docs/html/guide/components/bound-services.jd

page.title=Bound Services
parent.title=Services
parent.link=services.html
@jd:body


<div id="qv-wrapper">
<ol id="qv">
<h2>In this document</h2>
<ol>
  <li><a href="#Basics">The basics</a></li>
  <li><a href="#Creating">Creating a bound service</a>
    <ol>
      <li><a href="#Binder">Extending the Binder class</a></li>
      <li><a href="#Messenger">Using a Messenger</a></li>
    </ol>
  </li>
  <li><a href="#Binding">Binding to a service</a>
    <ol>
      <li><a href="#Additional_Notes">Additional notes</a></li>
    </ol>
  </li>
  <li><a href="#Lifecycle">Managing the lifecycle of a bound service</a></li>
</ol>

<h2>Key classes</h2>
<ol>
  <li>{@link android.app.Service}</li>
  <li>{@link android.content.ServiceConnection}</li>
  <li>{@link android.os.IBinder}</li>
</ol>

<h2>Samples</h2>
<ol>
  <li><a
 href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/RemoteService.html">
  {@code
      RemoteService}</a></li>
  <li><a
 href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/LocalService.html">
 {@code
      LocalService}</a></li>
</ol>

<h2>See also</h2>
<ol>
  <li><a href="{@docRoot}guide/components/services.html">Services</a></li>
</ol>
</div>


<p>A bound service is the server in a client-server interface. It allows components
(such as activities) to bind to the service, send requests, receive responses, and perform
interprocess communication (IPC). A bound service typically lives only while it serves another
application component and does not run in the background indefinitely.</p>

<p class="note"><strong>Note:</strong> If your app targets Android 5.0 (API level 21) or later,
it's recommended that you use the {@link android.app.job.JobScheduler} to execute background
 services. For more information about {@link android.app.job.JobScheduler}, see its
 {@link android.app.job.JobScheduler API-reference documentation}.</p>

<p>This document describes how to create a bound service, including how to bind
to the service from other application components. For additional information about services in
 general, such as how to deliver notifications from a service and set the service to run
 in the foreground, refer to the <a href="{@docRoot}guide/components/services.html">
 Services</a> document.</p>

<h2 id="Basics">The basics</h2>

<p>A bound service is an implementation of the {@link android.app.Service} class that allows
other applications to bind to it and interact with it. To provide binding for a
service, you must implement the {@link android.app.Service#onBind onBind()} callback method. This
method returns an {@link android.os.IBinder} object that defines the programming interface that
clients can use to interact with the service.</p>

<div class="sidebox-wrapper">
<div class="sidebox">
  <h3>Binding to a started service</h3>

<p>As discussed in the <a href="{@docRoot}guide/components/services.html">Services</a>
document, you can create a service that is both started and bound. That is, you can start a
 service by calling {@link android.content.Context#startService startService()}, which allows the
service to run indefinitely, and you can also allow a client to bind to the service by
 calling {@link
android.content.Context#bindService bindService()}.
  <p>If you do allow your service to be started and bound, then when the service has been
started, the system does <em>not</em> destroy the service when all clients unbind.
 Instead, you must
explicitly stop the service by calling {@link android.app.Service#stopSelf stopSelf()} or {@link
android.content.Context#stopService stopService()}.</p>

<p>Although you usually implement either {@link android.app.Service#onBind onBind()}
<em>or</em> {@link android.app.Service#onStartCommand onStartCommand()}, it's sometimes
 necessary to
implement both. For example, a music player might find it useful to allow its service to run
indefinitely and also provide binding. This way, an activity can start the service to play some
music and the music continues to play even if the user leaves the application. Then, when the user
returns to the application, the activity can bind to the service to regain control of
 playback.</p>

<p>For more information about the service lifecycle when adding binding to a started service,
 see <a href="#Lifecycle">Managing the lifecycle of a bound Service</a>.</p>
</div>
</div>

<p>A client can bind to a service by calling {@link android.content.Context#bindService
bindService()}. When it does, it must provide an implementation of {@link
android.content.ServiceConnection}, which monitors the connection with the service. The {@link
android.content.Context#bindService bindService()} method returns immediately without a
 value, but
when the Android system creates the connection between the
client and service, it calls {@link
android.content.ServiceConnection#onServiceConnected onServiceConnected()} on the {@link
android.content.ServiceConnection}, to deliver the {@link android.os.IBinder} that
the client can use to communicate with the service.</p>

