docs/html/guide/components/services.jd

page.title=Services
@jd:body

<div id="qv-wrapper">
<ol id="qv">
<h2>In this document</h2>
<ol>
<li><a href="#Basics">The basics</a></li>
<ol>
  <li><a href="#Declaring">Declaring a service in the manifest</a></li>
</ol>
<li><a href="#CreatingAService">Creating a started service</a>
  <ol>
    <li><a href="#ExtendingIntentService">Extending the IntentService class</a></li>
    <li><a href="#ExtendingService">Extending the Service class</a></li>
    <li><a href="#StartingAService">Starting a service</a></li>
    <li><a href="#Stopping">Stopping a service</a></li>
  </ol>
</li>
<li><a href="#CreatingBoundService">Creating a bound service</a></li>
<li><a href="#Notifications">Sending notifications to the user</a></li>
<li><a href="#Foreground">Running a service in the foreground</a></li>
<li><a href="#Lifecycle">Managing the lifecycle of a service</a>
<ol>
  <li><a href="#LifecycleCallbacks">Implementing the lifecycle callbacks</a></li>
</ol>
</li>
</ol>

<h2>Key classes</h2>
<ol>
  <li>{@link android.app.Service}</li>
  <li>{@link android.app.IntentService}</li>
</ol>

<h2>Samples</h2>
<ol>
  <li><a href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/ServiceStartArguments.html">{@code
      ServiceStartArguments}</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/bound-services.html">Bound Services</a></li>
</ol>

</div>

<p>A {@link android.app.Service} is an application component that can perform
long-running operations in the background, and it does not provide a user interface. Another
application component can start a service, and it continues to run in the background even if the
user switches to another application. Additionally, a component can bind to a service to
interact with it and even perform interprocess communication (IPC). For example, a service can
handle network transactions, play music, perform file I/O, or interact with a content provider, all
from the background.</p>

<p>These are the three different types of services:</p>

<dl>
  <dt>Scheduled</dt>
  <dd>A service is <em>scheduled</em> when an API such as the {@link android.app.job.JobScheduler},
  introduced in Android 5.0 (API level 21), launches the service. You can use the
  {@link android.app.job.JobScheduler} by registering jobs and specifying their requirements for
  network and timing. The system then gracefully schedules the jobs for execution at the
  appropriate times. The {@link android.app.job.JobScheduler} provides many methods to define
  service-execution conditions.
    <p class="note"><strong>Note:</strong> If your app targets Android 5.0 (API level 21), Google
    recommends that you use the {@link android.app.job.JobScheduler} to execute background
    services. For more information about using this class, see the
    {@link android.app.job.JobScheduler} reference documentation.</p></dd>
  <dt>Started</dt>
  <dd>A service is <em>started</em> when an application component (such as an activity)
  calls {@link android.content.Context#startService startService()}. After it's started, a
  service can run in the background indefinitely, even if the component that started it is
  destroyed. Usually, a started service performs a single operation and does not return a result to
  the caller. For example, it can download or upload a file over the network. When the operation is
  complete, the service should stop itself.</dd>
  <dt>Bound</dt>
  <dd>A service is <em>bound</em> when an application component binds to it by calling {@link
  android.content.Context#bindService bindService()}. A bound service offers a client-server
  interface that allows components to interact with the service, send requests, receive results,
  and even do so across processes with interprocess communication (IPC). A bound service runs only
  as long as another application component is bound to it. Multiple components can bind to the
  service at once, but when all of them unbind, the service is destroyed.</dd>
</dl>

<p>Although this documentation generally discusses started and bound services separately,
your service can work both ways&mdash;it can be started (to run indefinitely) and also allow
binding. It's simply a matter of whether you implement a couple of callback methods: {@link
android.app.Service#onStartCommand onStartCommand()} to allow components to start it and {@link
android.app.Service#onBind onBind()} to allow binding.</p>

<p>Regardless of whether your application is started, bound, or both, any application component
can use the service (even from a separate application) in the same way that any component can use
an activity&mdash;by starting it with an {@link android.content.Intent}. However, you can declare
the service as <em>private</em> in the manifest file and block access from other applications.
This is discussed more in the section about <a href="#Declaring">Declaring the service in the
manifest</a>.</p>

<p class="caution"><strong>Caution:</strong> A service runs in the
main thread of its hosting process; the service does <strong>not</strong> create its own
thread and does <strong>not</strong> run in a separate process unless you specify otherwise. If
your service is going to perform any CPU-intensive work or blocking operations, such as MP3
playback or networking, you should create a new thread within the service to complete that work.
By using a separate thread, you can reduce the risk of Application Not Responding (ANR) errors,
and the application's main thread can remain dedicated to user interaction with your
activities.</p>

