A bound service is the server in a client-server interface. A bound service allows components (such as activities) to bind to the service, send requests, receive responses, and even 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 shows you how to create a bound service, including how to bind to the service from other application components. However, you should also refer to theServices document for additional information about services in general, such as how to deliver notifications from a service, set the service to run in the foreground, and more.
A bound service is an implementation of the Service
class that allows other applications to bind to it and interact with it. To provide binding for a service, you must implement theonBind()
callback method. This method returns an IBinder
object that defines the programming interface that clients can use to interact with the service.
Binding to a Started Service
As discussed in the Services document, you can create a service that is both started and bound. That is, the service can be started by callingstartService()
, which allows the service to run indefinitely, and also allow a client to bind to the service by callingbindService()
.
If you do allow your service to be started and bound, then when the service has been started, the system doesnot destroy the service when all clients unbind. Instead, you must explicitly stop the service, by callingstopSelf()
or stopService()
.
Although you should usually implement either onBind()
or onStartCommand()
, it's sometimes necessary to implement both. For example, a music player might find it useful to allow its service to 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.
Be sure to read the section about Managing the Lifecycle of a Bound Service, for more information about the service lifecycle when adding binding to a started service.
A client can bind to the service by calling bindService()
. When it does, it must provide an implementation ofServiceConnection
, which monitors the connection with the service. ThebindService()
method returns immediately without a value, but when the Android system creates the connection between the client and service, it callsonServiceConnected()
on the ServiceConnection
, to deliver theIBinder
that the client can use to communicate with the service.
Multiple clients can connect to the service at once. However, the system calls your service'sonBind()
method to retrieve the IBinder
only when the first client binds. The system then delivers the sameIBinder
to any additional clients that bind, without callingonBind()
again.
When the last client unbinds from the service, the system destroys the service (unless the service was also started bystartService()
).
When you implement your bound service, the most important part is defining the interface that youronBind()
callback method returns. There are a few different ways you can define your service'sIBinder
interface and the following section discusses each technique.
Creating 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:
Extending the Binder class
If your service is private to your own application and runs in the same process as the client (which is common), you should 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 even the
Service
.
This is the preferred technique when your service is merely a background worker for your own application. The only reason you would not create your interface this way is because your service is used by other applications or across separate processes.
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, allowing the client to 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 theMessenger
queues all requests into a single thread so that you don't have to design your service to be thread-safe.
Using AIDL
AIDL (Android Interface Definition Language) performs all the work to decompose objects into primitives that the operating system can understand and marshall them across processes to perform IPC. The previous technique, using a
Messenger
, is actually based on AIDL as its underlying structure. As mentioned above, 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 capable of multi-threading and be built thread-safe.
To use AIDL directly, you must 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: Most applications should not use AIDL to create a bound service, because it may require multithreading capabilities and can result in a more complicated implementation. As such, AIDL is not suitable for most applications and this document does not discuss how to use it for your service. If you're certain that you need to use AIDL directly, see theAIDL document.
Extending the Binder class
If your service is used only by the local application and does not need to work across processes, then you can implement your ownBinder
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 would work 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 either:
- contains public methods that the client can call
- returns the current
Service
instance, which has public methods the client can call - or, 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 reason the service and client must be in the same application is so 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 does not perform any marshalling across processes.
For example, here's a service that provides clients access to methods in the service through aBinder
implementation:
public class LocalService extends Service { // Binder given to clients private final IBinder mBinder = 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 mBinder; } /** method for clients */ public int getRandomNumber() { return mGenerator.nextInt(100); } }
The LocalBinder
provides the getService()
method for clients to retrieve the current instance ofLocalService
. This allows clients to call public methods in the service. For example, clients can callgetRandomNumber()
from the service.
Here's an activity that binds to LocalService
and calls getRandomNumber()
when a button is clicked:
The above sample shows how the client binds to the service using an implementation ofServiceConnection
and the onServiceConnected()
callback. The next section provides more information about this process of binding to the service.
Note: The example above doesn't explicitly unbind from the service, but all clients should unbind at an appropriate time (such as when the activity pauses).
For more sample code, see the LocalService.java class and theLocalServiceActivities.java class in ApiDemos.
Using a Messenger
Compared to AIDL
When you need to perform IPC, using a Messenger
for your interface is simpler than implementing it with AIDL, becauseMessenger
queues all calls to the service, whereas, a pure AIDL interface sends simultaneous requests to the service, which must then handle multi-threading.
