onServiceConnected(ComponentName className, IBinder binder) {
s = ((LocalWordService.MyBinder) binder).getService();
Toast.makeText(MainActivity.this, "Connected",
Toast.LENGTH_SHORT).show();
}

public void onServiceDisconnected(ComponentName className) {
s = null;
}
};
private ArrayAdapter<String> adapter;
private List<String> wordList;

void doBindService() {
bindService(new Intent(this, LocalWordService.class), mConnection,
Context.BIND_AUTO_CREATE);
}

public void showServiceData(View view) {
if (s != null) {

Toast.makeText(this, "Number of elements" + s.getWordList().size(),
Toast.LENGTH_SHORT).show();
wordList.clear();
wordList.addAll(s.getWordList());
adapter.notifyDataSetChanged();
}
}
}

8. Thank you

Please help me to support this article:

9. Questions and Discussion

Before posting questions, please see the
vogella FAQ. If you have questions or find an error in this article please use the
www.vogella.com Google Group. I have created a short list

how to create good questions which might also help you.

10. Links and Literature

10.1. Source Code

Source Code of Examples

10.2. Android Resources

Android Development Tutorial

Android ListView and ListActivity

Android Location API and Google Maps

Android Intents

Android and Networking

Android Background processing with Threads and Asynchronous Task

Remote Messenger Service from Google

10.3. vogella Resources

vogella Training
Android and Eclipse Training from the vogella team

Android Tutorial
Introduction to Android Programming

GWT Tutorial
Program in Java and compile to JavaScript and HTML

Eclipse RCP Tutorial
Create native applications in Java

JUnit Tutorial
Test your application

Git Tutorial
Put everything you have under distributed version control system