Android Binder通信机制浅析

一.实现Binder简单版本（我们模仿AIDL方式生成的接口文件，来自己实现Binder.代替AIDL的创建方式）

1.定义接口

package netease.com.canvasdemo;import android.os.IBinder;import android.os.IInterface;import android.os.RemoteException;/** * Created by zhukkun on 7/25/16. */public interface IBank extends IInterface {    static final String DESCRIPTOR = "netease.com.canvasdemo.IBank";    static final int TRANSACTION_queryMoney = (IBinder.FIRST_CALL_TRANSACTION + 0);    public long queryMoney(int uid) throws RemoteException;}

2.Binder实现类

package netease.com.canvasdemo;import android.os.Binder;import android.os.IBinder;import android.os.IInterface;import android.os.Parcel;import android.os.Process;import android.os.RemoteException;import android.util.Log;/** * Created by zhukkun on 7/25/16. */public class BankImpl extends Binder implements IBank {    public BankImpl() {        Log.d("ClientActivity", "BankImpl Construct,pid"+Process.myPid()+"," + this);        this.attachInterface(this, DESCRIPTOR);    }    public static IBank asInterface(IBinder obj) {        if ((obj == null)) {            return null;        }        android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);        Log.d("ClientActivity", "queryInterface return iin:" + iin +",this,"+obj);        if (((iin != null) && (iin instanceof IBank))) {            return ((IBank) iin);        }        return new BankImpl.Proxy(obj);    }    @Override    public IInterface queryLocalInterface(String descriptor) {        Log.d("ClientActivity", "queryInterface" + descriptor);        Log.d("ClientActivity", "getInterfaceDescriptor" + getInterfaceDescriptor());        if (getInterfaceDescriptor().equals(descriptor)) {            return this;        }        return null;    }    @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_queryMoney: {                data.enforceInterface(DESCRIPTOR);                int uid = data.readInt();                long result = this.queryMoney(uid);                reply.writeNoException();                reply.writeLong(result);                return true;            }        }        return super.onTransact(code, data, reply, flags);    }    @Override    public long queryMoney(int uid) throws RemoteException {        Log.d("ClientActivity", "query Money,remote pid:" + Process.myPid());        return uid * 10l;    }    private static class Proxy implements IBank {        private IBinder mRemote;        Proxy(IBinder remote) {            mRemote = remote;        }        @Override        public IBinder asBinder() {            return mRemote;        }        public java.lang.String getInterfaceDescriptor() {            return DESCRIPTOR;        }        @Override        public long queryMoney(int uid) throws RemoteException {            Parcel data = Parcel.obtain();            Parcel reply = Parcel.obtain();            long result;            try {                data.writeInterfaceToken(DESCRIPTOR);                data.writeInt(uid);                mRemote.transact(TRANSACTION_queryMoney, data, reply, 0);                reply.readException();                result = reply.readLong();            } finally {                reply.recycle();                data.recycle();            }            return result;        }    }}

3.Service

package netease.com.canvasdemo;import android.app.Service;import android.content.Intent;import android.os.IBinder;import android.support.annotation.Nullable;import android.util.Log;/** * Created by zhukkun on 7/23/16. */public class MyService extends Service {    BankImpl mBank;    @Nullable    @Override    public IBinder onBind(Intent intent) {        mBank = new BankImpl();        Log.d("ClientActivity", "onBind return:" + mBank);        return mBank;    }}

4.Client

package netease.com.canvasdemo;import android.app.Activity;import android.content.ComponentName;import android.content.Intent;import android.content.ServiceConnection;import android.os.Bundle;import android.os.IBinder;import android.os.Process;import android.util.Log;/** * Created by zhukkun on 7/23/16. */public class ClientActivity extends Activity {    private IBank remote;    private ServiceConnection con = new ServiceConnection() {        @Override        public void onServiceConnected(ComponentName name, IBinder service) {            Log.d("ClientActivity", "onServiceConnected:" + service);            try {                remote =  BankImpl.asInterface(service);                Log.d("ClientActivity", "query Money,client pid:" + Process.myPid());                Log.d("ClientActivity", "money" + remote.queryMoney(10));            }catch (Exception e){                e.printStackTrace();            }        }        @Override        public void onServiceDisconnected(ComponentName name) {        }    };    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        Intent intent = new Intent("netease.com.aidldemo.MyService");        bindService(intent, con, BIND_AUTO_CREATE);    }    @Override    protected void onDestroy() {        super.onDestroy();        unbindService(con);    }}

