xref: /frameworks/base/docs/html/guide/components/services.jd

page.title=Services
@jd:body

<div id="qv-wrapper">
<ol id="qv">
<h2>Quickview</h2>
<ul>
  <li>A service can run in the background to perform work even while the user is in a different
application</li>
  <li>A service can allow other components to bind to it, in order to interact with it and
perform interprocess communication</li>
  <li>A service runs in the main thread of the application that hosts it, by default</li>
</ul>
<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 does not provide a user interface. Another
application component can start a service and it will continue 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 might
handle network transactions, play music, perform file I/O, or interact with a content provider, all
from the background.</p>

<p>A service can essentially take two forms:</p>

<dl>
  <dt>Started</dt>
  <dd>A service is "started" when an application component (such as an activity) starts it by
calling {@link android.content.Context#startService startService()}. Once 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 might download or upload a file over the network. When the operation is done, the
service should stop itself.</dd>
  <dt>Bound</dt>
  <dd>A service is "bound" 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, get 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 these two types of 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 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 private, 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&mdash;the service does <strong>not</strong> create its own thread
and does <strong>not</strong> run in a separate process (unless you specify otherwise). This means
that, if your service is going to do any CPU intensive work or blocking operations (such as MP3
playback or networking), you should create a new thread within the service to do that work. By using
a separate thread, you will 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. Thus, you should create a service only if that is what you
need.</p>
  <p>If you need to perform work outside your main thread, but only while the user is interacting
with your application, then you should probably instead create a new thread and not a service. 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()}, then 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 one
of its existing subclasses). In your implementation, you need to override some callback methods that
handle key aspects of the service lifecycle and provide a mechanism for components to bind to
the service, if appropriate. The most important callback methods you should override are:</p>

<dl>
  <dt>{@link android.app.Service#onStartCommand onStartCommand()}</dt>
    <dd>The system calls this method when another component, such as an activity,
requests that the service be started, by calling {@link android.content.Context#startService
startService()}. Once 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 done, 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 calls this method when another component wants to bind with the
service (such as to perform RPC), by calling {@link android.content.Context#bindService
bindService()}. 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, but if you don't want to allow binding, then you should return null.</dd>
  <dt>{@link android.app.Service#onCreate()}</dt>
    <dd>The system calls this method when the service is first created, to perform one-time setup
procedures (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 calls 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, receivers, etc. This is the last call 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()}), then the service
remains running 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), then the service runs
only as long as the component is bound to it. Once the service is unbound from all clients, the
system destroys it.</p>

<p>The Android system will force-stop 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, then it's less likely to be killed, and if the service is declared to <a
href="#Foreground">run in the foreground</a> (discussed later), then it will almost never be killed.
Otherwise, if the service was started and is long-running, then the system will lower its position
in the list of background tasks over time and the service will become highly susceptible to
killing&mdash;if your service is started, then 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 again (though this also depends on the value you return from {@link
android.app.Service#onStartCommand onStartCommand()}, as discussed later). 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 each type of service and how to use
it from other application components.</p>



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

<p>Like activities (and other components), you must declare all services in your application's
manifest file.</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. For example:</p>

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

<p>There are other attributes 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 permissions 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. Once
you publish your application, you should not change this name, because if you do, you might break
some functionality where explicit intents are used to reference your 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>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>Just like an activity, a service can define intent filters that allow other components to
invoke the service using implicit intents. By declaring intent filters, components
from any application installed on the user's device can potentially start your service if your
service declares an intent filter that matches the intent another application passes to {@link
android.content.Context#startService startService()}.</p>

<p>If you plan on using your service only locally (other applications do not use it), then you
don't need to (and should not) supply any intent filters. Without any intent filters, you must
start the service using an intent that explicitly names the service class. More information
about <a href="#StartingAService">starting a service</a> is discussed below.</p>

<p>Additionally, you can ensure that your service is private to your application only if
you include the <a
href="{@docRoot}guide/topics/manifest/service-element.html#exported">{@code android:exported}</a>
attribute and set it to {@code "false"}. This is effective even if your service supplies intent
filters.</p>

<p>For more information about creating intent filters for your service, see the <a
href="{@docRoot}guide/components/intents-filters.html">Intents and Intent Filters</a>
document.</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()}, resulting 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 and 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 done 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 done, the service stops itself and it is
destroyed.</p>

