AIDL and Messenger at the same time

Question

I try to write a demo about how to use the AIDL and the Messenger at the same time, but I have an error, and I don't know the reason.

MessengerDEMOActivity.java

public class MessengerDEMOActivity extends Activity {
    /** Called when the activity is first created. */

    private MessengerDEMOServiceConnection MDSconnection = null;
    private Messenger mMessenger = null;

    class IncomingHandler extends Handler {
            public void handleMessage(Message msg) {
            Bundle b = msg.getData();
            System.out.println("MESSENGER! " + b.getString("MESSENGER"));
        }
    }

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

        mMessenger = new Messenger(new IncomingHandler());

        MDSconnection = new MessengerDEMOServiceConnection(mMessenger);
        Intent intent = new Intent();
        intent.putExtra("ID", "AIDL");
        intent.setClassName("messenger.demo", messenger.demo.MessengerDEMOService.class.getName());
        bindService(intent, MDSconnection, Context.BIND_AUTO_CREATE);

    }
}

MessengerDEMOServiceConnection.java

public class MessengerDEMOServiceConnection implements ServiceConnection {

    private IMessengerDEMOService service = null;
    private Messenger mMessenger = null;

    public MessengerDEMOServiceConnection(Messenger mMessenger) {
        super();
        System.out.println("MessengerDEMOServiceConnection SocketServiceConnection()");
        this.mMessenger = mMessenger;
    }

    public void onServiceConnected(ComponentName name, IBinder boundService) {
        System.out.println("MessengerDEMOServiceConnection onServiceConnected()");
        service = IMessengerDEMOService.Stub.asInterface((IBinder) boundService);

        Messenger mService = new Messenger(boundService);

        Message msg = Message.obtain(null, 5);
        msg.replyTo = mMessenger;
        try {
            mService.send(msg); // line 31
        } catch (RemoteException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }

        try {
            service.foo();
        } catch (RemoteException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public void onServiceDisconnected(ComponentName name) {
        System.out.println("MessengerDEMOServiceConnection onServiceDisconnected()");
        service = null;
    }
}

MessengerDEMOService.java

public class MessengerDEMOService extends Service {

    private Messenger mMessenger = null;

    class IncomingHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            System.out.println("MessengerDEMOService IncomingHandler");
            Messenger activityMessenger = msg.replyTo;
            Message m = new Message();
            Bundle b = new Bundle();
            b.putString("MESSENGER", "blablabla");
            m.setData(b);
            try {
                activityMessenger.send(m);
            } catch (RemoteException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    public void onCreate() {
        System.out.println("MessengerDEMOService onCreate");
        mMessenger = new Messenger(new IncomingHandler());
    }

    public IBinder onBind(Intent intent) {

        System.out.println("MessengerDEMOService onBind()");

        if (intent.getExtras().getString("ID").equals("AIDL") == true) {
            System.out.println("MessengerDEMOService onBind() AIDL");
            return new IMessengerDEMOService.Stub() {
                public void foo() {
                    System.out.println("MessengerDEMOService onBind() foo()");
                }
            };
        }

        System.out.println("MessengerDEMOService onBind() MESSENGER");
        return mMessenger.getBinder();
    }
}

And the stack trace:

12-27 16:42:29.412: INFO/System.out(832): MessengerDEMOServiceConnection SocketServiceConnection()
12-27 16:42:29.442: INFO/System.out(832): MessengerDEMOService onCreate
12-27 16:42:29.452: INFO/System.out(832): MessengerDEMOService onBind()
12-27 16:42:29.452: INFO/System.out(832): MessengerDEMOService onBind() AIDL
12-27 16:42:29.542: INFO/System.out(832): MessengerDEMOServiceConnection onServiceConnected()
12-27 16:42:29.572: WARN/Parcel(832): **** enforceInterface() expected 'messenger.demo.IMessengerDEMOService' but read 'android.os.IMessenger'
12-27 16:42:29.572: DEBUG/AndroidRuntime(832): Shutting down VM
12-27 16:42:29.572: WARN/dalvikvm(832): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
12-27 16:42:29.592: ERROR/AndroidRuntime(832): FATAL EXCEPTION: main
12-27 16:42:29.592: ERROR/AndroidRuntime(832): java.lang.SecurityException: Binder invocation to an incorrect interface
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.os.Parcel.enforceInterface(Native Method)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at messenger.demo.IMessengerDEMOService$Stub.onTransact(IMessengerDEMOService.java:49)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.os.Binder.transact(Binder.java:249)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.os.IMessenger$Stub$Proxy.send(IMessenger.java:89)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.os.Messenger.send(Messenger.java:50)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at messenger.demo.MessengerDEMOServiceConnection.onServiceConnected(MessengerDEMOServiceConnection.java:31)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.app.ActivityThread$PackageInfo$ServiceDispatcher.doConnected(ActivityThread.java:1247)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.app.ActivityThread$PackageInfo$ServiceDispatcher$RunConnection.run(ActivityThread.java:1264)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.os.Handler.handleCallback(Handler.java:587)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.os.Handler.dispatchMessage(Handler.java:92)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.os.Looper.loop(Looper.java:123)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at android.app.ActivityThread.main(ActivityThread.java:4627)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at java.lang.reflect.Method.invokeNative(Native Method)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at java.lang.reflect.Method.invoke(Method.java:521)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
12-27 16:42:29.592: ERROR/AndroidRuntime(832):     at dalvik.system.NativeStart.main(Native Method)

Without these lines the code is working:

Message msg = Message.obtain(null, 5);
            msg.replyTo = mMessenger;
            try {
                mService.send(msg); // line 31
            } catch (RemoteException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }

It works without the AIDL and works the AIDL "alone". Can I call the onBinder more than one time?

Answer 1

Can I call the onBinder more than one time?

Simple answer: yes. Actually this does happen if two different applications connect to the same service.

So why doesn't this work? Because you're using two different interfaces for communication.

What happens in detail:

1. You connect to the service (AIDL)
2. You try to send a message by the messenger
3. Android sends this message to the service by AIDL
4. First thing to do is, that the given interfaces do match (by comparing an Interface Descriptor) (See you Stub.onTransact for details)
5. Android notices that these interfaces do not match and throws a SecurityException

There is no RemoteException thrown because the send() method isn't invoked yet.

Easy fix: catch the SecurityException. But I'd consider this extremly bad style (as always it is when your application logic relays on exceptions).

Not so easy fix: For this to work, both interfaces would need to have to same interface descriptor (defined in Stub.DESCRIPTOR in your generated AIDL class). However you'd need to implement the Proxy/Stub for IPC by yourself (not that bad).

Another fix: Create two ServiceConnection classes. One responsible for AIDL, one for the messenger.

Best fix (IMO): Decide for one technique at a time. Your solution feels like driving to work (alone) with two cars at the same time.