<p>Multiple clients can connect to a service simultaneously. However, the system calls your service's
{@link android.app.Service#onBind onBind()} method to retrieve the
 {@link android.os.IBinder} only
when the first client binds. The system then delivers the same {@link android.os.IBinder} to any
additional clients that bind, without calling {@link android.app.Service#onBind onBind()}
 again.</p>

<p>When the last client unbinds from the service, the system destroys the service, unless the
service was also started by {@link android.content.Context#startService startService()}.</p>

<p>The most important part of your bound service implementation is defining the interface
that your {@link android.app.Service#onBind onBind()} callback method returns. The following
section discusses several different ways that you can define your service's
 {@link android.os.IBinder} interface.</p>

<h2 id="Creating">Creating a bound service</h2>

<p>When creating a service that provides binding, you must provide an {@link android.os.IBinder}
that provides the programming interface that clients can use to interact with the service. There
are three ways you can define the interface:</p>

<dl>
  <dt><a href="#Binder">Extending the Binder class</a></dt>
  <dd>If your service is private to your own application and runs in the same process
  as the client
(which is common), you should create your interface by extending the {@link android.os.Binder}
 class
and returning an instance of it from
{@link android.app.Service#onBind onBind()}. The client receives the {@link android.os.Binder} and
can use it to directly access public methods available in either the {@link android.os.Binder}
implementation or the {@link android.app.Service}.
  <p>This is the preferred technique when your service is merely a background worker for your own
application. The only reason you would not create your interface this way is because
your service is used by other applications or across separate processes.</dd>

  <dt><a href="#Messenger">Using a Messenger</a></dt>
  <dd>If you need your interface to work across different processes, you can create
an interface for the service with a {@link android.os.Messenger}. In this manner, the service
defines a {@link android.os.Handler} that responds to different types of {@link
android.os.Message} objects. This {@link android.os.Handler}
is the basis for a {@link android.os.Messenger} that can then share an {@link android.os.IBinder}
with the client, allowing the client to send commands to the service using {@link
android.os.Message} objects. Additionally, the client can define a {@link android.os.Messenger} of
its own, so the service can send messages back.
  <p>This is the simplest way to perform interprocess communication (IPC), because the {@link
android.os.Messenger} queues all requests into a single thread so that you don't have to design
your service to be thread-safe.</p>
  </dd>

  <dt><a href="{@docRoot}guide/components/aidl.html">Using AIDL</a></dt>
  <dd>Android Interface Definition Language (AIDL) decomposes objects into
primitives that the operating system can understand and marshals them across processes to perform
IPC. The previous technique, using a {@link android.os.Messenger}, is actually based on AIDL as
its underlying structure. As mentioned above, the {@link android.os.Messenger} creates a queue of
all the client requests in a single thread, so the service receives requests one at a time. If,
however, you want your service to handle multiple requests simultaneously, then you can use AIDL
directly. In this case, your service must be thread-safe and capable of multi-threading.
  <p>To use AIDL directly, you must
create an {@code .aidl} file that defines the programming interface. The Android SDK tools use
this file to generate an abstract class that implements the interface and handles IPC, which you
can then extend within your service.</p>
  </dd>
</dl>

  <p class="note"><strong>Note:</strong> Most applications <em>shouldn't</em> use AIDL to
create a bound service, because it may require multithreading capabilities and
can result in a more complicated implementation. As such, AIDL is not suitable for
 most applications
and this document does not discuss how to use it for your service. If you're certain that you need
to use AIDL directly, see the <a href="{@docRoot}guide/components/aidl.html">AIDL</a>
document.</p>

<h3 id="Binder">Extending the Binder class</h3>

<p>If your service is used only by the local application and does not need to
 work across processes,
then you can implement your own {@link android.os.Binder} class that provides your client direct
access to public methods in the service.</p>

<p class="note"><strong>Note:</strong> This works only if the client and service are in the same
application and process, which is most common. For example, this would work well for a music
application that needs to bind an activity to its own service that's playing music in the
background.</p>