4.首先我们不在AndroidManifest中专门制定MyService的进程，尝试运行，来看下结果。

我们可以看到，Service 与Client运行在相同的进程中，并且在onBind中创建的实例，就是onServiceConnected中拿到的实例。

5.我们再来看看如果在AndroidManifest中指定MyService的进程，使得Service与Client运行在不同的进程中，运行来看下结果。

可以看到，服务端在onBind中创建的的进程是pid=25143,创建的IBinder实现实例是 BankImpl@52827de8，并且返回的也是这个实例。但是，在onServiceConnected中拿到的IBinder实现实例却是BinderProxy@52832fc8, 这是为了实现IPC，Android底层对IBinder进行了包装，里面的实现机制日后再分析，我们只要了解拿到的实现类不是onBind中创建的那一个就Ok了，这点在后面的分析中很重要。

我们首先得出一个结论：根据是否是跨进程，onServiceConnected回调中传回的IBinder实现实例是不同的，如果是同一进程，返回的就是Service中创建的实例，如果是跨进程，Android底层会对IBinder的实现实例进行一层包装，返回BinderProxy。

二.返回的IBinder实例的不同所造成的影响

private ServiceConnection con = new ServiceConnection() {        @Override        public void onServiceConnected(ComponentName name, IBinder service) {            Log.d("ClientActivity", "onServiceConnected:" + service);            try {                remote =  <span style="color:#ff0000;">BankImpl.asInterface(service);</span>                Log.d("ClientActivity", "query Money,client pid:" + Process.myPid());                Log.d("ClientActivity", "money" + remote.queryMoney(10));            }catch (Exception e){                e.printStackTrace();            }        }        @Override        public void onServiceDisconnected(ComponentName name) {        }    };

public class BankImpl extends Binder implements IBank {<span style="white-space:pre"></span>    public BankImpl() {        Log.d("ClientActivity", "BankImpl Construct,pid"+Process.myPid()+"," + this);        this.attachInterface(this, DESCRIPTOR); //设置当前的接口实例，和描述信息    }    public static IBank asInterface(IBinder obj) {        if ((obj == null)) {            return null;        }        android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);  //根据描述信息，取出接口实例        Log.d("ClientActivity", "queryInterface return iin:" + iin +",this,"+obj);        if (((iin != null) && (iin instanceof IBank))) {            return ((IBank) iin);        }        return new BankImpl.Proxy(obj);    } ...<span style="font-family: Arial, Helvetica, sans-serif;">}</span>

按照我们上面的例子 BankImpl()是在Service的onBind()中创建的，this.attachInterface(this, DESCRIPTOR) ，我们看下代码：

/**     * Convenience method for associating a specific interface with the Binder.     * After calling, queryLocalInterface() will be implemented for you     * to return the given owner IInterface when the corresponding     * descriptor is requested.     */    public void attachInterface(IInterface owner, String descriptor) {        mOwner = owner;        mDescriptor = descriptor;    }

再来看下，Binder的queryInterface

/** * Use information supplied to attachInterface() to return the * associated IInterface if it matches the requested * descriptor. */public IInterface queryLocalInterface(String descriptor) {    if (mDescriptor.equals(descriptor)) {        return mOwner;    }    return null;}

代码看完了以后，我们根据上面的运行catlog中进行分析。

1.在相同进程中，onBind中创建的实例就是onServiceConnect中返回的实例，所以能根据描述信息，直接返回该实例给Client，然后直接调用该实例的方法。

2.不同进程，因为返回的实例不是同一个，所以queryLocalInterface查找为null，故要将返回的实例再进行一次代理包装，把包装后的实例返回给Client进行调用。

三.使用Proxy来进行方法调用

private static class Proxy implements IBank {        private IBinder mRemote;        Proxy(IBinder remote) {            mRemote = remote;        }        @Override        public IBinder asBinder() {            return mRemote;        }        public java.lang.String getInterfaceDescriptor() {            return DESCRIPTOR;        }        @Override        public long queryMoney(int uid) throws RemoteException {            Parcel data = Parcel.obtain();            Parcel reply = Parcel.obtain();            long result;            try {                data.writeInterfaceToken(DESCRIPTOR);                data.writeInt(uid);                <span style="color:#ff0000;">mRemote.transact(TRANSACTION_queryMoney, data, reply, 0);</span>                reply.readException();                result = reply.readLong();            } finally {                reply.recycle();                data.recycle();            }            return result;        }    }

实现跨进程调用的关键语句，是上面红色的部分。通过把数据包装成Parcel，通过管道的形式传递到服务端进程中，

再看下BankImpl的onTransct()

@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_queryMoney: {                data.enforceInterface(DESCRIPTOR);                int uid = data.readInt();                long result = <span style="color:#ff0000;">this.queryMoney(uid);</span>                reply.writeNoException();                reply.writeLong(result);                return true;            }        }        return super.onTransact(code, data, reply, flags);    }

然后在onTransct()中进行参数的解析，在调用自身的方法，最后把结果写回管道里，传回客户端进程。

至于如何返回给客户端的IBinder，是如何进行包装，还有管道的具体实现，太过于底层了，以后有机会再分析。

参考文章：https://github.com/simple-android-framework/android_design_patterns_analysis/tree/master/proxy/singwhatiwanna