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Services

A 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.

These are the three different types of services:

Scheduled
A service is scheduled when an API such as the JobScheduler, introduced in Android 5.0 (API level 21), launches the service. You can use the 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 JobScheduler provides many methods to define service-execution conditions.

Note: If your app targets Android 5.0 (API level 21), Google recommends that you use the JobScheduler to execute background services. For more information about using this class, see the JobScheduler reference documentation.

Started
A service is started when an application component (such as an activity) calls 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.
Bound
A service is bound when an application component binds to it by calling 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.

Although this documentation generally discusses started and bound services separately, your service can work both ways—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: onStartCommand() to allow components to start it and onBind() to allow binding.

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—by starting it with an 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 Declaring the service in the manifest.

Caution: A service runs in the main thread of its hosting process; the service does not create its own thread and does not 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.

The basics

To create a service, you must create a subclass of 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:

onStartCommand()
The system invokes this method by calling 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 stopSelf() or stopService(). If you only want to provide binding, you don't need to implement this method.
onBind()
The system invokes this method by calling 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 IBinder. You must always implement this method; however, if you don't want to allow binding, you should return null.
onCreate()
The system invokes this method to perform one-time setup procedures when the service is initially created (before it calls either onStartCommand() or onBind()). If the service is already running, this method is not called.
onDestroy()
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.

If a component starts the service by calling startService() (which results in a call to onStartCommand()), the service continues to run until it stops itself with stopSelf() or another component stops it by calling stopService().

If a component calls bindService() to create the service and onStartCommand() is not 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.

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 run in the foreground, 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—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 onStartCommand(). For more information about when the system might destroy a service, see the Processes and Threading document.

In the following sections, you'll see how you can create the startService() and bindService() service methods, as well as how to use them from other application components.

Declaring a service in the manifest

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

To declare your service, add a <service> element as a child of the <application> element. Here is an example:

<manifest ... >
  ...
  <application ... >
      <service android:name=".ExampleService" />
      ...
  </application>
</manifest>

See the <service> element reference for more information about declaring your service in the manifest.

There are other attributes that you can include in the <service> 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 android:name attribute is the only required attribute—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, Things That Cannot Change).

Caution: To ensure that your app is secure, always use an explicit intent when starting a 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 bindService() with an implicit intent.

You can ensure that your service is available to only your app by including the android:exported attribute and setting it to false. This effectively stops other apps from starting your service, even when using an explicit intent.

Note: Users can see what services are running on their device. If they see a service that they don't recognize or trust, they can stop the service. In order to avoid having your service stopped accidentally by users, you need to add the android:description attribute to the <service> element in your app manifest. In the description, provide a short sentence explaining what the service does and what benefits it provides.

Creating a started service

A started service is one that another component starts by calling startService(), which results in a call to the service's onStartCommand() method.

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 stopSelf(), or another component can stop it by calling stopService().

An application component such as an activity can start the service by calling startService() and passing an Intent that specifies the service and includes any data for the service to use. The service receives this Intent in the onStartCommand() method.

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 startService(). The service receives the intent in onStartCommand(), connects to the Internet, and performs the database transaction. When the transaction is complete, the service stops itself and is destroyed.

Caution: 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.

Traditionally, there are two classes you can extend to create a started service:

Service
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.
IntentService
This is a subclass of 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 onHandleIntent(), which receives the intent for each start request so that you can complete the background work.

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

Extending the IntentService class

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 IntentService class.

The IntentService class does the following:

To complete the work that is provided by the client, implement onHandleIntent(). However, you also need to provide a small constructor for the service.

Here's an example implementation of IntentService:

public class HelloIntentService extends IntentService {

  /**
   * A constructor is required, and must call the super IntentService(String)
   * 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.
   */
  @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();
      }
  }
}

That's all you need: a constructor and an implementation of onHandleIntent().

If you decide to also override other callback methods, such as onCreate(), onStartCommand(), or onDestroy(), be sure to call the super implementation so that the IntentService can properly handle the life of the worker thread.

For example, onStartCommand() must return the default implementation, which is how the intent is delivered to onHandleIntent():

@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);
}

Besides onHandleIntent(), the only method from which you don't need to call the super class is onBind(). You need to implement this only if your service allows binding.

In the next section, you'll see how the same kind of service is implemented when extending the base Service class, which uses more code, but might be appropriate if you need to handle simultaneous start requests.

Extending the Service class

Using 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 Service class to handle each intent.

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

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);
      }
      @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);
      }
  }

  @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);
  }

  @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;
  }

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

  @Override
  public void onDestroy() {
    Toast.makeText(this, "service done", Toast.LENGTH_SHORT).show();
  }
}

As you can see, it's a lot more work than using IntentService.

However, because you handle each call to 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.