<p>Here's how to set it up:</p>
<ol>
  <li>In your service, create an instance of {@link android.os.Binder} that does
  one of the following:
    <ul>
      <li>Contains public methods that the client can call.</li>
      <li>Returns the current {@link android.app.Service} instance, which has public methods the
client can call.</li>
      <li>Returns an instance of another class hosted by the service with public methods the
client can call.</li>
    </ul>
  <li>Return this instance of {@link android.os.Binder} from the {@link
android.app.Service#onBind onBind()} callback method.</li>
  <li>In the client, receive the {@link android.os.Binder} from the {@link
android.content.ServiceConnection#onServiceConnected onServiceConnected()} callback method and
make calls to the bound service using the methods provided.</li>
</ol>

<p class="note"><strong>Note:</strong> The service and client must be in the same
application so that the client can cast the returned object and properly call its APIs.
 The service
and client must also be in the same process, because this technique does not perform any
marshaling across processes.</p>

<p>For example, here's a service that provides clients with access to methods in the service through
a {@link android.os.Binder} implementation:</p>

<pre>
public class LocalService extends Service {
    // Binder given to clients
    private final IBinder mBinder = new LocalBinder();
    // Random number generator
    private final Random mGenerator = new Random();

    /**
     * Class used for the client Binder.  Because we know this service always
     * runs in the same process as its clients, we don't need to deal with IPC.
     */
    public class LocalBinder extends Binder {
        LocalService getService() {
            // Return this instance of LocalService so clients can call public methods
            return LocalService.this;
        }
    }

    &#64;Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }

    /** method for clients */
    public int getRandomNumber() {
      return mGenerator.nextInt(100);
    }
}
</pre>

<p>The {@code LocalBinder} provides the {@code getService()} method for clients to retrieve the
current instance of {@code LocalService}. This allows clients to call public methods in the
service. For example, clients can call {@code getRandomNumber()} from the service.</p>

<p>Here's an activity that binds to {@code LocalService} and calls {@code getRandomNumber()}
when a button is clicked:</p>

<pre>
public class BindingActivity extends Activity {
    LocalService mService;
    boolean mBound = false;

    &#64;Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }

    &#64;Override
    protected void onStart() {
        super.onStart();
        // Bind to LocalService
        Intent intent = new Intent(this, LocalService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
    }

    &#64;Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }

    /** Called when a button is clicked (the button in the layout file attaches to
      * this method with the android:onClick attribute) */
    public void onButtonClick(View v) {
        if (mBound) {
            // Call a method from the LocalService.
            // However, if this call were something that might hang, then this request should
            // occur in a separate thread to avoid slowing down the activity performance.
            int num = mService.getRandomNumber();
            Toast.makeText(this, "number: " + num, Toast.LENGTH_SHORT).show();
        }
    }

    /** Defines callbacks for service binding, passed to bindService() */
    private ServiceConnection mConnection = new ServiceConnection() {

        &#64;Override
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // We've bound to LocalService, cast the IBinder and get LocalService instance
            LocalBinder binder = (LocalBinder) service;
            mService = binder.getService();
            mBound = true;
        }

        &#64;Override
        public void onServiceDisconnected(ComponentName arg0) {
            mBound = false;
        }
    };
}
</pre>

<p>The above sample shows how the client binds to the service using an implementation of
{@link android.content.ServiceConnection} and the {@link
android.content.ServiceConnection#onServiceConnected onServiceConnected()} callback. The next
section provides more information about this process of binding to the service.</p>

<p class="note"><strong>Note:</strong> In the example above, the
{@link android.app.Activity#onStop onStop()} method unbinds the client from the service. Clients
should unbind from services at appropriate times, as discussed in
<a href="#Additional_Notes">Additional notes</a>.
</p>

<p>For more sample code, see the <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/LocalService.html">
{@code
LocalService.java}</a> class and the <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/LocalServiceActivities.html">
{@code
LocalServiceActivities.java}</a> class in <a
href="{@docRoot}resources/samples/ApiDemos/index.html">ApiDemos</a>.</p>

<h3 id="Messenger">Using a Messenger</h3>

<div class="sidebox-wrapper">
<div class="sidebox">
  <h4>Compared to AIDL</h4>
  <p>When you need to perform IPC, using a {@link android.os.Messenger} for your interface is
simpler than using AIDL, because {@link android.os.Messenger} queues
all calls to the service. A pure AIDL interface sends simultaneous requests to the
service, which must then handle multi-threading.</p>
  <p>For most applications, the service doesn't need to perform multi-threading, so using a {@link
android.os.Messenger} allows the service to handle one call at a time. If it's important
that your service be multi-threaded, use <a
href="{@docRoot}guide/components/aidl.html">AIDL</a> to define your interface.</p>
</div>
</div>

