Q:

More than one activity starting/binding to a Service

From what I have read regarding Services I should be able to have more than one activity bind to the same Service, but it's not working. For what it's worth my BackgroundService extends Service.
From my MainActivity I start the Service as follows:
Intent backgroundService = new Intent(MainActivity.this, BackgroundService.class);
backgroundService.putExtra(BackgroundService.EXTRA_MESSENGER_MAINACTIVITY, new Messenger(mainHandler));
startService(backgroundService);

Then from another activity I try to bind this same Service as follows:
Intent backgroundService = new Intent(RcvMessages.this, BackgroundService.class);
backgroundService.putExtra(BackgroundService.EXTRA_MESSENGER_RCVMESSAGES, new Messenger(rcvHandler));
startService(backgroundService);

My understanding is that each binding to the Service will have it's own handler as indicated above. But it doesn't work. Only the first handler works. What am I doing wrong?
@Femi said "You are passing in a Messenger as an Intent extra to the BackgroundService class, but there is no built-in Android code that understands what to do with that Messenger: that has to be provided by the BackgroundService implementation. What is the code for BackgroundService, and what does it do with the Messenger instances it is passed?" @Femi have a look at my BackgroundSerive, particularly the onStartCommand():
/**************************************************************************************************
 * File: BackgroundService.java
 * Application: BackgroundService
 * Description: This file contains the background Service that is launched by the MainActivity's
 *     bind button.
 **************************************************************************************************/

package com.marie.mainactivity;

import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Intent;
import android.os.Bundle;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.util.Log;

/*
 * Class: BackgroundService
 * Purpose: Using the onStart() method the BackgroundService gets the reference to the
 *     Messenger instance that was passed to BackgroundService. The messenger is then
 *     used by the ServiceWorker() thread to send messages to the handler that is defined
 *     in the MainActivity class.
 */
public class BackgroundService extends Service {
    private NotificationManager notificationMgr;

    public static final String EXTRA_MESSENGER_MAINACTIVITY = "com.marie.mainactivity.EXTRA_MESSENGER_MAINACTIVITY";
    private Messenger messenger_mainactivity;

    public static final String EXTRA_MESSENGER_RCVMESSAGES = "com.marie.mainactivity.EXTRA_MESSENGER_RCVMESSAGES";
    private Messenger messenger_rcvmessages;

    public static final String EXTRA_SONG = "com.marie.mainactivity.EXTRA_SONG";
    int song = 0;

    @Override
    public void onCreate() {
        super.onCreate();

        notificationMgr = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);

        displayNotificationMessage("starting Background Service");

        Thread thr = new Thread(null, new ServiceWorker(), "BackgroundService");
        thr.start();
    }   

    /*
     * This is the ServiceWorker thread that passes messages to the handler defined in
     * the MainActivity class.
     * NOTE: Instead of passing messages to a handler in MainActivity I would like
     * it to pass messages to a handler defined in the RcvMessages activity.
     */
    class ServiceWorker implements Runnable
    {
        public void run() {
            // do background processing here... something simple

            // send a message to the handler defined in the MainActivity activity
            if (messenger_mainactivity != null)
            {
                try {
                    Message msg1 = Message.obtain();
                    msg1.obj = "SongData";
                    msg1.arg1 = 1000;
                    messenger_mainactivity.send(msg1);
                } catch (RemoteException e) {
                    e.printStackTrace();
                } catch (NullPointerException e) {
                    e.printStackTrace();
                }
            } else {
                Log.e("messenger_mainactivity", "null");
            }

            // send a message to the handler defined in the RcvMessages activity
            if (messenger_rcvmessages != null)
            {
                try {
                    Message msg1 = Message.obtain();
                    msg1.obj = "Song";
                    msg1.arg1 = song;
                    messenger_rcvmessages.send(msg1);
                } catch (RemoteException e) {
                    e.printStackTrace();
                } catch (NullPointerException e) {
                    e.printStackTrace();
                }
            } else {
                Log.e("messenger_rcvmessages", "null");
            }

            // stop the service when done...
            // BackgroundService.this.stopSelf();
            // Or use the unbindBtn in the MainActivity class.
        }
    }

    @Override
    public void onDestroy()
    {
        displayNotificationMessage("stopping Background Service");
        super.onDestroy();
    }

    /*
    @Override
    public void onStart(Intent intent, int startId) {
        super.onStart(intent, startId);
    }
    */