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

<div class="sidebox-wrapper">
<div class="sidebox">
  <h3>Should you use a service or a thread?</h3>
  <p>A service is simply a component that can run in the background, even when the user is not
interacting with your application, so you should create a service only if that is what you
need.</p>
  <p>If you must perform work outside of your main thread, but only while the user is interacting
with your application, you should instead create a new thread. For example, if you want to
play some music, but only while your activity is running, you might create
a thread in {@link android.app.Activity#onCreate onCreate()}, start running it in {@link
android.app.Activity#onStart onStart()}, and stop it in {@link android.app.Activity#onStop
onStop()}. Also consider using {@link android.os.AsyncTask} or {@link android.os.HandlerThread}
instead of the traditional {@link java.lang.Thread} class. See the <a
href="{@docRoot}guide/components/processes-and-threads.html#Threads">Processes and
Threading</a> document for more information about threads.</p>
  <p>Remember that if you do use a service, it still runs in your application's main thread by
default, so you should still create a new thread within the service if it performs intensive or
blocking operations.</p>
</div>
</div>

<p>To create a service, you must create a subclass of {@link android.app.Service} or use one
of its existing subclasses. In your implementation, you must override some callback methods that
handle key aspects of the service lifecycle and provide a mechanism that allows the components to
bind to the service, if appropriate. These are the most important callback methods that you should
override:</p>

<dl>
  <dt>{@link android.app.Service#onStartCommand onStartCommand()}</dt>
    <dd>The system invokes this method by calling {@link android.content.Context#startService
startService()} when another component (such as an activity) requests that the service be started.
When this method executes, the service is started and can run in the
background indefinitely. If you implement this, it is your responsibility to stop the service when
its work is complete by calling {@link android.app.Service#stopSelf stopSelf()} or {@link
android.content.Context#stopService stopService()}. If you only want to provide binding, you don't
need to implement this method.</dd>
  <dt>{@link android.app.Service#onBind onBind()}</dt>
    <dd>The system invokes this method by calling {@link android.content.Context#bindService
bindService()} when another component wants to bind with the service (such as to perform RPC).
In your implementation of this method, you must provide an interface that clients
use to communicate with the service by returning an {@link android.os.IBinder}. You must always
implement this method; however, if you don't want to allow binding, you should return
null.</dd>
  <dt>{@link android.app.Service#onCreate()}</dt>
    <dd>The system invokes this method to perform one-time setup procedures when the service is
initially created (before it calls either
{@link android.app.Service#onStartCommand onStartCommand()} or
{@link android.app.Service#onBind onBind()}). If the service is already running, this method is not
called.</dd>
  <dt>{@link android.app.Service#onDestroy()}</dt>
    <dd>The system invokes this method when the service is no longer used and is being destroyed.
Your service should implement this to clean up any resources such as threads, registered
listeners, or receivers. This is the last call that the service receives.</dd>
</dl>

<p>If a component starts the service by calling {@link
android.content.Context#startService startService()} (which results in a call to {@link
android.app.Service#onStartCommand onStartCommand()}), the service
continues to run until it stops itself with {@link android.app.Service#stopSelf()} or another
component stops it by calling {@link android.content.Context#stopService stopService()}.</p>

<p>If a component calls
{@link android.content.Context#bindService bindService()} to create the service and {@link
android.app.Service#onStartCommand onStartCommand()} is <em>not</em> called, the service runs
only as long as the component is bound to it. After the service is unbound from all of its clients,
the system destroys it.</p>

<p>The Android system force-stops a service only when memory is low and it must recover system
resources for the activity that has user focus. If the service is bound to an activity that has user
focus, it's less likely to be killed; if the service is declared to <a
href="#Foreground">run in the foreground</a>, it's rarely killed.
If the service is started and is long-running, the system lowers its position
in the list of background tasks over time, and the service becomes highly susceptible to
killing&mdash;if your service is started, you must design it to gracefully handle restarts
by the system. If the system kills your service, it restarts it as soon as resources become
available, but this also depends on the value that you return from {@link
android.app.Service#onStartCommand onStartCommand()}. For more information
about when the system might destroy a service, see the <a
href="{@docRoot}guide/components/processes-and-threads.html">Processes and Threading</a>
document.</p>

<p>In the following sections, you'll see how you can create the
{@link android.content.Context#startService startService()} and
{@link android.content.Context#bindService bindService()} service methods, as well as how to use
them from other application components.</p>

<h3 id="Declaring">Declaring a service in the manifest</h3>

<p>You must declare all services in your application's
manifest file, just as you do for activities and other components.</p>

