Android FrameWork 之Binder机制初识

public class  Binder extends Object implements IBinder

IBinder

为远程对象制定了基础的接口方法，轻量级进程内与进程间IPC调用的核心部分。该类为抽象类，并且android建议通过继承 Binder去实现接口，而不是直接的去实现IBinder接口。

IBinder的核心方法是：

    public boolean transact(int code, Parcel data, Parcel reply, int flags)
        throws RemoteException;

它与Binder中的onTransact方法对应，它允许你使用IBinder（客户端）、Binder( 服务端 )交互。

并且当客户端执行完transact()方法后并不会立刻回调，而是等待 服务端 onTransact()执行完毕后才会回调。

 /**
     * Default implementation is a stub that returns false.  You will want
     * to override this to do the appropriate unmarshalling of transactions.
     *
     * <p>If you want to call this, call transact().
     */
    protected boolean onTransact(int code, Parcel data, Parcel reply,
            int flags) throws RemoteException {
        if (code == INTERFACE_TRANSACTION) {
            reply.writeString(getInterfaceDescriptor());
            return true;
        } else if (code == DUMP_TRANSACTION) {
            ParcelFileDescriptor fd = data.readFileDescriptor();
            String[] args = data.readStringArray();
            if (fd != null) {
                try {
                    dump(fd.getFileDescriptor(), args);
                } finally {
                    try {
                        fd.close();
                    } catch (IOException e) {
                        // swallowed, not propagated back to the caller
                    }
                }
            }
            // Write the StrictMode header.
            if (reply != null) {
                reply.writeNoException();
            } else {
                StrictMode.clearGatheredViolations();
            }
            return true;
        }
        return false;
    }

例如，当进程A与进程B 通过IPC通讯。

进程A调用transact()方法，将Parcel发送给进程B，进程B接收到传入的Parcel后，调用Binder.onTransact()方法，并使用Parcel进行回复。在等待进程B回复的过程中，进程A是阻塞的。

绑定服务进行交互的几种方式

1.Extending  the binder class

应用场景

Service 是Applicatin私有的，并且与Application运行在同进程中（比较普遍），因为该方式不支持进程间通讯。

创建Binder与Service：

public class LocalService extends Service {
    // Binder given to clients
    private final IBinder mBinder = new LocalBinder();
    // Random number generator
    private final Random mGenerator = new Random();

    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);
    }
}

客户端调用：

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;// 获取本地的IBinder对象句柄
            mService = binder.getService();//通过IBinder句柄，获取Service的句柄
            mBound = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName arg0) {
            mBound = false;
        }
    };
}

2.Use Messenger

应用场景：

使用消息驱动，支持IPC(Interface Progress Communication)。

===Handler异步消息处理线程详解===

示例代码：

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();
    }
}

客户端调用：

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);//如果服务连接成功，则将服务端IBinder对象传递至客户端，mMessenger.getBinder();的值也是这里被初始化的；
            mBound = true;//这样做的好处是，1）将IBinder的处理封装在Messenger类中，2）由IPC调用转为本地的异步线程处理
        }

        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;
        }
    }
}

3.Use AIDL (Android Interface Define Language)

Android本身并不推荐使用AIDL，因为它需要支持多线程，使用它会使应用变得更复杂。

关于AIDL接口的定义：请查阅AIDL。

操作步骤：

1.新建xxx.aidl文件，定义接口；

2.Android AIDL工具将自动编译在Gen目录下生成对应的Java文件（使用Eclipse开发环境）。

package com.example.android;
interface IServerServiceAidl{
void startServer(String command);
void stopServer();
}

实际上完全可以手动去实现这部分内容（如下面），

相较于使用AIDL在Gen目录下自动生成的JAVA文件来说，本质上并无区别；

public interface IServerServiceAidl extends IInterface {
	/** Local-side IPC implementation stub class. */

	public void startServer(String command) throws RemoteException;

	public void stopServer() throws RemoteException;
}

public class Proxy implements IServerServiceAidl {
	private IBinder mRemote;

	Proxy(IBinder remote) {
		mRemote = remote;
	}

	@Override
	public IBinder asBinder() {
		return mRemote;
	}

	public String getInterfaceDescriptor() {
		return Stub.DESCRIPTOR;
	}

	@Override
	public void startServer(String command) throws RemoteException {
		Parcel _data = Parcel.obtain();
		Parcel _reply = Parcel.obtain();
		try {
			_data.writeInterfaceToken(Stub.DESCRIPTOR);
			_data.writeString(command);
			mRemote.transact(Stub.TRANSACTION_STARTSERVER, _data, _reply, 0);
			_reply.readException();
		} finally {
			_reply.recycle();
			_data.recycle();
		}
	}

	@Override
	public void stopServer() throws RemoteException {
		Parcel _data = Parcel.obtain();
		Parcel _reply = Parcel.obtain();
		try {
			_data.writeInterfaceToken(Stub.DESCRIPTOR);
			mRemote.transact(Stub.TRANSACTION_STOPSERVER, _data, _reply, 0);
			_reply.readException();
		} finally {
			_reply.recycle();
			_data.recycle();
		}
	}
}

public abstract class Stub extends Binder implements IServerServiceAidl {
	public static final String DESCRIPTOR = "IServerServiceAidl";
	public static final int TRANSACTION_STARTSERVER = (IBinder.FIRST_CALL_TRANSACTION + 0);
	public static final int TRANSACTION_STOPSERVER = (IBinder.FIRST_CALL_TRANSACTION + 1);

	/** Construct the stub at attach it to the interface. */
	public Stub() {
		this.attachInterface(this, DESCRIPTOR);
	}

	/**
	 * Cast an IBinder object into an IServerServiceAidl interface, generating a
	 * proxy if needed.
	 */
	public static IServerServiceAidl asInterface(IBinder obj) {
		if ((obj == null)) {
			return null;
		}
		IInterface iin = obj.queryLocalInterface(DESCRIPTOR);

		if (((iin != null) && (iin instanceof IServerServiceAidl))) {
			Log.d("debug", "Return :Local Binder Object");
			return ((IServerServiceAidl) iin);
		}
		Log.d("debug", "Return :Remote Binder Proxy Object");
		return new Proxy(obj);
	}

	@Override
	public IBinder asBinder() {
		return this;
	}

	@Override
	public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
			throws RemoteException {
		switch (code) {
		case INTERFACE_TRANSACTION: {
			reply.writeString(DESCRIPTOR);
			return true;
		}
		case TRANSACTION_STARTSERVER: {
			data.enforceInterface(DESCRIPTOR);
			String _arg0;
			_arg0 = data.readString();
			this.startServer(_arg0);
			reply.writeNoException();
			return true;
		}
		case TRANSACTION_STOPSERVER: {
			data.enforceInterface(DESCRIPTOR);
			this.stopServer();
			reply.writeNoException();
			return true;
		}
		}
		return super.onTransact(code, data, reply, flags);
	}

}

这里当使用到Binder时，将会先判断是否是本地的Binder，如果是本地的就直接使用了没有必要去远程ServiceManager去索取；

接下来就是通过Parcel交互数据的过程，这里需要校验下DESCRIPTOR，来判断请求者是否是真的想请求指定服务程序。

附上一些扩展阅读资料以帮助更好的理解Binder：

Android Binder RPC机制了解

Android Binder设计与实现 – 设计篇

Android系统进程间通信Binder机制在应用程序框架层的Java接口源代码分析

点击下载  mDemo-Binder  机制学习资源