A bound service is the server in a client-server interface. It lets components
such as activities bind to the service, send requests, receive responses, and perform
interprocess communication (IPC). A bound service typically lives only while it serves another
application component and does not run in the background indefinitely.
This document describes how to create a bound service, including how to bind
to the service from other application components. For additional information about services in
general, such as how to deliver notifications from a service and set the service to run
in the foreground, refer to the
Services overview
.
The basics
A bound service is an implementation of the
Service
class that lets
other applications bind to it and interact with it. To provide binding for a
service, you implement the
onBind()
callback method. This
method returns an
IBinder
object that defines the programming interface that
clients can use to interact with the service.
Bind to a started service
As discussed in the
Services overview
,
you can create a service that is both started and bound. That is, you can start a
service by calling
startService()
, which lets the
service run indefinitely. You can also let a client bind to the service by
calling
bindService()
.
If you let your service be started and bound, then when the service starts,
the system
doesn't
destroy the service when all clients unbind.
Instead, you must
explicitly stop the service by calling
stopSelf()
or
stopService()
.
Although you usually implement either
onBind()
or
onStartCommand()
, it's sometimes
necessary to
implement both. For example, a music player might find it useful to let its service run
indefinitely and also provide binding. This way, an activity can start the service to play some
music, and the music continues to play even if the user leaves the application. Then, when the user
returns to the application, the activity can bind to the service to regain control of
playback.
For more information about the service lifecycle when adding binding to a started service,
see the
Manage the lifecycle of a bound service
section.
A client binds to a service by calling
bindService()
. When it does, it must
provide an implementation of
ServiceConnection
, which
monitors the connection with the service. The return value of
bindService()
indicates whether the
requested service exists and whether the client is permitted access to it.
When
the Android system creates the connection between the client and service, it
calls
onServiceConnected()
on the
ServiceConnection
. The
onServiceConnected()
method includes an
IBinder
argument, which the client then uses to communicate with the bound service.
You can connect multiple clients to a service simultaneously. However, the
system caches the
IBinder
service communication channel.
In other words, the system calls the service's
onBind()
method to generate the
IBinder
only when the first
client binds. The system then delivers that same
IBinder
to
all additional clients that bind to that same service, without calling
onBind()
again.
When the last client unbinds from the service, the system destroys the service, unless the
service was started using
startService()
.
The most important part of your bound service implementation is defining the interface
that your
onBind()
callback method returns. The following
section discusses several ways that you can define your service's
IBinder
interface.
Create a bound service
When creating a service that provides binding, you must provide an
IBinder
that provides the programming interface that clients can use to interact with the service. There
are three ways you can define the interface:
- Extend the Binder class
- If your service is private to your own application and runs in the same process
as the client, which is common, create your interface by extending the
Binder
class
and returning an instance of it from
onBind()
. The client receives the
Binder
and
can use it to directly access public methods available in either the
Binder
implementation or the
Service
.
This is the preferred technique when your service is merely a background worker for your own
application. The only use case when this is not the preferred way to create your interface is
if your service is used by other applications or across separate processes.
- Use a Messenger
- If you need your interface to work across different processes, you can create
an interface for the service with a
Messenger
. In this manner, the service
defines a
Handler
that responds to different types of
Message
objects.
This
Handler
is the basis for a
Messenger
that can then share an
IBinder
with the client, letting the client send commands to the service using
Message
objects. Additionally, the client can define a
Messenger
of
its own, so the service can send messages back.
This is the simplest way to perform interprocess communication (IPC), because the
Messenger
queues all requests into a single thread so that you don't have to design
your service to be thread-safe.
- Use AIDL
- Android Interface Definition Language (AIDL) decomposes objects into
primitives that the operating system can understand and marshalls them across processes to perform
IPC. The previous technique, using a
Messenger
, is actually based on AIDL as
its underlying structure.
As mentioned in the preceding section, the
Messenger
creates a queue of
all the client requests in a single thread, so the service receives requests one at a time. If,
however, you want your service to handle multiple requests simultaneously, then you can use AIDL
directly. In this case, your service must be thread-safe and capable of multithreading.
To use AIDL directly,
create an
.aidl
file that defines the programming interface. The Android SDK tools use
this file to generate an abstract class that implements the interface and handles IPC, which you
can then extend within your service.
Note:
For most applications, AIDL isn't the best choice to
create a bound service, because it might require multithreading capabilities and
can result in a more complicated implementation. Therefore, this document does not discuss how
to use it for your service. If you're certain that you need
to use AIDL directly, see the
AIDL
document.
