深入理解Binder机制1-AIDL原理

Binder作为Android系统中重要的进程间通信方式，了解其基本的原理，对于分析问题具有重要的作用。由于Binder架构涉及的内容比较多，后面将会从应用层、框架层、Native层、内核层四个层次来说明Binder的原理。首先将从应用层的AIDL开始逐渐深入到内核层。整个系列的文章如下：

深入理解Binder机制1-AIDL原理
深入理解Binder机制2-注册服务addService
深入理解Binder机制3-获取服务getService
深入理解Binder机制4-bindService过程分析
深入理解Binder机制5-binder驱动分析
深入理解Binder机制6-总结篇

一、AIDL

在进行进程间通信时，需要将接口定义好，定义好之后创建aidl文件,将接口方法放在文件中。客户端和服务端，aidl文件要保持一致，包括包名。在build之后，会在客户端和服务端生成接口类。

1.aidl文件

[->IRemoteService.aidl]

// IRemoteService.aidl
package com.skytoby.server;interface IRemoteService {void addPhone(String name);boolean getPhone(String name);int getPid();
}

2.服务端

[->PhoneService.java]

package com.skytoby.server;import android.app.Service;
import android.content.Intent;
import android.os.IBinder;
import android.os.RemoteException;
import android.util.Log;
import java.util.ArrayList;
import java.util.List;public class PhoneService extends Service {List<String> phones = new ArrayList<>();//实现binder接口private IRemoteService.Stub mBinder = new IRemoteService.Stub() {@Overridepublic void addPhone(String name) throws RemoteException {Log.d("phone binder","server add phone:"+name);phones.add(name);}@Overridepublic boolean getPhone(String name) throws RemoteException {Log.d("phone binder","server get phone:"+name);if(phones.contains(name)){return true;}return false;}@Overridepublic int getPid() throws RemoteException {Log.d("phone binder","server getPid "+android.os.Process.myPid());return android.os.Process.myPid();}@Overridepublic void linkToDeath(IBinder.DeathRecipient recipient, int flags) {super.linkToDeath(recipient, flags);Log.d("phone binder","server getPid linkToDeath");}};@Overridepublic void onCreate() {Log.d("phone binder","server onCreate");super.onCreate();}@Overridepublic IBinder onBind(Intent intent) {Log.d("phone binder","server onBind");return mBinder;}@Overridepublic boolean onUnbind(Intent intent) {Log.d("phone binder","server onUnbind");return super.onUnbind(intent);}@Overridepublic void onDestroy() {Log.d("phone binder","server onDestroy");super.onDestroy();}
}

3.客户端

[->MainActivity.java]

package com.skytoby.client;import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.IBinder;
import android.os.RemoteException;
import android.support.design.widget.BottomNavigationView;
import android.support.v7.app.AppCompatActivity;
import android.support.annotation.NonNull;
import android.util.Log;
import android.view.MenuItem;
import android.widget.TextView;
import com.skytoby.server.IRemoteService;public class MainActivity extends AppCompatActivity {private TextView mTextMessage;private IRemoteService mService;private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener= new BottomNavigationView.OnNavigationItemSelectedListener() {@Overridepublic boolean onNavigationItemSelected(@NonNull MenuItem item) {switch (item.getItemId()) {case R.id.navigation_home:mTextMessage.setText(R.string.title_home);bindService();return true;case R.id.navigation_dashboard:mTextMessage.setText(R.string.title_dashboard);unbindService();return true;case R.id.navigation_notifications:mTextMessage.setText(R.string.title_notifications);killService();return true;}return false;}};@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);BottomNavigationView navView = findViewById(R.id.nav_view);mTextMessage = findViewById(R.id.message);navView.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);Log.d("phone binder","onCreate");}void bindService(){Intent intent = new Intent();intent.setComponent(new ComponentName("com.skytoby.server","com.skytoby.server.PhoneService"));bindService(intent, connection, Context.BIND_AUTO_CREATE);}void unbindService(){if(mService!=null){unbindService(connection);}}void killService(){try {if(mService!=null){android.os.Process.killProcess(mService.getPid());}} catch (RemoteException e) {e.printStackTrace();}}ServiceConnection connection = new ServiceConnection() {@Overridepublic void onServiceConnected(ComponentName name, IBinder service) {Log.d("phone binder","onServiceConnected");try {//获取代理mService = IRemoteService.Stub.asInterface(service);Log.d("phone binder","client getphone:"+mService.getPhone("apple"));mService.addPhone("apple");Log.d("phone binder","client getphone:"+mService.getPhone("apple"));} catch (RemoteException e) {e.printStackTrace();}}@Overridepublic void onServiceDisconnected(ComponentName name) {Log.d("binder","onServiceDisconnected");mService =  null;}};}

4.运行结果

点击客户端相应的控件，完成绑定服务，解绑服务，杀死service进程，日志如下：

16:09:10.394 27006-27006/com.skytoby.server D/phone binder: server onCreate
16:09:10.395 27006-27006/com.skytoby.server D/phone binder: server onBind
16:09:10.405 27006-27018/com.skytoby.server D/phone binder: server get phone:apple
16:09:10.407 27006-27018/com.skytoby.server D/phone binder: server add phone:apple
16:09:10.409 27006-27018/com.skytoby.server D/phone binder: server get phone:apple
16:16:55.497 27006-27006/com.skytoby.server D/phone binder: server onUnbind
16:16:55.502 27006-27006/com.skytoby.server D/phone binder: server onDestroy
16:16:57.955 27006-27020/com.skytoby.server D/phone binder: server getPid 27006