<p>If you need your service to communicate with remote processes, then you can use a
{@link android.os.Messenger} to provide the interface for your service. This technique allows
you to perform interprocess communication (IPC) without the need to use AIDL.</p>

<p>Here's a summary of how to use a {@link android.os.Messenger}:</p>

<ol>
  <li>The service implements a {@link android.os.Handler} that receives a callback for each
call from a client.</li>
  <li>The service uses the {@link android.os.Handler} to create a {@link android.os.Messenger}
  object
(which is a reference to the {@link android.os.Handler}).</li>
  <li>The {@link android.os.Messenger} creates an {@link android.os.IBinder} that the service
returns to clients from {@link android.app.Service#onBind onBind()}.</li>
  <li>Clients use the {@link android.os.IBinder} to instantiate the {@link android.os.Messenger}
(that references the service's {@link android.os.Handler}), which the client uses to send
{@link android.os.Message} objects to the service.</li>
  <li>The service receives each {@link android.os.Message} in its {@link
android.os.Handler}&mdash;specifically, in the {@link android.os.Handler#handleMessage
handleMessage()} method.</li>
</ol>


<p>In this way, there are no <em>methods</em> for the client to call on the service. Instead, the
client delivers <em>messages</em> ({@link android.os.Message} objects) that the service
 receives in
its {@link android.os.Handler}.</p>

<p>Here's a simple example service that uses a {@link android.os.Messenger} interface:</p>

<pre>
public class MessengerService extends Service {
    /** Command to the service to display a message */
    static final int MSG_SAY_HELLO = 1;

    /**
     * Handler of incoming messages from clients.
     */
    class IncomingHandler extends Handler {
        &#64;Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_SAY_HELLO:
                    Toast.makeText(getApplicationContext(), "hello!", Toast.LENGTH_SHORT).show();
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }

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

    /**
     * When binding to the service, we return an interface to our messenger
     * for sending messages to the service.
     */
    &#64;Override
    public IBinder onBind(Intent intent) {
        Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT).show();
        return mMessenger.getBinder();
    }
}
</pre>

<p>Notice that the {@link android.os.Handler#handleMessage handleMessage()} method in the
{@link android.os.Handler} is where the service receives the incoming {@link android.os.Message}
and decides what to do, based on the {@link android.os.Message#what} member.</p>

<p>All that a client needs to do is create a {@link android.os.Messenger} based on the {@link
android.os.IBinder} returned by the service and send a message using {@link
android.os.Messenger#send send()}. For example, here's a simple activity that binds to the
service and delivers the {@code MSG_SAY_HELLO} message to the service:</p>

<pre>
public class ActivityMessenger extends Activity {
    /** Messenger for communicating with the service. */
    Messenger mService = null;

    /** Flag indicating whether we have called bind on the service. */
    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 object we can use to
            // interact with the service.  We are communicating with the
            // service using a Messenger, so here we get a client-side
            // representation of that from the raw IBinder object.
            mService = new Messenger(service);
            mBound = true;
        }

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

    public void sayHello(View v) {
        if (!mBound) return;
        // Create and send a message to the service, using a supported 'what' value
        Message msg = Message.obtain(null, MessengerService.MSG_SAY_HELLO, 0, 0);
        try {
            mService.send(msg);
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }

    &#64;Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }

    &#64;Override
    protected void onStart() {
        super.onStart();
        // Bind to the service
        bindService(new Intent(this, MessengerService.class), mConnection,
            Context.BIND_AUTO_CREATE);
    }

    &#64;Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }
}
</pre>

<p>Notice that this example does not show how the service can respond to the client.
 If you want the
service to respond, you need to also create a {@link android.os.Messenger} in the client.
When the client receives the {@link android.content.ServiceConnection#onServiceConnected
onServiceConnected()} callback, it sends a {@link android.os.Message} to the service that includes
the client's {@link android.os.Messenger} in the {@link android.os.Message#replyTo} parameter
of the {@link android.os.Messenger#send send()} method.</p>