<p>To declare your service, add a <a
href="{@docRoot}guide/topics/manifest/service-element.html">{@code &lt;service&gt;}</a> element
as a child of the <a
href="{@docRoot}guide/topics/manifest/application-element.html">{@code &lt;application&gt;}</a>
element. Here is an example:</p>

<pre>
&lt;manifest ... &gt;
  ...
  &lt;application ... &gt;
      &lt;service android:name=".ExampleService" /&gt;
      ...
  &lt;/application&gt;
&lt;/manifest&gt;
</pre>

<p>See the <a
href="{@docRoot}guide/topics/manifest/service-element.html">{@code &lt;service&gt;}</a> element
reference for more information about declaring your service in the manifest.</p>

<p>There are other attributes that you can include in the <a
href="{@docRoot}guide/topics/manifest/service-element.html">{@code &lt;service&gt;}</a> element to
define properties such as the permissions that are required to start the service and the process in
which the service should run. The <a
href="{@docRoot}guide/topics/manifest/service-element.html#nm">{@code android:name}</a>
attribute is the only required attribute&mdash;it specifies the class name of the service. After
you publish your application, leave this name unchanged to avoid the risk of breaking
code due to dependence on explicit intents to start or bind the service (read the blog post, <a
href="http://android-developers.blogspot.com/2011/06/things-that-cannot-change.html">Things
That Cannot Change</a>).

<p class="caution"><strong>Caution</strong>: To ensure that your app is secure, always use an
explicit intent when starting a {@link android.app.Service} and do not declare intent filters for
your services. Using an implicit intent to start a service is a security hazard because you cannot
be certain of the service that will respond to the intent, and the user cannot 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>

<p>You can ensure that your service is available to only your app by
including the <a
href="{@docRoot}guide/topics/manifest/service-element.html#exported">{@code android:exported}</a>
attribute and setting it to {@code false}. This effectively stops other apps from starting your
service, even when using an explicit intent.</p>

<h2 id="CreatingStartedService">Creating a started service</h2>

<p>A started service is one that another component starts by calling {@link
android.content.Context#startService startService()}, which results in a call to the service's
{@link android.app.Service#onStartCommand onStartCommand()} method.</p>

<p>When a service is started, it has a lifecycle that's independent of the
component that started it. The service can run in the background indefinitely, even if
the component that started it is destroyed. As such, the service should stop itself when its job
is complete by calling {@link android.app.Service#stopSelf stopSelf()}, or another component can
stop it by calling {@link android.content.Context#stopService stopService()}.</p>

<p>An application component such as an activity can start the service by calling {@link
android.content.Context#startService startService()} and passing an {@link android.content.Intent}
that specifies the service and includes any data for the service to use. The service receives
this {@link android.content.Intent} in the {@link android.app.Service#onStartCommand
onStartCommand()} method.</p>

<p>For instance, suppose an activity needs to save some data to an online database. The activity
can start a companion service and deliver it the data to save by passing an intent to {@link
android.content.Context#startService startService()}. The service receives the intent in {@link
android.app.Service#onStartCommand onStartCommand()}, connects to the Internet, and performs the
database transaction. When the transaction is complete, the service stops itself and is
destroyed.</p>

<p class="caution"><strong>Caution:</strong> A service runs in the same process as the application
in which it is declared and in the main thread of that application by default. If your service
performs intensive or blocking operations while the user interacts with an activity from the same
application, the service slows down activity performance. To avoid impacting application
performance, start a new thread inside the service.</p>

<p>Traditionally, there are two classes you can extend to create a started service:</p>

<dl>
  <dt>{@link android.app.Service}</dt>
  <dd>This is the base class for all services. When you extend this class, it's important to
create a new thread in which the service can complete all of its work; the service uses your
application's main thread by default, which can slow the performance of any activity that your
application is running.</dd>
  <dt>{@link android.app.IntentService}</dt>
  <dd>This is a subclass of {@link android.app.Service} that uses a worker thread to handle all of
the start requests, one at a time. This is the best option if you don't require that your service
handle multiple requests simultaneously. Implement {@link
android.app.IntentService#onHandleIntent onHandleIntent()}, which receives the intent for each
start request so that you can complete the background work.</dd>
</dl>

<p>The following sections describe how you can implement your service using either one for these
classes.</p>

<h3 id="ExtendingIntentService">Extending the IntentService class</h3>

<p>Because most of the started services don't need to handle multiple requests simultaneously
(which can actually be a dangerous multi-threading scenario), it's best that you
implement your service using the {@link android.app.IntentService} class.</p>