Extend the Binder class
If only the local application uses your service and it doesn't need to
work across processes,
then you can implement your own
Binder
class that provides your client direct
access to public methods in the service.
Note:
This works only if the client and service are in the same
application and process, which is most common. For example, this works well for a music
application that needs to bind an activity to its own service that's playing music in the
background.
Here's how to set it up:
- In your service, create an instance of
Binder
that does
one of the following:
- Contains public methods that the client can call.
- Returns the current
Service
instance, which has public methods the
client can call.
- Returns an instance of another class hosted by the service with public methods the
client can call.
- Return this instance of
Binder
from the
onBind()
callback method.
- In the client, receive the
Binder
from the
onServiceConnected()
callback method and
make calls to the bound service using the methods provided.
Note:
The service and client must be in the same
application so that the client can cast the returned object and properly call its APIs.
The service
and client must also be in the same process, because this technique doesn't perform any
marshalling across processes.
For example, here's a service that provides clients with access to methods in the service through
a
Binder
implementation:
Kotlin
class LocalService : Service() {
// Binder given to clients.
private val binder = LocalBinder()
// Random number generator.
private val mGenerator = Random()
/** Method for clients. */
val randomNumber: Int
get() = mGenerator.nextInt(100)
/**
* Class used for the client Binder. Because we know this service always
* runs in the same process as its clients, we don't need to deal with IPC.
*/
inner class LocalBinder : Binder() {
// Return this instance of LocalService so clients can call public methods.
fun getService(): LocalService = this@LocalService
}
override fun onBind(intent: Intent): IBinder {
return binder
}
}
Java
public class LocalService extends Service {
// Binder given to clients.
private final IBinder binder = new LocalBinder();
// Random number generator.
private final Random mGenerator = new Random();
/**
* Class used for the client Binder. Because we know this service always
* runs in the same process as its clients, we don't need to deal with IPC.
*/
public class LocalBinder extends Binder {
LocalService getService() {
// Return this instance of LocalService so clients can call public methods.
return LocalService.this;
}
}
@Override
public IBinder onBind(Intent intent) {
return binder;
}
/** Method for clients. */
public int getRandomNumber() {
return mGenerator.nextInt(100);
}
}
The
LocalBinder
provides the
getService()
method for clients to retrieve the
current instance of
LocalService
. This lets clients call public methods in the
service. For example, clients can call
getRandomNumber()
from the service.
Here's an activity that binds to
LocalService
and calls
getRandomNumber()
when a button is clicked:
Kotlin
class BindingActivity : Activity() {
private lateinit var mService: LocalService
private var mBound: Boolean = false
/** Defines callbacks for service binding, passed to bindService(). */
private val connection = object : ServiceConnection {
override fun onServiceConnected(className: ComponentName, service: IBinder) {
// We've bound to LocalService, cast the IBinder and get LocalService instance.
val binder = service as LocalService.LocalBinder
mService = binder.getService()
mBound = true
}
override fun onServiceDisconnected(arg0: ComponentName) {
mBound = false
}
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.main)
}
override fun onStart() {
super.onStart()
// Bind to LocalService.
Intent(this, LocalService::class.java).also { intent ->
bindService(intent, connection, Context.BIND_AUTO_CREATE)
}
}
override fun onStop() {
super.onStop()
unbindService(connection)
mBound = false
}
/** Called when a button is clicked (the button in the layout file attaches to
* this method with the android:onClick attribute). */
fun onButtonClick(v: View) {
if (mBound) {
// Call a method from the LocalService.
// However, if this call is something that might hang, then put this request
// in a separate thread to avoid slowing down the activity performance.
val num: Int = mService.randomNumber
Toast.makeText(this, "number: $num", Toast.LENGTH_SHORT).show()
}
}
}
Java
public class BindingActivity extends Activity {
LocalService mService;
boolean mBound = false;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
@Override
protected void onStart() {
super.onStart();
// Bind to LocalService.
Intent intent = new Intent(this, LocalService.class);
bindService(intent, connection, Context.BIND_AUTO_CREATE);
}
@Override
protected void onStop() {
super.onStop();
unbindService(connection);
mBound = false;
}
/** Called when a button is clicked (the button in the layout file attaches to
* this method with the android:onClick attribute). */
public void onButtonClick(View v) {
if (mBound) {
// Call a method from the LocalService.
// However, if this call is something that might hang, then put this request
// in a separate thread to avoid slowing down the activity performance.
int num = mService.getRandomNumber();
Toast.makeText(this, "number: " + num, Toast.LENGTH_SHORT).show();
}
}
/** Defines callbacks for service binding, passed to bindService(). */
private ServiceConnection connection = new ServiceConnection() {
@Override
public void onServiceConnected(ComponentName className,
IBinder service) {
// We've bound to LocalService, cast the IBinder and get LocalService instance.