<p>You can see an example of how to provide two-way messaging in the <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/MessengerService.html">
{@code
MessengerService.java}</a> (service) and <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/MessengerServiceActivities.html">
{@code
MessengerServiceActivities.java}</a> (client) samples.</p>

<h2 id="Binding">Binding to a service</h2>

<p>Application components (clients) can bind to a service by calling
{@link android.content.Context#bindService bindService()}. The Android
system then calls the service's {@link android.app.Service#onBind
onBind()} method, which returns an {@link android.os.IBinder} for interacting with
 the service.</p>

<p>The binding is asynchronous, and {@link android.content.Context#bindService
bindService()} returns immediately without <em>not</em> returning the {@link android.os.IBinder} to
the client. To receive the {@link android.os.IBinder}, the client must create an
 instance of {@link
android.content.ServiceConnection} and pass it to {@link android.content.Context#bindService
bindService()}. The {@link android.content.ServiceConnection} includes a callback method that the
system calls to deliver the {@link android.os.IBinder}.</p>

<p class="note"><strong>Note:</strong> Only activities, services, and content providers can bind
to a service&mdash;you <strong>can't</strong> bind to a service from a broadcast receiver.</p>

<p>To bind to a service from your client, follow these steps: </p>
<ol>
  <li>Implement {@link android.content.ServiceConnection}.
    <p>Your implementation must override two callback methods:</p>
    <dl>
      <dt>{@link android.content.ServiceConnection#onServiceConnected onServiceConnected()}</dt>
        <dd>The system calls this to deliver the {@link android.os.IBinder} returned by
the service's {@link android.app.Service#onBind onBind()} method.</dd>
      <dt>{@link android.content.ServiceConnection#onServiceDisconnected
onServiceDisconnected()}</dt>
        <dd>The Android system calls this when the connection to the service is unexpectedly
lost, such as when the service has crashed or has been killed. This is <em>not</em>
 called when the
client unbinds.</dd>
    </dl>
  </li>
  <li>Call {@link
android.content.Context#bindService bindService()}, passing the {@link
android.content.ServiceConnection} implementation. </li>
  <li>When the system calls your {@link android.content.ServiceConnection#onServiceConnected
onServiceConnected()} callback method, you can begin making calls to the service, using
the methods defined by the interface.</li>
  <li>To disconnect from the service, call {@link
android.content.Context#unbindService unbindService()}.
  <p>If your client is still bound to a service when your app destroys the client, destruction
causes the client to unbind. It is better practice to unbind the client as soon as it is done
interacting with the service. Doing so allows the idle service to shut down. For more information
about appropriate times to bind and unbind, see <a href="#Additional_Notes">Additional notes</a>.
</p>
  </li>
</ol>

<p>The following example connects the client to the service created above by
<a href="#Binder">extending the Binder class</a>, so all it must do is cast the returned
{@link android.os.IBinder} to the {@code LocalService} class and request the {@code
LocalService} instance:</p>

<pre>
LocalService mService;
private ServiceConnection mConnection = new ServiceConnection() {
    // Called when the connection with the service is established
    public void onServiceConnected(ComponentName className, IBinder service) {
        // Because we have bound to an explicit
        // service that is running in our own process, we can
        // cast its IBinder to a concrete class and directly access it.
        LocalBinder binder = (LocalBinder) service;
        mService = binder.getService();
        mBound = true;
    }

    // Called when the connection with the service disconnects unexpectedly
    public void onServiceDisconnected(ComponentName className) {
        Log.e(TAG, "onServiceDisconnected");
        mBound = false;
    }
};
</pre>

<p>With this {@link android.content.ServiceConnection}, the client can bind to a service
 by passing
it to {@link android.content.Context#bindService bindService()}, as shown in the following example:</p>

<pre>
Intent intent = new Intent(this, LocalService.class);
bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
</pre>

<ul>
  <li>The first parameter of {@link android.content.Context#bindService bindService()} is an
{@link android.content.Intent} that explicitly names the service to bind.
<p class="caution"><strong>Caution:</strong> If you use an intent to bind to a
 {@link android.app.Service}, ensure that your app is secure by using an <a href="{@docRoot}guide/components/intents-filters.html#Types">explicit</a>
intent. Using an implicit intent to start a service is a
security hazard because you can't be certain what service will respond to the intent,
and the user can't see which service starts. Beginning with Android 5.0 (API level 21),
 the system
throws an exception if you call {@link android.content.Context#bindService bindService()}
with an implicit intent.</p>
</li>