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

        Bundle extras = intent.getExtras();
        Messenger messenger;

        messenger = (Messenger)extras.get(EXTRA_MESSENGER_MAINACTIVITY);
        if (messenger != null) {
            messenger_mainactivity = messenger;
        }

        messenger = (Messenger)extras.get(EXTRA_MESSENGER_RCVMESSAGES);
        if (messenger != null) {
            messenger_rcvmessages = messenger;
            song = extras.getInt(EXTRA_SONG);
        }

        return START_NOT_STICKY;
    }

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

    private void displayNotificationMessage(String message)
    {
        Notification notification = new Notification(R.drawable.note, message, System.currentTimeMillis());

        PendingIntent contentIntent = PendingIntent.getActivity(this, 0, new Intent(this, MainActivity.class), 0);

        notification.setLatestEventInfo(this, "Background Service", message, contentIntent);

        notificationMgr.notify(R.id.app_notification_id, notification);
    }
}

Here's the whole code:
/**************************************************************************************************
 * File: MainActivity.java
 * Application: BackgroundService
 * Description: This file contains the main activity that is run when the BackgroundService
 *     application is launched.
 **************************************************************************************************/

package com.marie.mainactivity;

import com.marie.mainactivity.BackgroundService;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.os.Messenger;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

/*
 * Class: MainActivity
 * Purpose: Using a button, the MainActivity class starts the backgroundService and
 *     the RcvMessages activity. Using another button MainActivity stops the backgroundService.
 *     NOTE: RcvMessages is only a stub that does nothing but display a simple message.
 * Handler: MainActivity defines and provides a reference to "handler" for the backgroundService.
 */
public class MainActivity extends Activity {
    private static final String TAG = "MainActivity";

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

        /*
         * The bind button: bindBtn
         * Clicking this button starts the background Service and launches the
         * RcvMessages activity. NOTE: RcvMessages is only a stub so far.
         */
        Button bindBtn = (Button)findViewById(R.id.bindBtn);
        bindBtn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // Start the background Service for sending canned messages to the handler as a test.
                Log.d(TAG, "starting service");
                Intent backgroundService = new Intent(MainActivity.this, BackgroundService.class);
                backgroundService.putExtra(BackgroundService.EXTRA_MESSENGER_MAINACTIVITY, new Messenger(mainHandler));
                startService(backgroundService);

                // Start the RcvMessages activity to receive messages from the handler. But how???
                Intent messages = new Intent(MainActivity.this, RcvMessages.class);
                messages.putExtra(RcvMessages.EXTRA_SONG, 10);
                startActivity(messages);
            }
        });

        /*
         * The unbind button: unbindBtn
         * Clicking this button stops the background Service.
         */
        Button unbindBtn = (Button)findViewById(R.id.unbindBtn);
        unbindBtn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {

                // Stop the background Service
                Intent backgroundService = new Intent(MainActivity.this, BackgroundService.class);
                stopService(backgroundService);
            }
        });
    }

    /*
     * This is the handler to be passed to the background Service via a Messenger.
     * NOTE: I want it to send messages to my RcvMessages activity.
     */
    private Handler mainHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            // simple handler test (does not send messages to RcvMessages activity
            String obj = (String) msg.obj;
            int arg1 = msg.arg1;
            Log.i("handleMessge_mainactivity", "obj: " + obj + " " + "arg1: " + arg1);  
        }
    };
}

/**************************************************************************************************
 * File: BackgroundService.java
 * Application: BackgroundService
 * Description: This file contains the background Service that is launched by the MainActivity's
 *     bind button.
 **************************************************************************************************/

package com.marie.mainactivity;

import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Intent;
import android.os.Bundle;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.util.Log;

/*
 * Class: BackgroundService
 * Purpose: Using the onStart() method the BackgroundService gets the reference to the
 *     Messenger instance that was passed to BackgroundService. The messenger is then
 *     used by the ServiceWorker() thread to send messages to the handler that is defined
 *     in the MainActivity class.
 */
public class BackgroundService extends Service {
    private NotificationManager notificationMgr;

    public static final String EXTRA_MESSENGER_MAINACTIVITY = "com.marie.mainactivity.EXTRA_MESSENGER_MAINACTIVITY";
    private Messenger messenger_mainactivity;

    public static final String EXTRA_MESSENGER_RCVMESSAGES = "com.marie.mainactivity.EXTRA_MESSENGER_RCVMESSAGES";
    private Messenger messenger_rcvmessages;

    public static final String EXTRA_SONG = "com.marie.mainactivity.EXTRA_SONG";
    int song = 0;

    @Override
    public void onCreate() {
        super.onCreate();

        notificationMgr = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);

        displayNotificationMessage("starting Background Service");

        Thread thr = new Thread(null, new ServiceWorker(), "BackgroundService");
        thr.start();
    }   

    /*
     * This is the ServiceWorker thread that passes messages to the handler defined in
     * the MainActivity class.
     * NOTE: Instead of passing messages to a handler in MainActivity I would like
     * it to pass messages to a handler defined in the RcvMessages activity.
     */
    class ServiceWorker implements Runnable
    {
        public void run() {
            // do background processing here... something simple

