绑定服务是客户端--服务器接口中的服务器。组件（如activity）和服务进行绑定后，可以发送请求、接收响应、执行进程间通信（IPC）。不会无限期在后台运行。

要提供服务绑定，必须实现onBind()回调方法，该方法返回的IBinder对象定义了客户端用来与服务进行交互的编程接口。

客户端可以通过调用bindService()绑定到服务。调用时，必须提供ServiceConnection的实现，后者会监控与服务的连接，当Android系统创建客户端与服务之间的连接时，会对ServiceConnection回调onServiceConnected()，向客户端传递用来与服务通信的IBinder。当实现绑定服务的时候，最重要的环节是定义onBind()回调方法返回的接口。

创建绑定服务

必须提供IBinder，用以提供客户端与服务进行交互的接口。

@Override
public IBinder onBind(Intent arg0) {
return sBinder;

}

客户端绑定到服务步骤:

1.实现ServiceConnection，重写两个回调方法：onServiceConnected()---系统会调用该方法以传递服务的onBind()返回的IBinder；onServiceDisconnected()---Android系统会在与服务的连接以外中断（或者随着activity 的生命周期stop）时调用该方法，当客户端取消绑定的时候，不会回调该方法

2.调用bindService()，传递ServiceConnection

3.当系统调用onServiceConnected()的回调方法时，可以使用接口定义的方法开始调用服务

4.要断开与服务的连接，请调用unBindService()

如果应用在客户端与服务仍然绑定的状态下被销毁了，则销毁会导致客户端取消绑定。

扩展binder类：通过扩展Binder类并重onBind()返回它的一个实例创建接口，客户端收到Binder后可以利用它直接访问Binder实现中的，甚至service中可用的公共方法

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);
    }
}

LocalBinder为客户端提供了getService方法，以检索LocalService的当前实例。这样客户端便可调用服务中的公用方法。如客户端可以调用服务中的getRandomNumber()方法。

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;
        }
    };
}

上例说明客户端如何使用ServiceConnection的实现和onServiceConnected()的回调绑定到服务，点击按键，会调用服务中的公共方法getRandomNumber()。客户端应在适当的时机进行取消绑定，如例子中在onStop()的时候进行取消服务绑定。

使用Messenger：如果想让接口跨不同进程工作，可以使用Messenger为服务创建接口。服务可以这种方式定义对应于不同类型Message对象的handler。该Handler是Messenger的基础，后者可以与客户端分享一个IBinder，从而让客户端利用Message对象向服务发送命令。

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();
    }
}

服务就是在Handler的handleMessage()方法中接收传入的message，并根据what成员决定下一步操作。客户端只需要根据服务返回的IBinder创建一个Messenger，然后利用send发送一条消息：

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;
        }
    }
}

使用AIDL：AIDL（Android接口定义语言）执行所有将对象分解成原语的工作，操作系统可以识别这些原语并将它们编组到各进程中以执行IPC。如果想让服务同时处理多个请求，可以直接使用AIDL，这种情况下服务必须具备多线程处理能力，并采用线程安全式设计。如果需要使用AIDL，必须创建一个定义编程接口的.aidl 文件。AndroidSDK工具利用该文件生成一个实现接口并处理IPC的抽象类，随后可在服务内对其进行扩展。