android - Binding to a service from another app

Question

I wrote two apps (target Gingerbread). Let say app1 and app2. App1 has two services started with "BOOT_COMPLETED" and they are started with the return value START_STICKY. They run in separate threads. To make a long story short. One of the service is watching for incoming data on a serial port (a kind of proxy for app communicating with interfaces on the other end of the serial port). The other has a listener watching some system status and waiting for some "instructions" from other apps. I know they are running well because they are listed in the running services and I added some code that forces them to do some stuff when some specific data come from the serial port.

Now the problem: I wrote app2. It tries to bind to one of the service in app1. I used android-developper documentation and implemented a bidirectional communication between the service in app1 and app2. Since I just have a small amount of very simple data to send, I used a messenger, as suggested. I basically just use the "what, arg1 and arg2" I did not use the AIDL interface as the documentation was suggesting.

Here is the section of the androidmanifest declaring the service in app1 I try to bind too.

    <service android:name=".ModemWatcherService"
              android:label="@string/app_name"
              android:exported="true">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
            <!-- Service name -->
            <action android:name="com.admetric.modemwatcher.Service" />
        </intent-filter>
    </service>

Then, here are the few method dealing with this issue in app1:

    @Override
public IBinder onBind(Intent intent) {
    Log.d(TAG, "entering onBind");
    return mMessenger.getBinder();
}

/**
 * Handler of incoming messages from clients.
 */
class IncomingHandler extends Handler {
    @Override
    public void handleMessage(Message msg) {
        String logMessage = "Received meaasge what= %d, arg1= %d, arg2= %d" + String.valueOf(msg.what) + String.valueOf(msg.arg1) + String.valueOf( msg.arg2);
        Log.d(TAG, logMessage);
        switch (msg.what) {
            case MSG_REGISTER_CLIENT:
                mClients.add(msg.replyTo);
                break;
            case MSG_UNREGISTER_CLIENT:
                mClients.remove(msg.replyTo);
                break;
            case .....  
             more code here for the application
            default:
                super.handleMessage(msg);
        }
    }
}


@Override
public void onCreate() {
    mHandler = new Handler();
    startSignalLevelListener();
    Log.i(TAG, "Just did onCreated");
    // Display a notification about us starting.  We put an icon in the status bar.
    // showNotification();
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    Log.i(TAG, "Received start id " + startId + ": " + intent);
    // We want this service to continue running until it is explicitly
    // stopped, so return sticky.
    return START_STICKY;
}

For app2, here is the relevant code to establish the binding with the bidirectional communication:

public final class ComWithIoMcu extends Service {
private static final String TAG = "ComWithIoMcu";
/** Messenger for communicating with service. */
static Messenger mServiceMcu = null;
/** Flag indicating whether we have called bind on the service. */
boolean mIsBound;

/**
 * Command to the service to register a client, receiving callbacks
 * from the service.  The Message's replyTo field must be a Messenger of
 * the client where callbacks should be sent.
 */
static final int MSG_REGISTER_CLIENT = 1;

/**
 * Command to the service to unregister a client, ot stop receiving callbacks
 * from the service.  The Message's replyTo field must be a Messenger of
 * the client as previously given with MSG_REGISTER_CLIENT.
 */
static final int MSG_UNREGISTER_CLIENT = 2;
/**
 * Command to forward a string command to the I/O MCU
 */    
public static final int MSG_SEND_STRING_TO_IOMCU = 3;
/** List of supported commands
 * 
 */
   ...... more code ....

/**
 * Handler of incoming messages from service.
 */
class IncomingHandler extends Handler {
    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case MSG_UNSOL_MESSAGE:
                Log.d(TAG, "Received from service: " + msg.arg1);
                break;
            default:
                super.handleMessage(msg);
        }
    }
}

/**
 * Target we publish for clients to send messages to IncomingHandler.
 */
final Messenger mMessenger = new Messenger(new IncomingHandler());
boolean mBound;

/**
 * Class for interacting with the main interface of the service.
 */
private ServiceConnection mConnection = new ServiceConnection() {
    public void onServiceConnected(ComponentName className,
            IBinder service) {
        // This is called when the connection with the service has been
        // established, giving us the service object we can use to
        // interact with the service.  We are communicating with our
        // service through an IDL interface, so get a client-side
        // representation of that from the raw service object.
        mServiceMcu = new Messenger(service);
        Log.d(TAG, "Attached.");