LocalBinder binder = (LocalBinder) service;
mService = binder.getService();
mBound = true;
}
@Override
public void onServiceDisconnected(ComponentName arg0) {
mBound = false;
}
};
}
The preceding sample shows how the client binds to the service using an implementation of
ServiceConnection
and the
onServiceConnected()
callback. The next
section provides more information about this process of binding to the service.
Note:
In the preceding example, the
onStop()
method unbinds the client from the service.
Unbind clients from services at appropriate times, as discussed in the
Additional notes
section.
For more sample code, see the
LocalService.java
class and the
LocalServiceActivities.java
class in
ApiDemos
.
Use a Messenger
If you need your service to communicate with remote processes, then you can use a
Messenger
to provide the interface for your service. This technique lets
you perform interprocess communication (IPC) without the need to use AIDL.
Using a
Messenger
for your interface is
simpler than using AIDL because
Messenger
queues
all calls to the service. A pure AIDL interface sends simultaneous requests to the
service, which must then handle multithreading.
For most applications, the service doesn't need to perform multithreading, so using a
Messenger
lets the service handle one call at a time. If it's important
that your service be multithreaded, use
AIDL
to define your interface.
Here's a summary of how to use a
Messenger
:
- The service implements a
Handler
that receives a callback for each
call from a client.
- The service uses the
Handler
to create a
Messenger
object
(which is a reference to the
Handler
).
- The
Messenger
creates an
IBinder
that the service
returns to clients from
onBind()
.
- Clients use the
IBinder
to instantiate the
Messenger
(that references the service's
Handler
), which the client uses to send
Message
objects to the service.
- The service receives each
Message
in its
Handler
—specifically, in the
handleMessage()
method.
In this way, there are no
methods
for the client to call on the service. Instead, the
client delivers
messages
(
Message
objects) that the service
receives in
its
Handler
.
Here's a simple example service that uses a
Messenger
interface:
Kotlin
/** Command to the service to display a message. */
private const val MSG_SAY_HELLO = 1
class MessengerService : Service() {
/**
* Target we publish for clients to send messages to IncomingHandler.
*/
private lateinit var mMessenger: Messenger
/**
* Handler of incoming messages from clients.
*/
internal class IncomingHandler(
context: Context,
private val applicationContext: Context = context.applicationContext
) : Handler() {
override fun handleMessage(msg: Message) {
when (msg.what) {
MSG_SAY_HELLO ->
Toast.makeText(applicationContext, "hello!", Toast.LENGTH_SHORT).show()
else -> super.handleMessage(msg)
}
}
}
/**
* When binding to the service, we return an interface to our messenger
* for sending messages to the service.
*/
override fun onBind(intent: Intent): IBinder? {
Toast.makeText(applicationContext, "binding", Toast.LENGTH_SHORT).show()
mMessenger = Messenger(IncomingHandler(this))
return mMessenger.binder
}
}
Java
public class MessengerService extends Service {
/**
* Command to the service to display a message.
*/
static final int MSG_SAY_HELLO = 1;
/**
* Handler of incoming messages from clients.
*/
static class IncomingHandler extends Handler {
private Context applicationContext;
IncomingHandler(Context context) {
applicationContext = context.getApplicationContext();
}
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_SAY_HELLO:
Toast.makeText(applicationContext, "hello!", Toast.LENGTH_SHORT).show();
break;
default:
super.handleMessage(msg);
}
}
}
/**
* Target we publish for clients to send messages to IncomingHandler.
*/
Messenger mMessenger;
/**
* When binding to the service, we return an interface to our messenger
* for sending messages to the service.
*/
@Override
public IBinder onBind(Intent intent) {
Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT).show();
mMessenger = new Messenger(new IncomingHandler(this));
return mMessenger.getBinder();
}
}
The
handleMessage()
method in the
Handler
is where the service receives the incoming
Message
and decides what to do, based on the
what
member.
All that a client needs to do is create a
Messenger
based on the
IBinder
returned by the service and send a message using
send()
. For example, here's an activity that binds to the
service and delivers the
MSG_SAY_HELLO
message to the service:
Kotlin
class ActivityMessenger : Activity() {
/** Messenger for communicating with the service. */
private var mService: Messenger? = null
/** Flag indicating whether we have called bind on the service. */
private var bound: Boolean = false
/**
* Class for interacting with the main interface of the service.
