Archive for September, 2009

Written by: Igor Darkov, Software Developer of Device Team, Apriorit Inc.

In this article I’ve described:

Services are long running background processes provided by Android. They could be used for background tasks execution. Tasks can be different: background calculations, backup procedures, internet communications, etc. Services can be started on the system requests and they can communicate with other processes using the Android IPC channels technology. The Android system can control the service lifecycle depending on the client requests, memory and CPU usage. Note that the service has lower priority than any process which is visible for the user.

Let’s develop the simple example service. It will show scheduled and requested notifications to user. Service should be managed using the service request, communicated from the simple Android Activity and from the PC.

First we need to install and prepare environment:

After the environment is prepared we can create Eclipse Android project. It will include sources, resources, generated files and the Android manifest.

First of all we need to implement service class. It should be inherited from the android.app.Service (http://developer.android.com/reference/android/app/Service.html) base class. Each service class must have the corresponding declaration in its package’s manifest. Manifest declaration will be described later. Services, like the other application objects, run in the main thread of their hosting process. If you need to do some intensive work, you should do it in another thread.

In the service class we should implement abstract method onBind. Also we override some other methods:

To send system log output we can use static methods of the android.util.Log class (http://developer.android.com/reference/android/util/Log.html). To browse system logs on PC you can use ADB utility command: adb logcat.

The notification feature is implemented in our service as the special runnable object. It could be used from the other threads and processes. The service class has method showNotification, which can display message to user using the Toast.makeText call. The runnable object also uses it:

Code will be executed in the service thread. To execute runnable method we can use the special object android.os.Handler. There are two main uses for the Handler: to schedule messages and runnables to be executed as some point in the future; and to place an action to be performed on a different thread than your own. Each Handler instance is associated with a single thread and that thread’s message queue. To show notification we should set message and call post() method of the Handler’s object.

Each application runs in its own process. Sometimes you need to pass objects between processes and call some service methods. These operations can be performed using IPC. On the Android platform, one process can not normally access the memory of another process. So they have to decompose their objects into primitives that can be understood by the operating system , and “marshall” the object across that boundary for developer.

The AIDL IPC mechanism is used in Android devices. It is interface-based, similar to COM or Corba, but is lighter . It uses a proxy class to pass values between the client and the implementation.

AIDL (Android Interface Definition Language) is an IDL language used to generate code that enables two processes on an Android-powered device to communicate using IPC. If you have the code in one process (for example, in Activity) that needs to call methods of the object in another process (for example, Service), you can use AIDL to generate code to marshall the parameters.

Service interface example showed below supports only one sendNotification call:

The IBinder interface for a remotable object is used by clients to perform IPC. Client can communicate with the service by calling Context’s bindService(). The IBinder implementation could be retrieved from the onBind method. The INotifyService interface implementation is based on the android.os.Binder class (http://developer.android.com/reference/android/os/Binder.html):

As it was described above, the notifications could be send using the Handler object’s post() method call. The NotificaionRunnable object is passed as the method’s parameter.

On the client side we can request IBinder object and work with it as with the INotifyService interface.  To connect to the service the android.content.ServiceConnection interface implementation can be used. Two methods should be defined: onServiceConnected, onServiceDisconnected:

The bindService method can be called from the client Activity context to connect to the service:

The unbindService method can be called from the client Activity context to disconnect from the service:

Broadcasts are the way applications and system components can communicate. Also we can use broadcasts to control service from the PC. The messages are sent as Intents, and the system handles dispatching them, including starting receivers.

Intents can be broadcasted to BroadcastReceivers, allowing messaging between applications. By registering a BroadcastReceiver in application’s AndroidManifest.xml (using tag) you can have your application’s receiver class started and called whenever someone sends you a broadcast. Activity Manager uses the IntentFilters, applications register to figure out which program should be used for a given broadcast.

Let’s develop the receiver that will start and stop notify service on request. The base class android.content.BroadcastReceiver should be used for these purposes (http://developer.android.com/reference/android/content/BroadcastReceiver.html):

To send broadcast from the client application we use the Context.sendBroadcast call. I will describe how to use receiver and send broadcasts from the PC in chapter 2.

Every application must have an AndroidManifest.xml file in its root directory. The manifest contains essential information about the application to the Android system, the system must have this information before it can run any of the application’s code. The core components of an application (its activities, services, and broadcast receivers) are activated by intents. An intent is a bundle of information (an Intent object) describing a desired action — including the data to be acted upon, the category of component that should perform the action, and other pertinent instructions. Android locates an appropriate component to respond to the intent, starts the new instance of the component if one is needed, and passes it to the Intent object.

We should describe 2 components for our service:

Services like the other applications for the Android platform can be installed from the special package with the .apk extension. Android package contains all required binary files and the manifest.

Before installing the service from the PC we should enable the USB Debugging option in the device Settings-Applications-Development menu and then connect device to PC via the USB.

On the PC side we will use the ADB utility which is available in the Android SDK tools directory. The ADB utility supports several optional command-line arguments that provide powerful features, such as copying files to and from the device. The shell command-line argument lets you connect to the phone itself and issue rudimentary shell commands.

We will use several commands:

I’ll describe the package installation process in details. It consists of several steps which are performed by the ADB utility install command:

To be able to start and stop the NotifyService from the PC we can use the “am” (Activity Manager?) utility which is present on the Android device. The command line parameters are described in the Appendix II. The “am” utility can send system broadcast intents. Our service has the broadcast receiver which will be launched by the system request.

To start NotifyService we can execute remote shell command:

To stop the NotifyService we can execute remote shell command:

Note, that the NotifyServiceStart and NotifyServiceStop intents were described in the manifest file inside the … tag. Other requests will not start the receiver.

http://developer.android.com/sdk/1.5_r2/installing.html

http://developer.android.com/reference/classes.html

https://www.isecpartners.com/files/iSEC_Securing_Android_Apps.pdf

http://developer.android.com/guide/developing/tools/aidl.html