<p>The {@link android.app.IntentService} class does the following:</p>

<ul>
  <li>It creates a default worker thread that executes all of the intents that are delivered to
{@link android.app.Service#onStartCommand onStartCommand()}, separate from your application's main
thread.</li>
  <li>Creates a work queue that passes one intent at a time to your {@link
android.app.IntentService#onHandleIntent onHandleIntent()} implementation, so you never have to
worry about multi-threading.</li>
  <li>Stops the service after all of the start requests are handled, so you never have to call
{@link android.app.Service#stopSelf}.</li>
  <li>Provides a default implementation of {@link android.app.IntentService#onBind onBind()}
  that returns null.</li>
  <li>Provides a default implementation of {@link android.app.IntentService#onStartCommand
onStartCommand()} that sends the intent to the work queue and then to your {@link
android.app.IntentService#onHandleIntent onHandleIntent()} implementation.</li>
</ul>

<p>To complete the work that is provided by the client, implement {@link
android.app.IntentService#onHandleIntent onHandleIntent()}.
However, you also need to provide a small constructor for the service.</p>

<p>Here's an example implementation of {@link android.app.IntentService}:</p>

<pre>
public class HelloIntentService extends IntentService {

  /**
   * A constructor is required, and must call the super {@link android.app.IntentService#IntentService}
   * constructor with a name for the worker thread.
   */
  public HelloIntentService() {
      super("HelloIntentService");
  }

  /**
   * The IntentService calls this method from the default worker thread with
   * the intent that started the service. When this method returns, IntentService
   * stops the service, as appropriate.
   */
  &#64;Override
  protected void onHandleIntent(Intent intent) {
      // Normally we would do some work here, like download a file.
      // For our sample, we just sleep for 5 seconds.
      try {
          Thread.sleep(5000);
      } catch (InterruptedException e) {
          // Restore interrupt status.
          Thread.currentThread().interrupt();
      }
  }
}
</pre>

<p>That's all you need: a constructor and an implementation of {@link
android.app.IntentService#onHandleIntent onHandleIntent()}.</p>

<p>If you decide to also override other callback methods, such as {@link
android.app.IntentService#onCreate onCreate()}, {@link
android.app.IntentService#onStartCommand onStartCommand()}, or {@link
android.app.IntentService#onDestroy onDestroy()}, be sure to call the super implementation so
that the {@link android.app.IntentService} can properly handle the life of the worker thread.</p>

<p>For example, {@link android.app.IntentService#onStartCommand onStartCommand()} must return
the default implementation, which is how the intent is delivered to {@link
android.app.IntentService#onHandleIntent onHandleIntent()}:</p>

<pre>
&#64;Override
public int onStartCommand(Intent intent, int flags, int startId) {
    Toast.makeText(this, "service starting", Toast.LENGTH_SHORT).show();
    return super.onStartCommand(intent,flags,startId);
}
</pre>

<p>Besides {@link android.app.IntentService#onHandleIntent onHandleIntent()}, the only method
from which you don't need to call the super class is {@link android.app.IntentService#onBind
onBind()}. You need to implement this only if your service allows binding.</p>

<p>In the next section, you'll see how the same kind of service is implemented when extending
the base {@link android.app.Service} class, which uses more code, but might be
appropriate if you need to handle simultaneous start requests.</p>

<h3 id="ExtendingService">Extending the Service class</h3>

<p>Using {@link android.app.IntentService} makes your
implementation of a started service very simple. If, however, you require your service to
perform multi-threading (instead of processing start requests through a work queue), you
can extend the {@link android.app.Service} class to handle each intent.</p>

<p>For comparison, the following example code shows an implementation of the {@link
android.app.Service} class that performs the same work as the previous example using {@link
android.app.IntentService}. That is, for each start request, it uses a worker thread to perform the
job and processes only one request at a time.</p>

<pre>
public class HelloService extends Service {
  private Looper mServiceLooper;
  private ServiceHandler mServiceHandler;

  // Handler that receives messages from the thread
  private final class ServiceHandler extends Handler {
      public ServiceHandler(Looper looper) {
          super(looper);
      }
      &#64;Override
      public void handleMessage(Message msg) {
          // Normally we would do some work here, like download a file.
          // For our sample, we just sleep for 5 seconds.
          try {
              Thread.sleep(5000);
          } catch (InterruptedException e) {
              // Restore interrupt status.
              Thread.currentThread().interrupt();
          }
          // Stop the service using the startId, so that we don't stop
          // the service in the middle of handling another job
          stopSelf(msg.arg1);
      }
  }