        // We want to monitor the service for as long as we are
        // connected to it.
        try {
            Message msg = Message.obtain(null,
                    MSG_REGISTER_CLIENT);
            msg.replyTo = mMessenger;
            mServiceMcu.send(msg);

        } catch (RemoteException e) {
            // In this case the service has crashed before we could even
            // do anything with it; we can count on soon being
            // disconnected (and then reconnected if it can be restarted)
            // so there is no need to do anything here.
            Log.e(TAG, "ModemWatcherService is not running");
        }
    }

    public void onServiceDisconnected(ComponentName className) {
        // This is called when the connection with the service has been
        // unexpectedly disconnected -- that is, its process crashed.
        mServiceMcu = null;
        mBound = false; 


    }
};

void doBindService() {
    // Establish a connection with the service.  We use an explicit
    // class name because there is no reason to be able to let other
    // applications replace our component.
    //bindService(new Intent(this, MessengerService.class), mConnection, Context.BIND_AUTO_CREATE);
    try {
        Intent intentForMcuService = new Intent();
        Log.d(TAG, "Before init intent.componentName");
        intentForMcuService.setComponent(new ComponentName("com.admetric.modemwatcher", "ModemWatcherService"));
        Log.d(TAG, "Before bindService");
        if (bindService(intentForMcuService, mConnection, 0)){
            Log.d(TAG, "Binding to Modem Watcher returned true");
        } else {
            Log.d(TAG, "Binding to Modem Watcher returned false");
        }
    } catch (SecurityException e) {
        Log.e(TAG, "can't bind to ModemWatcherService, check permission in Manifest");
    }
    mIsBound = true;
    Log.d(TAG, "Binding.");
}

void doUnbindService() {
    if (mIsBound) {
        // If we have received the service, and hence registered with
        // it, then now is the time to unregister.
        if (mServiceMcu != null) {
            try {
                Message msg = Message.obtain(null, MSG_UNREGISTER_CLIENT);
                msg.replyTo = mMessenger;
                mServiceMcu.send(msg);
            } catch (RemoteException e) {
                // There is nothing special we need to do if the service
                // has crashed.
            }
        }

        // Detach our existing connection.
        unbindService(mConnection);
        mIsBound = false;
        Log.d(TAG, "Unbinding.");
    }
}

Looking at the running services, I can see that the service I created in app2 is running. Logcat shows me that I try to bind the the ModemWatcherService but it is not found. Here is the interesting section of logcat

12-05 17:22:59.884 D/ComWithIoMcu(  547): Before init intent.componentName
12-05 17:22:59.884 D/ComWithIoMcu(  547): Before bindService
12-05 17:22:59.888 D/ComWithIoMcu(  547): Binding to Modem Watcher returned false
12-05 17:22:59.888 D/ComWithIoMcu(  547): Binding.
12-05 17:22:59.888 W/ActivityManager(   89): Unable to start service Intent { cmp=com.admetric.modemwatcher/ModemWatcherService }: not found

My first thought was that I was missing a permission but bindService() can trow security exceptions and in this case it doesn't so, I checked and it returns false for an unknown reason. Also, I know that in app1, the onBind is never called proving that the binding never occurred. So the logcat message "not found" make sense but I declared that service public in its manifest. It's probably a simple mistake but I've been on this issu for a while now and I did not find why. Any idea why app2 can't find the service in app1 ? I used just cut and paste for names so I would not do stupid mistyping mistakes in names. Am I missing permissions of some sort? Do I need to do some extra step to publish the service for the whole system? That's the first time I try to access something in one app from another app so, I might have missed something.

Answer 1

Your ComponentName is incorrectly constructed. When passing in the class name is must be fully qualified like so:

intentForMcuService.setComponent(new ComponentName("com.admetric.modemwatcher",
        "com.admetric.modemwatcher.ModemWatcherService"));

Another thing, if you are referencing a Service outside the boundaries of the application, it's probably best not to use ComponentName to reference it, even if it does work correctly. A more common approach would be to create a custom ACTION string for your Intent and have the Service filter that action.