For most applications, the service doesn't need to perform multi-threading, so using aMessenger
allows the service to handle one call at a time. If it's important that your service be multi-threaded, then you should useAIDL to define your interface.
If you need your service to communicate with remote processes, then you can use aMessenger
to provide the interface for your service. This technique allows you to perform interprocess communication (IPC) without the need to use AIDL.
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
Handler
is used to create a Messenger
object (which is a reference to theHandler
). - The
Messenger
creates an IBinder
that the service returns to clients fromonBind()
. - Clients use the
IBinder
to instantiate the Messenger
(that references the service's Handler
), which the client uses to sendMessage
objects to the service. - The service receives each
Message
in its Handler
—specifically, in thehandleMessage()
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 itsHandler
.
Here's a simple example service that uses a Messenger
interface:
Notice that the handleMessage()
method in the Handler
is where the service receives the incoming Message
and decides what to do, based on thewhat
member.
All that a client needs to do is create a Messenger
based on theIBinder
returned by the service and send a message usingsend()
. For example, here's a simple activity that binds to the service and delivers theMSG_SAY_HELLO
message to the service:
Notice that this example does not show how the service can respond to the client. If you want the service to respond, then you need to also create aMessenger
in the client. Then when the client receives theonServiceConnected()
callback, it sends a Message
to the service that includes the client'sMessenger
in the replyTo
parameter of thesend()
method.
You can see an example of how to provide two-way messaging in the MessengerService.java (service) andMessengerServiceActivities.java (client) samples.
Binding to a Service
Application components (clients) can bind to a service by calling bindService()
. The Android system then calls the service'sonBind()
method, which returns an IBinder
for interacting with the service.
The binding is asynchronous. bindService()
returns immediately and doesnot return the IBinder
to the client. To receive theIBinder
, the client must create an instance of ServiceConnection
and pass it to bindService()
. TheServiceConnection
includes a callback method that the system calls to deliver theIBinder
.
Note: Only activities, services, and content providers can bind to a service—youcannot bind to a service from a broadcast receiver.
So, to bind to a service from your client, you must:
- 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 has crashed or has been killed. This is not called when the client unbinds. - Call
bindService()
, passing the ServiceConnection
implementation. - 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()
. When your client is destroyed, it will unbind from the service, but you should always unbind when you're done interacting with the service or when your activity pauses so that the service can shutdown while its not being used. (Appropriate times to bind and unbind is discussed more below.)
For example, the following snippet connects the client to the service created above byextending the Binder class, so all it must do is cast the returnedIBinder
to the LocalService
class and request theLocalService
instance:
With this ServiceConnection
, the client can bind to a service by passing this it tobindService()
. For example:
Intent intent = new Intent(this, LocalService.class); bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
- The first parameter of
bindService()
is an Intent
that explicitly names the service to bind (thought the intent could be implicit). - The second parameter is the
ServiceConnection
object. - The third parameter is a flag indicating options for the binding. It should usually be
BIND_AUTO_CREATE
in order to create the service if its not already alive. Other possible values areBIND_DEBUG_UNBIND
and BIND_NOT_FOREGROUND
, or0
for none.
Additional notes
Here are some important notes about binding to a service:
- You should 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 should usually pair the binding and unbinding during matching bring-up and tear-down moments of the client's lifecycle. For example:
- If you only need to interact with the service while your activity is visible, you should bind during
onStart()
and unbind during onStop()
. - If you want your activity to receive responses even while it is stopped in the background, then you can 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 if the service is in another process, then you increase the weight of the process and it becomes more likely that the system will kill it.
Note: You should usually not bind and unbind during your activity'sonResume()
and onPause()
, because these callbacks occur at every lifecycle transition and you should 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 may 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 Activities document.)
For more sample code, showing how to bind to a service, see the RemoteService.java class inApiDemos.
Managing the Lifecycle of a Bound Service
When a service is unbound from all clients, the Android system destroys it (unless it was also started withonStartCommand()
). As such, 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 to bestarted. In this case, the service runs until the service stops itself withstopSelf()
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 youronUnbind()
method, you can optionally return true
if you would like to receive a call toonRebind()
the next time a client binds to the service (instead of receiving a call toonBind()
). onRebind()
returns void, but the client still receives theIBinder
in its onServiceConnected()
callback. Below, figure 1 illustrates the logic for this kind of lifecycle.
Figure 1. The lifecycle for a service that is started and also allows binding.