            // send a message to the handler defined in the MainActivity activity
            if (messenger_mainactivity != null)
            {
                try {
                    Message msg1 = Message.obtain();
                    msg1.obj = "SongData";
                    msg1.arg1 = 1000;
                    messenger_mainactivity.send(msg1);
                } catch (RemoteException e) {
                    e.printStackTrace();
                } catch (NullPointerException e) {
                    e.printStackTrace();
                }
            } else {
                Log.e("messenger_mainactivity", "null");
            }

            // send a message to the handler defined in the RcvMessages activity
            if (messenger_rcvmessages != null)
            {
                try {
                    Message msg1 = Message.obtain();
                    msg1.obj = "Song";
                    msg1.arg1 = song;
                    messenger_rcvmessages.send(msg1);
                } catch (RemoteException e) {
                    e.printStackTrace();
                } catch (NullPointerException e) {
                    e.printStackTrace();
                }
            } else {
                Log.e("messenger_rcvmessages", "null");
            }

            // stop the service when done...
            // BackgroundService.this.stopSelf();
            // Or use the unbindBtn in the MainActivity class.
        }
    }

    @Override
    public void onDestroy()
    {
        displayNotificationMessage("stopping Background Service");
        super.onDestroy();
    }

    /*
    @Override
    public void onStart(Intent intent, int startId) {
        super.onStart(intent, startId);
    }
    */

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

        Bundle extras = intent.getExtras();
        Messenger messenger;

        messenger = (Messenger)extras.get(EXTRA_MESSENGER_MAINACTIVITY);
        if (messenger != null) {
            messenger_mainactivity = messenger;
        }

        messenger = (Messenger)extras.get(EXTRA_MESSENGER_RCVMESSAGES);
        if (messenger != null) {
            messenger_rcvmessages = messenger;
            song = extras.getInt(EXTRA_SONG);
        }

        return START_NOT_STICKY;
    }

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

    private void displayNotificationMessage(String message)
    {
        Notification notification = new Notification(R.drawable.note, message, System.currentTimeMillis());

        PendingIntent contentIntent = PendingIntent.getActivity(this, 0, new Intent(this, MainActivity.class), 0);

        notification.setLatestEventInfo(this, "Background Service", message, contentIntent);

        notificationMgr.notify(R.id.app_notification_id, notification);
    }
}

/**************************************************************************************************
 * File: RcvMessages.java
 * Application: BackgroundService
 * Description: This file contains stub code that displays a test message in an EditText.
 **************************************************************************************************/

package com.marie.mainactivity;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.os.Messenger;
import android.text.InputType;
import android.util.Log;
import android.widget.EditText;

/*
 * Class: RcvMessages
 * Purpose: RcvMessages is stub code that I want to extend in some way to receive messages from
 *     the background Service.
 *     NOTE: I don't know who to do this.
 */
public class RcvMessages extends Activity {

    public static final String EXTRA_SONG = "com.marie.mainactivity.EXTRA_SONG";
    EditText myText;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.messages);

        myText = (EditText)findViewById(R.id.my_text);

        myText.setSingleLine();
        myText.setInputType(InputType.TYPE_NULL);

        // Display a simple test message for now.
        // myText.setText("RcvMessages here");
    }

    @Override
    public void onStart(){
        super.onStart();
        Bundle extras = getIntent().getExtras();
        int song = (Integer) extras.get(EXTRA_SONG);
        //myText.setText("song " + song);

        // Start the background Service for sending canned messages to the handler as a test.
        Intent backgroundService = new Intent(RcvMessages.this, BackgroundService.class);
        backgroundService.putExtra(BackgroundService.EXTRA_MESSENGER_RCVMESSAGES, new Messenger(rcvHandler));
        backgroundService.putExtra(BackgroundService.EXTRA_SONG, song);
        bindService(backgroundService);
    }

    private void bindService(Intent backgroundService) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onStop() {
        super.onStop();
        myText.setText("");
    }

    /*
     * This is the handler to be passed to the background Service via a Messenger.
     * NOTE: I want it to send messages to my RcvMessages activity.
     */
    private Handler rcvHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            // simple handler test
            String song = (String) msg.obj;
            int songNum = msg.arg1;
            Log.i("handleMessage_rcvmessages", "songNum: " + songNum);  

            myText.setText(song + " " + songNum);
        }
    };

}

A:

You are not binding to the service (that is done using bindService) you are attempting to start the service. If you want to bind you should use bindService and read up on that behavior from the javadoc and the link you specified.
As far as independent Handlers not working: that depends on the implementation of BackgroundService.