  &#64;Override
  public void onCreate() {
    // Start up the thread running the service.  Note that we create a
    // separate thread because the service normally runs in the process's
    // main thread, which we don't want to block.  We also make it
    // background priority so CPU-intensive work will not disrupt our UI.
    HandlerThread thread = new HandlerThread("ServiceStartArguments",
            Process.THREAD_PRIORITY_BACKGROUND);
    thread.start();

    // Get the HandlerThread's Looper and use it for our Handler
    mServiceLooper = thread.getLooper();
    mServiceHandler = new ServiceHandler(mServiceLooper);
  }

  &#64;Override
  public int onStartCommand(Intent intent, int flags, int startId) {
      Toast.makeText(this, "service starting", Toast.LENGTH_SHORT).show();

      // For each start request, send a message to start a job and deliver the
      // start ID so we know which request we're stopping when we finish the job
      Message msg = mServiceHandler.obtainMessage();
      msg.arg1 = startId;
      mServiceHandler.sendMessage(msg);

      // If we get killed, after returning from here, restart
      return START_STICKY;
  }

  &#64;Override
  public IBinder onBind(Intent intent) {
      // We don't provide binding, so return null
      return null;
  }

  &#64;Override
  public void onDestroy() {
    Toast.makeText(this, "service done", Toast.LENGTH_SHORT).show();
  }
}
</pre>

<p>As you can see, it's a lot more work than using {@link android.app.IntentService}.</p>

<p>However, because you handle each call to {@link android.app.Service#onStartCommand
onStartCommand()} yourself, you can perform multiple requests simultaneously. That's not what
this example does, but if that's what you want, you can create a new thread for each
request and run them right away instead of waiting for the previous request to finish.</p>

<p>Notice that the {@link android.app.Service#onStartCommand onStartCommand()} method must return an
integer. The integer is a value that describes how the system should continue the service in the
event that the system kills it. The default implementation for {@link
android.app.IntentService} handles this for you, but you are able to modify it. The return value
from {@link android.app.Service#onStartCommand onStartCommand()} must be one of the following
constants:</p>

<dl>
  <dt>{@link android.app.Service#START_NOT_STICKY}</dt>
    <dd>If the system kills the service after {@link android.app.Service#onStartCommand
onStartCommand()} returns, <em>do not</em> recreate the service unless there are pending
intents to deliver. This is the safest option to avoid running your service when not necessary
and when your application can simply restart any unfinished jobs.</dd>
  <dt>{@link android.app.Service#START_STICKY}</dt>
    <dd>If the system kills the service after {@link android.app.Service#onStartCommand
onStartCommand()} returns, recreate the service and call {@link
android.app.Service#onStartCommand onStartCommand()}, but <em>do not</em> redeliver the last intent.
Instead, the system calls {@link android.app.Service#onStartCommand onStartCommand()} with a
null intent unless there are pending intents to start the service. In that case,
those intents are delivered. This is suitable for media players (or similar services) that are not
executing commands but are running indefinitely and waiting for a job.</dd>
  <dt>{@link android.app.Service#START_REDELIVER_INTENT}</dt>
    <dd>If the system kills the service after {@link android.app.Service#onStartCommand
onStartCommand()} returns, recreate the service and call {@link
android.app.Service#onStartCommand onStartCommand()} with the last intent that was delivered to the
service. Any pending intents are delivered in turn. This is suitable for services that are
actively performing a job that should be immediately resumed, such as downloading a file.</dd>
</dl>
<p>For more details about these return values, see the linked reference documentation for each
constant.</p>

<h3 id="StartingAService">Starting a service</h3>

<p>You can start a service from an activity or other application component by passing an
{@link android.content.Intent} (specifying the service to start) to {@link
android.content.Context#startService startService()}. The Android system calls the service's {@link
android.app.Service#onStartCommand onStartCommand()} method and passes it the {@link
android.content.Intent}.

<p class="note"><strong>Note</strong>: Never call
{@link android.app.Service#onStartCommand onStartCommand()} directly.</p>

<p>For example, an activity can start the example service in the previous section ({@code
HelloService}) using an explicit intent with {@link android.content.Context#startService
startService()}, as shown here:</p>

<pre>
Intent intent = new Intent(this, HelloService.class);
startService(intent);
</pre>

<p>The {@link android.content.Context#startService startService()} method returns immediately, and
the Android system calls the service's {@link android.app.Service#onStartCommand
onStartCommand()} method. If the service is not already running, the system first calls {@link
android.app.Service#onCreate onCreate()}, and then it calls
{@link android.app.Service#onStartCommand onStartCommand()}.</p>

