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# Bound Services

## Quickview

- A bound service allows other components to bind to it, in order to interact with it and perform interprocess communication
- A bound service is destroyed once all clients unbind, unless the service was also started

## In this document

1. The Basics
2. Creating a Bound Service
   1. Extending the Binder class
   2. Using a Messenger
3. Binding to a Service
4. Managing the Lifecycle of a Bound Service

## Key classes

1. `Service`
2. `ServiceConnection`
3. `IBinder`

## Samples

1. `RemoteService`
2. `LocalService`

## See also

1. Services

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.

When a service is unbound from all clients, the Android system destroys it (unless it was also started with `onStartCommand()`). 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 be 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 would like to receive a call to `onRebind()` the next time a client binds to the service (instead of receiving a call to `onBind()`). `onRebind()` returns void, but the client still receives the `IBinder` in its `onServiceConnected()` callback. Below, figure 1 illustrates the logic for this kind of lifecycle.

For more information about the lifecycle of an started service, see the Services document.

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