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.
A service can essentially take two forms:
Although this documentation generally discusses these two types of 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 callback methods: {@link android.app.Service#onStartCommand onStartCommand()} to allow components to start it and {@link android.app.Service#onBind 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 {@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 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). 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.
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.
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 Processes and Threading document for more information about threads.
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.
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:
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()}.
If a component calls {@link android.content.Context#bindService bindService()} to create the service (and {@link android.app.Service#onStartCommand onStartCommand()} is not 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.
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 run in the foreground (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—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 Processes and Threading document.
In the following sections, you'll see how you can create each type of service and how to use it from other application components.
Like activities (and other components), you must declare all services in your application's manifest file.
To declare your service, add a {@code <service>} element as a child of the {@code <application>} element. For example:
<manifest ... >
...
<application ... >
<service android:name=".ExampleService" />
...
</application>
</manifest>
See the {@code <service>} element reference for more information about declaring your service in the manifest.
There are other attributes you can include in the {@code <service>} element to define properties such as permissions required to start the service and the process in which the service should run. The {@code android:name} attribute is the only required attribute—it specifies the class name of the service. Once you publish your application, you should not change this name, because if you do, you risk breaking code due to dependence on explicit intents to start or bind the service (read the blog post, Things That Cannot Change).
To ensure your app is secure, always use an explicit intent when starting or binding your {@link android.app.Service} and do not declare intent filters for the service. If it's critical that you allow for some amount of ambiguity as to which service starts, you can supply intent filters for your services and exclude the component name from the {@link android.content.Intent}, but you then must set the package for the intent with {@link android.content.Intent#setPackage setPackage()}, which provides sufficient disambiguation for the target service.
Additionally, you can ensure that your service is available to only your app by including the {@code android:exported} attribute and setting it to {@code "false"}. This effectively stops other apps from starting your service, even when using an explicit intent.
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.
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()}.
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.
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.
Caution: 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.
Traditionally, there are two classes you can extend to create a started service:
The following sections describe how you can implement your service using either one for these classes.
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.
The {@link android.app.IntentService} does the following:
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.)
Here's an example implementation of {@link android.app.IntentService}:
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.
*/
@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() < endTime) {
synchronized (this) {
try {
wait(endTime - System.currentTimeMillis());
} catch (Exception e) {
}
}
}
}
}
That's all you need: a constructor and an implementation of {@link android.app.IntentService#onHandleIntent onHandleIntent()}.
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.
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()}):
@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 {@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).
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.
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.
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.
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.
long endTime = System.currentTimeMillis() + 5*1000;
while (System.currentTimeMillis() < 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);
}
}
@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 {@link android.app.IntentService}.
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).
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:
For more details about these return values, see the linked reference documentation for each constant.
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.)
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()}:
Intent intent = new Intent(this, HelloService.class); startService(intent);
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()}.
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.
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.
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()}.
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.
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 startId
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.
Caution: 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()}.
For more information about the lifecycle of a service, see the section below about Managing the Lifecycle of a Service.
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 start it by calling {@link android.content.Context#startService startService()}).
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).
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 not need to stop a bound service in the way you must when the service is started through {@link android.app.Service#onStartCommand onStartCommand()}).
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.
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.
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 Bound Services.
Once running, a service 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 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).
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).
See the Toast Notifications or Status Bar Notifications developer guides for more information.
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.
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 {@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:
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 you give to {@link android.app.Service#startForeground startForeground()} must not be 0.
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 not stop the service. However, if you stop the service while it's still running in the foreground, then the notification is also removed.
For more information about notifications, see Creating Status Bar Notifications.
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 two different paths:
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..
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 not need to stop itself.)
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.
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 {@link android.app.Service#onCreate onCreate}() {
// The service is being created
}
@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 mStartMode;
}
@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 mBinder;
}
@Override
public boolean {@link android.app.Service#onUnbind onUnbind}(Intent intent) {
// All clients have unbound with {@link android.content.Context#unbindService unbindService()}
return mAllowRebind;
}
@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
}
@Override
public void {@link android.app.Service#onDestroy 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 {@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()}.
By implementing these methods, you can monitor two nested loops of the service's lifecycle:
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()}.
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.
Note: 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—{@link android.app.Service#onDestroy onDestroy()} is the only callback received.
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()}).
For more information about creating a service that provides binding, see the Bound Services document, which includes more information about the {@link android.app.Service#onRebind onRebind()} callback method in the section about Managing the Lifecycle of a Bound Service.