<p>If the service does not also provide binding, the intent that is delivered with {@link
android.content.Context#startService startService()} is the only mode of communication between the
application component and the service. However, if you want the service to send a result back,
the client that starts the service can create a {@link android.app.PendingIntent} for a broadcast
(with {@link android.app.PendingIntent#getBroadcast getBroadcast()}) and deliver it to the service
in the {@link android.content.Intent} that starts the service. The service can then use the
broadcast to deliver a result.</p>

<p>Multiple requests to start the service result in multiple corresponding calls to the service's
{@link android.app.Service#onStartCommand onStartCommand()}. However, only one request to stop
the service (with {@link android.app.Service#stopSelf stopSelf()} or {@link
android.content.Context#stopService stopService()}) is required to stop it.</p>

<h3 id="Stopping">Stopping a service</h3>

<p>A started service must manage its own lifecycle. That is, the system does not stop or
destroy the service unless it must recover system memory and the service
continues to run after {@link android.app.Service#onStartCommand onStartCommand()} returns. The
service must stop itself by calling {@link android.app.Service#stopSelf stopSelf()}, or another
component can stop it by calling {@link android.content.Context#stopService stopService()}.</p>

<p>Once requested to stop with {@link android.app.Service#stopSelf stopSelf()} or {@link
android.content.Context#stopService stopService()}, the system destroys the service as soon as
possible.</p>

<p>If your service handles multiple requests to {@link
android.app.Service#onStartCommand onStartCommand()} concurrently, you shouldn't stop the
service when you're done processing a start request, as you might have received a new
start request (stopping at the end of the first request would terminate the second one). To avoid
this problem, you can use {@link android.app.Service#stopSelf(int)} to ensure that your request to
stop the service is always based on the most recent start request. That is, when you call {@link
android.app.Service#stopSelf(int)}, you pass the ID of the start request (the <code>startId</code>
delivered to {@link android.app.Service#onStartCommand onStartCommand()}) to which your stop request
corresponds. Then, if the service receives a new start request before you are able to call {@link
android.app.Service#stopSelf(int)}, the ID does not match and the service does not stop.</p>

<p class="caution"><strong>Caution:</strong> To avoid wasting system resources and consuming
battery power, ensure that your application stops its services when it's done working.
If necessary, other components can stop the service by calling {@link
android.content.Context#stopService stopService()}. Even if you enable binding for the service,
you must always stop the service yourself if it ever receives a call to {@link
android.app.Service#onStartCommand onStartCommand()}.</p>

<p>For more information about the lifecycle of a service, see the section below about <a
href="#Lifecycle">Managing the Lifecycle of a Service</a>.</p>

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

<p>A bound service is one that allows application components to bind to it by calling {@link
android.content.Context#bindService bindService()} to create a long-standing connection.
It generally doesn't allow components to <em>start</em> it by calling {@link
android.content.Context#startService startService()}.</p>

<p>Create a bound service when you want to interact with the service from activities
and other components in your application or to expose some of your application's functionality to
other applications through interprocess communication (IPC).</p>

<p>To create a bound service, implement the {@link
android.app.Service#onBind onBind()} callback method to return an {@link android.os.IBinder} that
defines the interface for communication with the service. Other application components can then call
{@link android.content.Context#bindService bindService()} to retrieve the interface and
begin calling methods on the service. The service lives only to serve the application component that
is bound to it, so when there are no components bound to the service, the system destroys it.
You do <em>not</em> need to stop a bound service in the same way that you must when the service is
started through {@link android.app.Service#onStartCommand onStartCommand()}.</p>

<p>To create a bound service, you must define the interface that specifies how a client can
communicate with the service. This interface between the service
and a client must be an implementation of {@link android.os.IBinder} and is what your service must
return from the {@link android.app.Service#onBind
onBind()} callback method. After the client receives the {@link android.os.IBinder}, it can begin
interacting with the service through that interface.</p>

<p>Multiple clients can bind to the service simultaneously. When a client is done interacting with
the service, it calls {@link android.content.Context#unbindService unbindService()} to unbind.
When there are no clients bound to the service, the system destroys the service.</p>

<p>There are multiple ways to implement a bound service, and the implementation is more
complicated than a started service. For these reasons, the bound service discussion appears in a
separate document about <a
href="{@docRoot}guide/components/bound-services.html">Bound Services</a>.</p>

<h2 id="Notifications">Sending notifications to the user</h2>

<p>When a service is running, it can notify the user of events using <a
href="{@docRoot}guide/topics/ui/notifiers/toasts.html">Toast Notifications</a> or <a
href="{@docRoot}guide/topics/ui/notifiers/notifications.html">Status Bar Notifications</a>.</p>

<p>A toast notification is a message that appears on the surface of the current window for only a
moment before disappearing. A status bar notification provides an icon in the status bar with a
message, which the user can select in order to take an action (such as start an activity).</p>