Notice that the 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 IntentService handles this for you, but you are able to modify it. The return value from onStartCommand() must be one of the following constants:

START_NOT_STICKY
If the system kills the service after onStartCommand() returns, do not 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.
START_STICKY
If the system kills the service after onStartCommand() returns, recreate the service and call onStartCommand(), but do not redeliver the last intent. Instead, the system calls 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.
START_REDELIVER_INTENT
If the system kills the service after onStartCommand() returns, recreate the service and call 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.

For more details about these return values, see the linked reference documentation for each constant.

Starting a service

You can start a service from an activity or other application component by passing an Intent (specifying the service to start) to startService(). The Android system calls the service's onStartCommand() method and passes it the Intent.

Note: Never call onStartCommand() directly.

For example, an activity can start the example service in the previous section (HelloService) using an explicit intent with startService(), as shown here:

Intent intent = new Intent(this, HelloService.class);
startService(intent);

The startService() method returns immediately, and the Android system calls the service's onStartCommand() method. If the service is not already running, the system first calls onCreate(), and then it calls onStartCommand().

If the service does not also provide binding, the intent that is delivered with 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 PendingIntent for a broadcast (with getBroadcast()) and deliver it to the service in the Intent that starts the service. The service can then use the broadcast to deliver a result.

Multiple requests to start the service result in multiple corresponding calls to the service's onStartCommand(). However, only one request to stop the service (with stopSelf() or stopService()) is required to stop it.

Stopping a service

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 onStartCommand() returns. The service must stop itself by calling stopSelf(), or another component can stop it by calling stopService().

Once requested to stop with stopSelf() or stopService(), the system destroys the service as soon as possible.

If your service handles multiple requests to 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 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 stopSelf(int), you pass the ID of the start request (the startId delivered to onStartCommand()) to which your stop request corresponds. Then, if the service receives a new start request before you are able to call stopSelf(int), the ID does not match and the service does not stop.

Caution: 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 stopService(). Even if you enable binding for the service, you must always stop the service yourself if it ever receives a call to onStartCommand().

For more information about the lifecycle of a service, see the section below about Managing the Lifecycle of a Service.

Creating a bound service

A bound service is one that allows application components to bind to it by calling bindService() to create a long-standing connection. It generally doesn't allow components to start it by calling startService().

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).

To create a bound service, implement the onBind() callback method to return an IBinder that defines the interface for communication with the service. Other application components can then call 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 not need to stop a bound service in the same way that you must when the service is started through onStartCommand().

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 IBinder and is what your service must return from the onBind() callback method. After the client receives the IBinder, it can begin interacting with the service through that interface.

Multiple clients can bind to the service simultaneously. When a client is done interacting with the service, it calls unbindService() to unbind. When there are no clients bound to the service, the system destroys the service.

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 Bound Services.

Sending notifications to the user

When a service is running, it can notify the user of events using Toast Notifications or Status Bar Notifications.

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).

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).

See the Toast Notifications or Status Bar Notifications developer guides for more information.

Running a service in the foreground

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 Ongoing heading. This means that the notification cannot be dismissed unless the service is either stopped or removed from the foreground.

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.

To request that your service run in the foreground, call startForeground(). This method takes two parameters: an integer that uniquely identifies the notification and the Notification for the status bar. Here is an example:

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);

Caution: The integer ID that you give to startForeground() must not be 0.

To remove the service from the foreground, call stopForeground(). This method takes a boolean, which indicates whether to remove the status bar notification as well. This method does not stop the service. However, if you stop the service while it's still running in the foreground, the notification is also removed.

For more information about notifications, see Creating Status Bar Notifications.

Managing the lifecycle of a service

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.

The service lifecycle—from when it's created to when it's destroyed—can follow either of these two paths:

These two paths are not entirely separate. You can bind to a service that is already started with startService(). For example, you can start a background music service by calling startService() with an 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 bindService(). In cases such as this, stopService() or stopSelf() doesn't actually stop the service until all of the clients unbind.

Implementing the lifecycle callbacks

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:

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

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

Note: Unlike the activity lifecycle callback methods, you are not required to call the superclass implementation of these callback methods.

Figure 2. The service lifecycle. The diagram on the left shows the lifecycle when the service is created with startService() and the diagram on the right shows the lifecycle when the service is created with bindService().

Figure 2 illustrates the typical callback methods for a service. Although the figure separates services that are created by startService() from those created by 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 onStartCommand() (by a client calling startService()) can still receive a call to onBind() (when a client calls bindService()).

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

Note: Although a started service is stopped by a call to either stopSelf() or stopService(), there is not a respective callback for the service (there's no onStop() callback). Unless the service is bound to a client, the system destroys it when the service is stopped—onDestroy() is the only callback received.

For more information about creating a service that provides binding, see the Bound Services document, which includes more information about the onRebind() callback method in the section about Managing the lifecycle of a bound service.

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