二、AIDL原理分析

1. IRemoteService类

aidl生成一个对应的IRemoteService.java，其原理还是利用了framework binder的架构，具体的内部原理后面介绍，先分析下这个生成类，其流程如下：

AIDL接口：继承IInterface。
Stub类：Binder的实现类，服务端通过这个类来提供服务。
Proxy类：服务器的本地代理，客户端通过这个类调用服务器的方法。
asInterface()：客户端调用，将服务端的返回的Binder对象，转换成客户端所需要的AIDL接口类型对象。返回对象：

1.若客户端和服务端位于同一进程，则直接返回Stub对象本身；

2.否则，返回的是系统封装后的Stub.proxy对象。

asBinder()：根据当前调用情况返回代理Proxy的Binder对象。
onTransact()：运行服务端的Binder线程池中，当客户端发起跨进程请求时，远程请求会通过系统底层封装后交由此方法来处理。
transact()：运行在客户端，当客户端发起远程请求的同时将当前线程挂起。之后调用服务端的onTransact()直到远程请求返回，当前线程才继续执行。

/** This file is auto-generated.  DO NOT MODIFY.* Original file: G:\\Android studio\\Aidl\\client\\src\\main\\aidl\\com\\skytoby\\server\\IRemoteService.aidl*/
package com.skytoby.server;public interface IRemoteService extends android.os.IInterface {/*** Local-side IPC implementation stub class.*/public static abstract class Stub extends android.os.Binder implements com.skytoby.server.IRemoteService {private static final java.lang.String DESCRIPTOR = "com.skytoby.server.IRemoteService";/*** Construct the stub at attach it to the interface.*/public Stub() {this.attachInterface(this, DESCRIPTOR);}/*** Cast an IBinder object into an com.skytoby.server.IRemoteService interface,* generating a proxy if needed.*/public static com.skytoby.server.IRemoteService asInterface(android.os.IBinder obj) {if ((obj == null)) {return null;}android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);if (((iin != null) && (iin instanceof com.skytoby.server.IRemoteService))) {return ((com.skytoby.server.IRemoteService) iin);}return new com.skytoby.server.IRemoteService.Stub.Proxy(obj);}@Overridepublic android.os.IBinder asBinder() {return this;}@Overridepublic boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException {java.lang.String descriptor = DESCRIPTOR;switch (code) {case INTERFACE_TRANSACTION: {reply.writeString(descriptor);return true;}case TRANSACTION_addPhone: {data.enforceInterface(descriptor);java.lang.String _arg0;_arg0 = data.readString();this.addPhone(_arg0);reply.writeNoException();return true;}case TRANSACTION_getPhone: {data.enforceInterface(descriptor);java.lang.String _arg0;_arg0 = data.readString();boolean _result = this.getPhone(_arg0);reply.writeNoException();reply.writeInt(((_result) ? (1) : (0)));return true;}case TRANSACTION_getPid: {data.enforceInterface(descriptor);int _result = this.getPid();reply.writeNoException();reply.writeInt(_result);return true;}default: {return super.onTransact(code, data, reply, flags);}}}private static class Proxy implements com.skytoby.server.IRemoteService {private android.os.IBinder mRemote;Proxy(android.os.IBinder remote) {mRemote = remote;}@Overridepublic android.os.IBinder asBinder() {return mRemote;}public java.lang.String getInterfaceDescriptor() {return DESCRIPTOR;}@Overridepublic void addPhone(java.lang.String name) throws android.os.RemoteException {android.os.Parcel _data = android.os.Parcel.obtain();android.os.Parcel _reply = android.os.Parcel.obtain();try {_data.writeInterfaceToken(DESCRIPTOR);_data.writeString(name);mRemote.transact(Stub.TRANSACTION_addPhone, _data, _reply, 0);_reply.readException();} finally {_reply.recycle();_data.recycle();}}@Overridepublic boolean getPhone(java.lang.String name) throws android.os.RemoteException {android.os.Parcel _data = android.os.Parcel.obtain();android.os.Parcel _reply = android.os.Parcel.obtain();boolean _result;try {_data.writeInterfaceToken(DESCRIPTOR);_data.writeString(name);mRemote.transact(Stub.TRANSACTION_getPhone, _data, _reply, 0);_reply.readException();_result = (0 != _reply.readInt());} finally {_reply.recycle();_data.recycle();}return _result;}@Overridepublic int getPid() throws android.os.RemoteException {android.os.Parcel _data = android.os.Parcel.obtain();android.os.Parcel _reply = android.os.Parcel.obtain();int _result;try {_data.writeInterfaceToken(DESCRIPTOR);mRemote.transact(Stub.TRANSACTION_getPid, _data, _reply, 0);_reply.readException();_result = _reply.readInt();} finally {_reply.recycle();_data.recycle();}return _result;}}static final int TRANSACTION_addPhone = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);static final int TRANSACTION_getPhone = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);static final int TRANSACTION_getPid = (android.os.IBinder.FIRST_CALL_TRANSACTION + 2);}public void addPhone(java.lang.String name) throws android.os.RemoteException;public boolean getPhone(java.lang.String name) throws android.os.RemoteException;public int getPid() throws android.os.RemoteException;
}

2. 原理

对于应用层来说bindService之后就可以和服务端进行交互了，可以不用里面具体的操作如何，这样的设计大大降低了使用了难度，对于binderService的具体的过程将在后面分析，下面是其分层次的调用图。