<p>Usually, a status bar notification is the best technique to use when background work such as
a file download has completed, and the user can now act on it. When the user
selects the notification from the expanded view, the notification can start an activity
(such as to display the downloaded file).</p>

<p>See the <a
href="{@docRoot}guide/topics/ui/notifiers/toasts.html">Toast Notifications</a> or <a
href="{@docRoot}guide/topics/ui/notifiers/notifications.html">Status Bar Notifications</a>
developer guides for more information.</p>

<h2 id="Foreground">Running a service in the foreground</h2>

<p>A foreground service is a service that the
user is actively aware of and is not a candidate for the system to kill when low on memory. A
foreground service must provide a notification for the status bar, which is placed under the
<em>Ongoing</em> heading. This means that the notification cannot be dismissed unless the service
is either stopped or removed from the foreground.</p>

<p>For example, a music player that plays music from a service should be set to run in the
foreground, because the user is explicitly aware
of its operation. The notification in the status bar might indicate the current song and allow
the user to launch an activity to interact with the music player.</p>

<p>To request that your service run in the foreground, call {@link
android.app.Service#startForeground startForeground()}. This method takes two parameters: an
integer that uniquely identifies the notification and the {@link
android.app.Notification} for the status bar. Here is an example:</p>

<pre>
Notification notification = new Notification(R.drawable.icon, getText(R.string.ticker_text),
        System.currentTimeMillis());
Intent notificationIntent = new Intent(this, ExampleActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
notification.setLatestEventInfo(this, getText(R.string.notification_title),
        getText(R.string.notification_message), pendingIntent);
startForeground(ONGOING_NOTIFICATION_ID, notification);
</pre>

<p class="caution"><strong>Caution:</strong> The integer ID that you give to {@link
android.app.Service#startForeground startForeground()} must not be 0.</p>

<p>To remove the service from the foreground, call {@link
android.app.Service#stopForeground stopForeground()}. This method takes a boolean, which indicates
whether to remove the status bar notification as well. This method does <em>not</em> stop the
service. However, if you stop the service while it's still running in the foreground, the
notification is also removed.</p>

<p>For more information about notifications, see <a
href="{@docRoot}guide/topics/ui/notifiers/notifications.html">Creating Status Bar
Notifications</a>.</p>

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

<p>The lifecycle of a service is much simpler than that of an activity. However, it's even more
important that you pay close attention to how your service is created and destroyed because a
service can run in the background without the user being aware.</p>

<p>The service lifecycle&mdash;from when it's created to when it's destroyed&mdash;can follow
either of these two paths:</p>

<ul>
<li>A started service
  <p>The service is created when another component calls {@link
android.content.Context#startService startService()}. The service then runs indefinitely and must
stop itself by calling {@link
android.app.Service#stopSelf() stopSelf()}. Another component can also stop the
service by calling {@link android.content.Context#stopService
stopService()}. When the service is stopped, the system destroys it.</p></li>

<li>A bound service
  <p>The service is created when another component (a client) calls {@link
android.content.Context#bindService bindService()}. The client then communicates with the service
through an {@link android.os.IBinder} interface. The client can close the connection by calling
{@link android.content.Context#unbindService unbindService()}. Multiple clients can bind to
the same service and when all of them unbind, the system destroys the service. The service
does <em>not</em> need to stop itself.</p></li>
</ul>

<p>These two paths are not entirely separate. You can bind to a service that is already
started with {@link android.content.Context#startService startService()}. For example, you can
start a background music service by calling {@link android.content.Context#startService
startService()} with an {@link android.content.Intent} that identifies the music to play. Later,
possibly when the user wants to exercise some control over the player or get information about the
current song, an activity can bind to the service by calling {@link
android.content.Context#bindService bindService()}. In cases such as this, {@link
android.content.Context#stopService stopService()} or {@link android.app.Service#stopSelf
stopSelf()} doesn't actually stop the service until all of the clients unbind.</p>

<h3 id="LifecycleCallbacks">Implementing the lifecycle callbacks</h3>

<p>Like an activity, a service has lifecycle callback methods that you can implement to monitor
changes in the service's state and perform work at the appropriate times. The following skeleton
service demonstrates each of the lifecycle methods:</p>

<pre>
public class ExampleService extends Service {
    int mStartMode;       // indicates how to behave if the service is killed
    IBinder mBinder;      // interface for clients that bind
    boolean mAllowRebind; // indicates whether onRebind should be used

