Bound Services at Android

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 the Services 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.

The Basics

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 the onBind() 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 calling startService(), which allows the service to run indefinitely, and also allow a client to bind to the service by calling bindService().

If you do allow your service to be started and bound, then when the service has been started, the system does not destroy the service when all clients unbind. Instead, you must explicitly stop the service, by calling stopSelf() 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 of ServiceConnection, which monitors the connection with the service. The bindService() method returns immediately without a value, but when the Android system creates the connection between the client and service, it calls onServiceConnected() on the ServiceConnection, to deliver the IBinder 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's onBind() method to retrieve the IBinder only when the first client binds. The system then delivers the same IBinder to any additional clients that bind, without calling onBind() again.

When the last client unbinds from the service, the system destroys the service (unless the service was also started by startService()).

When you implement your bound service, the most important part is defining the interface that your onBind() callback method returns. There are a few different ways you can define your service's IBinder 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.

Using 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, 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 the Messenger 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 the AIDL 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 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 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:

1. 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
2. Return this instance of Binder from the onBind() callback method.
3. 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 a Binder 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 of LocalService. This allows clients to 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:

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, mConnection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            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 were something that might hang, then this request should
            // occur 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 mConnection = 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 above 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: 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 the LocalServiceActivities.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, because Messenger 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 a Messenger allows the service to handle one call at a time. If it's important that your service be multi-threaded, then you should use AIDL to define your interface.

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 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 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:

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.
     */
    class IncomingHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_SAY_HELLO:
                    Toast.makeText(getApplicationContext(), "hello!", Toast.LENGTH_SHORT).show();
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }

    /**
     * Target we publish for clients to send messages to IncomingHandler.
     */
    final Messenger mMessenger = new Messenger(new IncomingHandler());

    /**
     * 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();
        return mMessenger.getBinder();
    }
}

Notice that 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 a simple activity that binds to the service and delivers the MSG_SAY_HELLO message to the service:

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 mBound;

    /**
     * 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);
            mBound = 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;
            mBound = false;
        }
    };

    public void sayHello(View v) {
        if (!mBound) 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 (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }
}

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 a Messenger in the client. Then 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.

Binding 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. bindService() returns immediately and does not return 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 cannot bind to a service from a broadcast receiver.

So, to bind to a service from your client, you must:

1. 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.

2. Call bindService(), passing the ServiceConnection implementation.
3. When the system calls your onServiceConnected() callback method, you can begin making calls to the service, using the methods defined by the interface.
4. 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 by extending the Binder class, so all it must do is cast the returned IBinder to the LocalService class and request the LocalService instance:

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 this it to bindService(). 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 are BIND_DEBUG_UNBIND and BIND_NOT_FOREGROUND, or 0 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's onResume() 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 in ApiDemos.

Managing the Lifecycle of a Bound Service

Figure 1. The lifecycle for a service that is started and also allows binding.