<p class="caution"><strong>Caution:</strong> A services runs in the same process as the application
in which it is declared and in the main thread of that application, by default. So, if your service
performs intensive or blocking operations while the user interacts with an activity from the same
application, the service will slow down activity performance. To avoid impacting application
performance, you should 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 that
you create a new thread in which to do all the service's work, because the service uses your
application's main thread, by default, which could slow the performance of any activity 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
start requests, one at a time. This is the best option if you don't require that your service
handle multiple requests simultaneously. All you need to do is implement {@link
android.app.IntentService#onHandleIntent onHandleIntent()}, which receives the intent for each
start request so you can do 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 started services don't need to handle multiple requests simultaneously
(which can actually be a dangerous multi-threading scenario), it's probably best if you
implement your service using the {@link android.app.IntentService} class.</p>

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

<ul>
  <li>Creates a default worker thread that executes all intents 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 start requests have been handled, so you never have to call
{@link android.app.Service#stopSelf}.</li>
  <li>Provides 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>All this adds up to the fact that all you need to do is implement {@link
android.app.IntentService#onHandleIntent onHandleIntent()} to do the work provided by the
client. (Though, 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.
      long endTime = System.currentTimeMillis() + 5*1000;
      while (System.currentTimeMillis() &lt; endTime) {
          synchronized (this) {
              try {
                  wait(endTime - System.currentTimeMillis());
              } catch (Exception e) {
              }
          }
      }
  }
}
</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 gets 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()} (but you only need to implement that 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 is a lot more code, but which might be
appropriate if you need to handle simultaneous start requests.</p>


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

<p>As you saw in the previous section, 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), then you
can extend the {@link android.app.Service} class to handle each intent.</p>

<p>For comparison, the following example code is an implementation of the {@link
android.app.Service} class that performs the exact same work as the example above 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.
          long endTime = System.currentTimeMillis() + 5*1000;
          while (System.currentTimeMillis() &lt; endTime) {
              synchronized (this) {
                  try {
                      wait(endTime - System.currentTimeMillis());
                  } catch (Exception e) {
                  }
              }
          }
          // 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, then 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 (as discussed above, the default implementation for {@link
android.app.IntentService} handles this for you, though 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 were pending intents to start the service, in which case,
those intents are delivered. This is suitable for media players (or similar services) that are not
executing commands, but 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}. (You should 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
HelloSevice}) using an explicit intent with {@link android.content.Context#startService
startService()}:</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()}, then calls {@link android.app.Service#onStartCommand
onStartCommand()}.</p>

<p>If the service does not also provide binding, the intent 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, then
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. So,
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>However, if your service handles multiple requests to {@link
android.app.Service#onStartCommand onStartCommand()} concurrently, then you shouldn't stop the
service when you're done processing a start request, because you might have since 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 received a new start request before you were able to call {@link
android.app.Service#stopSelf(int)}, then the ID will not match and the service will not stop.</p>

<p class="caution"><strong>Caution:</strong> It's important that your application stops its services
when it's done working, to avoid wasting system resources and consuming battery power. 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 received 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()} in order to create a long-standing connection
(and generally does not allow components to <em>start</em> it by calling {@link
android.content.Context#startService startService()}).</p>

<p>You should 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, you must 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 way you must when the service is started
through {@link android.app.Service#onStartCommand onStartCommand()}).</p>

<p>To create a bound service, the first thing you must do is 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. Once 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 at once. When a client is done interacting with the
service, it calls {@link android.content.Context#unbindService unbindService()} to unbind. Once
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, so 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>Once running, a service 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 a
moment then disappears, while 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 when some background work has completed
(such as a file completed
downloading) 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 view 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's considered to be something the
user is actively aware of and thus 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
"Ongoing" heading, which 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. For 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 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, indicating
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, then 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 two
different 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. That is, you can bind to a service that was already
started with {@link android.content.Context#startService startService()}. For example, a background
music service could be started 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 like this, {@link
android.content.Context#stopService stopService()} or {@link android.app.Service#stopSelf
stopSelf()} does not actually stop the service until all 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>By implementing these methods, you can monitor two nested loops of the service's lifecycle: </p>

<ul>
<li>The <strong>entire lifetime</strong> of a service happens between the time {@link
android.app.Service#onCreate onCreate()} is called and the time {@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 could create the thread where the music will be played in {@link
android.app.Service#onCreate onCreate()}, then stop the thread in {@link
android.app.Service#onDestroy onDestroy()}.

<p>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()}, respectively.
<p>If the service is started, the active lifetime ends 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). So, 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>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.
So, 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>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>
-->