    &#64;Override
    public void {@link android.app.Service#onCreate onCreate}() {
        // The service is being created
    }
    &#64;Override
    public int {@link android.app.Service#onStartCommand onStartCommand}(Intent intent, int flags, int startId) {
        // The service is starting, due to a call to {@link android.content.Context#startService startService()}
        return <em>mStartMode</em>;
    }
    &#64;Override
    public IBinder {@link android.app.Service#onBind onBind}(Intent intent) {
        // A client is binding to the service with {@link android.content.Context#bindService bindService()}
        return <em>mBinder</em>;
    }
    &#64;Override
    public boolean {@link android.app.Service#onUnbind onUnbind}(Intent intent) {
        // All clients have unbound with {@link android.content.Context#unbindService unbindService()}
        return <em>mAllowRebind</em>;
    }
    &#64;Override
    public void {@link android.app.Service#onRebind onRebind}(Intent intent) {
        // A client is binding to the service with {@link android.content.Context#bindService bindService()},
        // after onUnbind() has already been called
    }
    &#64;Override
    public void {@link android.app.Service#onDestroy onDestroy}() {
        // The service is no longer used and is being destroyed
    }
}
</pre>

<p class="note"><strong>Note:</strong> Unlike the activity lifecycle callback methods, you are
<em>not</em> required to call the superclass implementation of these callback methods.</p>

<img src="{@docRoot}images/service_lifecycle.png" alt="" />
<p class="img-caption"><strong>Figure 2.</strong> The service lifecycle. The diagram on the left
shows the lifecycle when the service is created with {@link android.content.Context#startService
startService()} and the diagram on the right shows the lifecycle when the service is created
with {@link android.content.Context#bindService bindService()}.</p>

<p>Figure 2 illustrates the typical callback methods for a service. Although the figure separates
services that are created by {@link android.content.Context#startService startService()} from those
created by {@link android.content.Context#bindService bindService()}, keep
in mind that any service, no matter how it's started, can potentially allow clients to bind to it.
A service that was initially started with {@link android.app.Service#onStartCommand
onStartCommand()} (by a client calling {@link android.content.Context#startService startService()})
can still receive a call to {@link android.app.Service#onBind onBind()} (when a client calls
{@link android.content.Context#bindService bindService()}).</p>

<p>By implementing these methods, you can monitor these two nested loops of the service's
lifecycle:</p>

<ul>
<li>The <strong>entire lifetime</strong> of a service occurs between the time that {@link
android.app.Service#onCreate onCreate()} is called and the time that {@link
android.app.Service#onDestroy} returns. Like an activity, a service does its initial setup in
{@link android.app.Service#onCreate onCreate()} and releases all remaining resources in {@link
android.app.Service#onDestroy onDestroy()}. For example, a
music playback service can create the thread where the music is played in {@link
android.app.Service#onCreate onCreate()}, and then it can stop the thread in {@link
android.app.Service#onDestroy onDestroy()}.

<p class="note"><strong>Note</strong>: The {@link android.app.Service#onCreate onCreate()}
and {@link android.app.Service#onDestroy onDestroy()} methods are called for all services, whether
they're created by {@link android.content.Context#startService startService()} or {@link
android.content.Context#bindService bindService()}.</p></li>

<li>The <strong>active lifetime</strong> of a service begins with a call to either {@link
android.app.Service#onStartCommand onStartCommand()} or {@link android.app.Service#onBind onBind()}.
Each method is handed the {@link
android.content.Intent} that was passed to either {@link android.content.Context#startService
startService()} or {@link android.content.Context#bindService bindService()}.
<p>If the service is started, the active lifetime ends at the same time that the entire lifetime
ends (the service is still active even after {@link android.app.Service#onStartCommand
onStartCommand()} returns). If the service is bound, the active lifetime ends when {@link
android.app.Service#onUnbind onUnbind()} returns.</p>
</li>
</ul>

<p class="note"><strong>Note:</strong> Although a started service is stopped by a call to
either {@link android.app.Service#stopSelf stopSelf()} or {@link
android.content.Context#stopService stopService()}, there is not a respective callback for the
service (there's no {@code onStop()} callback). Unless the service is bound to a client,
the system destroys it when the service is stopped&mdash;{@link
android.app.Service#onDestroy onDestroy()} is the only callback received.</p>

<p>For more information about creating a service that provides binding, see the <a
href="{@docRoot}guide/components/bound-services.html">Bound Services</a> document,
which includes more information about the {@link android.app.Service#onRebind onRebind()}
callback method in the section about <a
href="{@docRoot}guide/components/bound-services.html#Lifecycle">Managing the lifecycle of
a bound service</a>.</p>

<!--
<h2>Beginner's Path</h2>

<p>To learn how to query data from the system or other applications (such as contacts or media
stored on the device), continue with the <b><a
href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a></b>
document.</p>
-->