*/
private val mConnection = object : ServiceConnection {
override fun onServiceConnected(className: ComponentName, service: IBinder) {
// This is called when the connection with the service has been
// established, giving us the object we can use to
// interact with the service. We are communicating with the
// service using a Messenger, so here we get a client-side
// representation of that from the raw IBinder object.
mService = Messenger(service)
bound = true
}
override fun onServiceDisconnected(className: ComponentName) {
// This is called when the connection with the service has been
// unexpectedly disconnected—that is, its process crashed.
mService = null
bound = false
}
}
fun sayHello(v: View) {
if (!bound) return
// Create and send a message to the service, using a supported 'what' value.
val msg: Message = Message.obtain(null, MSG_SAY_HELLO, 0, 0)
try {
mService?.send(msg)
} catch (e: RemoteException) {
e.printStackTrace()
}
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.main)
}
override fun onStart() {
super.onStart()
// Bind to the service.
Intent(this, MessengerService::class.java).also { intent ->
bindService(intent, mConnection, Context.BIND_AUTO_CREATE)
}
}
override fun onStop() {
super.onStop()
// Unbind from the service.
if (bound) {
unbindService(mConnection)
bound = false
}
}
}
Java
public class ActivityMessenger extends Activity {
/** Messenger for communicating with the service. */
Messenger mService = null;
/** Flag indicating whether we have called bind on the service. */
boolean bound;
/**
* Class for interacting with the main interface of the service.
*/
private ServiceConnection mConnection = new ServiceConnection() {
public void onServiceConnected(ComponentName className, IBinder service) {
// This is called when the connection with the service has been
// established, giving us the object we can use to
// interact with the service. We are communicating with the
// service using a Messenger, so here we get a client-side
// representation of that from the raw IBinder object.
mService = new Messenger(service);
bound = true;
}
public void onServiceDisconnected(ComponentName className) {
// This is called when the connection with the service has been
// unexpectedly disconnected—that is, its process crashed.
mService = null;
bound = false;
}
};
public void sayHello(View v) {
if (!bound) return;
// Create and send a message to the service, using a supported 'what' value.
Message msg = Message.obtain(null, MessengerService.MSG_SAY_HELLO, 0, 0);
try {
mService.send(msg);
} catch (RemoteException e) {
e.printStackTrace();
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
@Override
protected void onStart() {
super.onStart();
// Bind to the service.
bindService(new Intent(this, MessengerService.class), mConnection,
Context.BIND_AUTO_CREATE);
}
@Override
protected void onStop() {
super.onStop();
// Unbind from the service.
if (bound) {
unbindService(mConnection);
bound = false;
}
}
}
This example doesn't show how the service can respond to the client.
If you want the
service to respond, you need to also create a
Messenger
in the client.
When the client receives the
onServiceConnected()
callback, it sends a
Message
to the service that includes
the client's
Messenger
in the
replyTo
parameter
of the
send()
method.
You can see an example of how to provide two-way messaging in the
MessengerService.java
(service) and
MessengerServiceActivities.java
(client) samples.
Bind to a service
Application components (clients) can bind to a service by calling
bindService()
. The Android
system then calls the service's
onBind()
method, which returns an
IBinder
for interacting with
the service.
The binding is asynchronous, and
bindService()
returns immediately
without
returning the
IBinder
to
the client. To receive the
IBinder
, the client must create an
instance of
ServiceConnection
and pass it to
bindService()
. The
ServiceConnection
includes a callback method that the
system calls to deliver the
IBinder
.
Note:
Only activities, services, and content providers can bind
to a service—you
can't
bind to a service from a broadcast receiver.
To bind to a service from your client, follow these steps:
- Implement
ServiceConnection
.
Your implementation must override two callback methods:
onServiceConnected()
- The system calls this to deliver the
IBinder
returned by
the service's
onBind()
method.
onServiceDisconnected()
- The Android system calls this when the connection to the service is unexpectedly
lost, such as when the service crashes or is killed. This is
not
called when the
client unbinds.
- Call
bindService()
, passing the
ServiceConnection
implementation.
Note:
If the method returns false, your
client does not have a valid connection to the service. However, do call
unbindService()
in your client. Otherwise, your client keeps the service from
shutting down when it is idle.
- When the system calls your
onServiceConnected()
callback method, you can begin making calls to the service, using
the methods defined by the interface.
- To disconnect from the service, call
unbindService()
.
If your client is still bound to a service when your app destroys the client, destruction
causes the client to unbind. It is a better practice to unbind the client as soon as it is done
interacting with the service. Doing so lets the idle service shut down. For more information
about appropriate times to bind and unbind, see the
Additional notes
section.