<li>The second parameter is the {@link android.content.ServiceConnection} object.</li>
<li>The third parameter is a flag indicating options for the binding. It should usually be {@link
android.content.Context#BIND_AUTO_CREATE} in order to create the service if it's not already
 alive.
Other possible values are {@link android.content.Context#BIND_DEBUG_UNBIND}
and {@link android.content.Context#BIND_NOT_FOREGROUND}, or {@code 0} for none.</li>
</ul>


<h3 id="Additional_Notes">Additional notes</h3>

<p>Here are some important notes about binding to a service:</p>
<ul>
  <li>You should always trap {@link android.os.DeadObjectException} exceptions, which are thrown
when the connection has broken. This is the only exception thrown by remote methods.</li>
  <li>Objects are reference counted across processes. </li>
  <li>You usually pair the binding and unbinding during
matching bring-up and tear-down moments of the client's lifecycle, as described in the
 following examples:
    <ul>
      <li>If you need to interact with the service only while your activity is visible, you
should bind during {@link android.app.Activity#onStart onStart()} and unbind during {@link
android.app.Activity#onStop onStop()}.</li>
      <li>If you want your activity to receive responses even while it is stopped in the
background, then you can bind during {@link android.app.Activity#onCreate onCreate()} and unbind
during {@link android.app.Activity#onDestroy onDestroy()}. Beware that this implies that your
activity needs to use the service the entire time it's running (even in the background), so if
the service is in another process, then you increase the weight of the process and it becomes
more likely that the system will kill it.</li>
    </ul>
    <p class="note"><strong>Note:</strong> You <em>don't</em> usually bind and unbind
during your activity's {@link android.app.Activity#onResume onResume()} and {@link
android.app.Activity#onPause onPause()}, because these callbacks occur at every
 lifecycle transition
and you should keep the processing that occurs at these transitions to a minimum. Also, if
multiple activities in your application bind to the same service and there is a
 transition between
two of those activities, the service may be destroyed and recreated as the current
 activity unbinds
(during pause) before the next one binds (during resume). This activity transition for how
activities coordinate their lifecycles is described in the <a
href="{@docRoot}guide/components/activities.html#CoordinatingActivities">Activities</a>
document.</p>
</ul>

<p>For more sample code, showing how to bind to a service, see the <a
href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/RemoteService.html">
{@code
RemoteService.java}</a> class in <a
href="{@docRoot}resources/samples/ApiDemos/index.html">ApiDemos</a>.</p>

<h2 id="Lifecycle">Managing the lifecycle of a bound service</h2>

<p>When a service is unbound from all clients, the Android system destroys it (unless it was also
started with {@link android.app.Service#onStartCommand onStartCommand()}). As such, you don't have
to manage the lifecycle of your service if it's purely a bound
service&mdash;the Android system manages it for you based on whether it is bound to
 any clients.</p>

<p>However, if you choose to implement the {@link android.app.Service#onStartCommand
onStartCommand()} callback method, then you must explicitly stop the service, because the
service is now considered to be <em>started</em>. In this case, the service runs until the service
stops itself with {@link android.app.Service#stopSelf()} or another component calls {@link
android.content.Context#stopService stopService()}, regardless of whether it is bound to any
clients.</p>

<p>Additionally, if your service is started and accepts binding, then when the system calls
your {@link android.app.Service#onUnbind onUnbind()} method, you can optionally return
{@code true} if you would like to receive a call to {@link android.app.Service#onRebind
onRebind()} the next time a client binds to the service. {@link android.app.Service#onRebind
onRebind()} returns void, but the client still receives the {@link android.os.IBinder} in its
{@link android.content.ServiceConnection#onServiceConnected onServiceConnected()} callback.
The following figure illustrates the logic for this kind of lifecycle.</p>

<img src="{@docRoot}images/fundamentals/service_binding_tree_lifecycle.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> The lifecycle for a service that is started
and also allows binding.</p>

<p>For more information about the lifecycle of a started service, see the <a
href="{@docRoot}guide/components/services.html#Lifecycle">Services</a> document.</p>