A
Service
is an
application component
that can perform
long-running operations in the background. It does not provide a user interface. Once
started, a service might continue running for some time, even after 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.
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. You should run any blocking operations on
a
separate thread
within the service to avoid Application
Not Responding (ANR) errors.
Types of Services
These are the three different types of services:
- Foreground
-
A foreground service performs some operation that is noticeable to the
user. For example, an audio app would use a foreground service to play an
audio track. Foreground services must display a
Notification
.
Foreground services continue running even when the user isn't interacting
with the app.
When you use a foreground service, you must display a notification so that
users are actively aware that the service is running. This notification cannot
be dismissed unless the service is either stopped or removed from the
foreground.
Learn more about how to configure
foreground services
in your
app.
Note:
The
WorkManager
API offers a flexible way of scheduling tasks, and is
able to
run
these jobs as foreground services
if needed. In many cases, using
WorkManager is preferable to using foreground services directly.
- Background
- A background service performs an operation that isn't directly noticed by
the user. For example, if an app used a service to compact its storage,
that would usually be a background service.
Note:
If your app targets API level 26 or higher, the system imposes
restrictions on running background
services
when the app itself isn't in the foreground. In most
situations, for example, you shouldn't
access location information from the
background
. Instead,
schedule tasks using
WorkManager
.
- 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 service 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
.
Choosing between a service and a thread
A service is simply a component that can run in the background, even when the user is not
interacting with your application, so you should create a service only if that is what you
need.
If you must perform work outside of your main thread, but only while the user is interacting
with your application, you should instead create a new thread in the context of another application
component. For example, if you want to play some music, but only while your activity is running,
you might create a thread in
onCreate()
,
start running it in
onStart()
,
and stop it in
onStop()
.
Also consider using thread pools and executors from the
java.util.concurrent
package
or
Kotlin coroutines
instead of the traditional
Thread
class. See the
Threading on Android
document for more information about
moving execution to background 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.
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 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 don't 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 responds 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.
The
Service
class 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.
The Android framework also provides the
IntentService
subclass of
Service
that uses a
worker thread to handle all of the start requests, one at a time. Using this class is
not
recommended
for new apps as it will not work well starting with Android 8 Oreo, due to the
introduction of
Background execution limits
.
Moreover, it's deprecated starting with Android 11.
You can use
JobIntentService
as a
replacement for
IntentService
that is compatible with newer versions of Android.
The following sections describe how you can implement your own custom service, however you should
strongly consider using WorkManager instead for most use cases. Consult the
guide to background processing on Android
to see if there is a solution that fits your needs.
Extending the Service class
You can extend the
Service
class
to handle each incoming intent. Here's how a basic implementation might look:
Kotlin
class HelloService : Service() {
private var serviceLooper: Looper? = null
private var serviceHandler: ServiceHandler? = null
// Handler that receives messages from the thread
private inner class ServiceHandler(looper: Looper) : Handler(looper) {
override fun handleMessage(msg: Message) {
// 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 (e: InterruptedException) {
// 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 fun 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("ServiceStartArguments", Process.THREAD_PRIORITY_BACKGROUND).apply {
start()
// Get the HandlerThread's Looper and use it for our Handler
serviceLooper = looper
serviceHandler = ServiceHandler(looper)
}
}
override fun onStartCommand(intent: Intent, flags: Int, startId: Int): Int {
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
serviceHandler?.obtainMessage()?.also { msg ->
msg.arg1 = startId
serviceHandler?.sendMessage(msg)
}
// If we get killed, after returning from here, restart
return START_STICKY
}
override fun onBind(intent: Intent): IBinder? {
// We don't provide binding, so return null
return null
}
override fun onDestroy() {
Toast.makeText(this, "service done", Toast.LENGTH_SHORT).show()
}
}
Java
public class HelloService extends Service {
private Looper serviceLooper;
private ServiceHandler serviceHandler;
// 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 doesn't 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
serviceLooper = thread.getLooper();
serviceHandler = new ServiceHandler(serviceLooper);
}
@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 = serviceHandler.obtainMessage();
msg.arg1 = startId;
serviceHandler.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();
}
}
The example code handles all incoming calls in
onStartCommand()
and posts the work to a
Handler
running on a background thread. It works just like an
IntentService
and processes all requests serially, one after another.
You could change the code to run the work on a thread pool, for example, if you'd like to run multiple requests simultaneously.
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 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
to
startService()
or
startForegroundService()
. The
Android system calls the service's
onStartCommand()
method and passes it the
Intent
,
which specifies which service to start.
Note
: If your app targets API level 26 or higher, the system
imposes restrictions on using or creating background services unless the app
itself is in the foreground. If an app needs to create a foreground service,
the app should call
startForegroundService()
. That method creates a background service, but the
method signals to the system that the service will promote itself to the
foreground. Once the service has been created, the service must call its
startForeground()
method within
five seconds.
For example, an activity can start the example service in the previous section (
HelloService
) using an explicit intent with
startService()
, as shown here:
Kotlin
startService(Intent(this, HelloService::class.java))
Java
startService(new Intent(this, HelloService.class));
The
startService()
method returns immediately, and
the Android system calls the service's
onStartCommand()
method. If the service isn't already running, the system first calls
onCreate()
, and then it calls
onStartCommand()
.
If the service doesn't 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 doesn't 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 doesn't match and the service doesn't 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
snackbar notifications
or
status bar notifications
.
A snackbar 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).
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:
- A started service
The service is created when another component calls
startService()
. The service then runs indefinitely and must
stop itself by calling
stopSelf()
. Another component can also stop the
service by calling
stopService()
. When the service is stopped, the system destroys it.
- A bound service
The service is created when another component (a client) calls
bindService()
. The client then communicates with the service
through an
IBinder
interface. The client can close the connection by calling
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 aren't 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:
Kotlin
class ExampleService : Service() {
private var startMode: Int = 0 // indicates how to behave if the service is killed
private var binder: IBinder? = null // interface for clients that bind
private var allowRebind: Boolean = false // indicates whether onRebind should be used
override fun
onCreate
() {
// The service is being created
}
override fun
onStartCommand
(intent: Intent?, flags: Int, startId: Int): Int {
// The service is starting, due to a call to startService()
return
startMode
}
override fun
onBind
(intent: Intent): IBinder? {
// A client is binding to the service with bindService()
return
binder
}
override fun
onUnbind
(intent: Intent): Boolean {
// All clients have unbound with unbindService()
return
allowRebind
}
override fun
onRebind
(intent: Intent) {
// A client is binding to the service with bindService(),
// after onUnbind() has already been called
}
override fun
onDestroy
() {
// The service is no longer used and is being destroyed
}
}
Java
public class ExampleService extends Service {
int startMode; // indicates how to behave if the service is killed
IBinder binder; // interface for clients that bind
boolean allowRebind; // 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
startMode
;
}
@Override
public IBinder
onBind
(Intent intent) {
// A client is binding to the service with
bindService()
return
binder
;
}
@Override
public boolean
onUnbind
(Intent intent) {
// All clients have unbound with
unbindService()
return
allowRebind
;
}
@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 isn't 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
.