The following example connects the client to the service created previously by
extending the Binder class
, so all it needs to do is cast the returned
IBinder
to the
LocalBinder
class and request the
LocalService
instance:
Kotlin
var mService: LocalService
val mConnection = object : ServiceConnection {
// Called when the connection with the service is established.
override fun onServiceConnected(className: ComponentName, service: IBinder) {
// Because we have bound to an explicit
// service that is running in our own process, we can
// cast its IBinder to a concrete class and directly access it.
val binder = service as LocalService.LocalBinder
mService = binder.getService()
mBound = true
}
// Called when the connection with the service disconnects unexpectedly.
override fun onServiceDisconnected(className: ComponentName) {
Log.e(TAG, "onServiceDisconnected")
mBound = false
}
}
Java
LocalService mService;
private ServiceConnection mConnection = new ServiceConnection() {
// Called when the connection with the service is established.
public void onServiceConnected(ComponentName className, IBinder service) {
// Because we have bound to an explicit
// service that is running in our own process, we can
// cast its IBinder to a concrete class and directly access it.
LocalBinder binder = (LocalBinder) service;
mService = binder.getService();
mBound = true;
}
// Called when the connection with the service disconnects unexpectedly.
public void onServiceDisconnected(ComponentName className) {
Log.e(TAG, "onServiceDisconnected");
mBound = false;
}
};
With this
ServiceConnection
, the client can bind to a service
by passing
it to
bindService()
, as shown in the following example:
Kotlin
Intent(this, LocalService::class.java).also { intent ->
bindService(intent, connection, Context.BIND_AUTO_CREATE)
}
Java
Intent intent = new Intent(this, LocalService.class);
bindService(intent, connection, Context.BIND_AUTO_CREATE);
- The first parameter of
bindService()
is an
Intent
that explicitly names the service to bind.
Caution:
If you use an intent to bind to a
Service
, make sure your app is secure by using an
explicit
intent. Using an implicit intent to start a service is a
security hazard because you can't be certain what service responds to the intent,
and the user can't 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.
- The second parameter is the
ServiceConnection
object.
- The third parameter is a flag indicating options for the binding—usually
BIND_AUTO_CREATE
, to create the service if it's not already
alive.
Other possible values are
BIND_DEBUG_UNBIND
,
BIND_NOT_FOREGROUND
, or
0
for none.
Additional notes
Here are some important notes about binding to a service:
- Always trap
DeadObjectException
exceptions, which are thrown
when the connection has broken. This is the only exception thrown by remote methods.
- Objects are reference counted across processes.
- You usually pair the binding and unbinding during the
matching bring-up and tear-down moments of the client's lifecycle, as described in the
following examples:
- If you need to interact with the service only while your activity is visible, bind during
onStart()
and unbind during
onStop()
.
- If you want your activity to receive responses even while it is stopped in the
background, bind during
onCreate()
and unbind
during
onDestroy()
. Beware that this implies that your
activity needs to use the service the entire time it's running, even in the background, so when
the service is in another process, then you increase the weight of the process and it is
more likely to be killed by the system.
Note:
You
don't
usually bind and unbind
during your activity's
onResume()
and
onPause()
callbacks, because these callbacks occur at every
lifecycle transition.
Keep the processing that occurs at these transitions to a minimum.
Also, if
multiple activities in your application bind to the same service and there is a
transition between
two of those activities, the service might be destroyed and recreated as the current
activity unbinds
(during pause) before the next one binds (during resume). This activity transition for how
activities coordinate their lifecycles is described in
The activity lifecycle
.
For more sample code showing how to bind to a service, see the
RemoteService.java
class in
ApiDemos
.
Manage the lifecycle of a bound service
When a service is unbound from all clients, the Android system destroys it
(unless it was started using
startService()
).
So, you don't have to manage the lifecycle of your service if it's
purely a bound service. The Android system manages it for you based on
whether it is bound to any clients.
However, if you choose to implement the
onStartCommand()
callback method, then you must explicitly stop the service, because the
service is now considered
started
. In this case, the service runs until the service
stops itself with
stopSelf()
or another component calls
stopService()
, regardless of whether it is bound to any
clients.
Additionally, if your service is started and accepts binding, then when the system calls
your
onUnbind()
method, you can optionally return
true
if you want to receive a call to
onRebind()
the next time a client binds to the service.
onRebind()
returns void, but the client still receives the
IBinder
in its
onServiceConnected()
callback.
The following figure illustrates the logic for this kind of lifecycle.
Figure 1.
The lifecycle for a service that is started
and also allows binding.
For more information about the lifecycle of a started service, see the
Services overview
.