前言

對于Android來說，Binder的重要性怎么說都不為過。不管是我們的四大組件Activity、Service、BroadcastReceiver、ContentProvider，還是經常在應用中使用到的各種ServiceManager，其背后都是Binder在支撐。然而Binder機制又不是三言兩語能夠描述得清楚的，因此本文通過對一個簡單的AIDL Demo進行分析，讓讀者對Binder有個初步的認識，要想深入了解Binder背后的原理，可以參考最后的延伸閱讀。

Demo

首先我們通過AIDL新建一個跨進程通信的Demo，然后在代碼中簡單分析Binder的運行過程。

Server Module

我們先新建一個提供接口的AIDL服務端module，服務端主要提供AddBook和getBookList兩個功能，其目錄如下：

p2.png

• IBookManager.AIDL

// IAIDLServer.aidl package com.nancyyihao.aidlserver; import com.nancyyihao.aidlserver.Book;// Declare any non-default types here with import statementsinterface IBookManager {List<Book> getBookList();void addBook(in Book book); }

• Book.java

package com.nancyyihao.aidlserver;import android.os.Parcel; import android.os.Parcelable;/*** Created by jumper on 2016/9/7.*/ public class Book implements Parcelable {private String bookName;private int bookId;public Book(){}public Book(int bookId, String bookName){this.bookId = bookId ;this.bookName = bookName ;}public Book(Parcel parcel){bookName = parcel.readString();bookId = parcel.readInt();}public String getBookName() {return bookName;}public void setBookName(String bookName) {this.bookName = bookName;}public int getBookId() {return bookId;}public void setBookId(int bookId) {this.bookId = bookId;}@Overridepublic int describeContents() {return 0;}@Overridepublic void writeToParcel(Parcel dest, int flags) {dest.writeString(bookName);dest.writeInt(bookId);}public static final Parcelable.Creator<Book> CREATOR = new Creator<Book>(){@Overridepublic Book createFromParcel(Parcel source) {return new Book(source);}@Overridepublic Book[] newArray(int size) {return new Book[size];}}; }

• Book.AIDL

package com.nancyyihao.aidlserver; parcelable Book;

• BookManagerService

package com.nancyyihao.aidlserver;import android.app.Service; import android.content.Intent; import android.os.Binder; import android.os.IBinder; import android.os.RemoteException;import java.util.List; import java.util.concurrent.CopyOnWriteArrayList;/*** Created by jumper on 2016/9/7.*/ public class BookManagerService extends Service {private static final String TAG = "BMS";private CopyOnWriteArrayList<Book> mBookList = new CopyOnWriteArrayList<>();@Overridepublic IBinder onBind(Intent intent) {return mBinder;}@Overridepublic void onCreate() {super.onCreate();// to distinguish with client module, we set the book id different from client modulemBookList.add(new Book(3,"Android"));mBookList.add(new Book(4,"iOS"));}private Binder mBinder = new IBookManager.Stub() {@Overridepublic List<Book> getBookList() throws RemoteException {return mBookList;}@Overridepublic void addBook(Book book) throws RemoteException {mBookList.add(book);}} ; }

Client Module

把Server module的代碼拷貝一份(AIDL包名不能變)，然后新建一個MainActivity即可

• MainActivity

package com.nancyyihao.aidlserver;import android.content.ComponentName; import android.content.Intent; import android.content.ServiceConnection; import android.os.Bundle; import android.os.IBinder; import android.support.v7.app.AppCompatActivity; import android.util.Log;import com.nancyyihao.R;import java.util.List;public class MainActivity extends AppCompatActivity {private static final String TAG = "MainActivity";private ServiceConnection mConnection = new ServiceConnection() {@Overridepublic void onServiceConnected(ComponentName name, IBinder service) {IBookManager bookManager = IBookManager.Stub.asInterface(service);Log.e("trace", "onServiceConnected");try {List<Book> bookList = bookManager.getBookList();Log.e(TAG, "query book list, list type:" + bookList.getClass().getCanonicalName());Log.e(TAG, "query book list:" + bookList.toString());} catch (Exception e) {e.printStackTrace();}}@Overridepublic void onServiceDisconnected(ComponentName name) {}};@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);final Intent intent = new Intent();intent.setAction("com.nancyyihao.startservice");intent.setPackage("com.nancyyihao.aidlserver"); // server's package nameLog.e("trace", "bindService");bindService(intent, mConnection, BIND_AUTO_CREATE);}@Overrideprotected void onDestroy() {super.onDestroy();unbindService(mConnection);} }

分析

把代碼寫好后，build一下，就能看到自動生成了一個IBookManager.Java文件

• IBookManager.Java

p3.png

package com.nancyyihao.aidlserver; // Declare any non-default types here with import statementspublic interface IBookManager extends android.os.IInterface {/*** Local-side IPC implementation stub class.*/public static abstract class Stub extends android.os.Binder implements com.nancyyihao.aidlserver.IBookManager {private static final java.lang.String DESCRIPTOR = "com.nancyyihao.aidlserver.IBookManager"; // Binder Indentifierpublic Stub() {this.attachInterface(this, DESCRIPTOR);}/*** Cast an IBinder object into an com.nancyyihao.aidlserver.IBookManager interface,* generating a proxy if needed.*/public static com.nancyyihao.aidlserver.IBookManager asInterface(android.os.IBinder obj) {if ((obj == null)) {return null;}android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);if (((iin != null) && (iin instanceof com.nancyyihao.aidlserver.IBookManager))) {return ((com.nancyyihao.aidlserver.IBookManager) iin); // local Binder}return new com.nancyyihao.aidlserver.IBookManager.Stub.Proxy(obj); // remote Binder}@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 {switch (code) {case INTERFACE_TRANSACTION: {reply.writeString(DESCRIPTOR);return true;}case TRANSACTION_getBookList: {data.enforceInterface(DESCRIPTOR);java.util.List<com.nancyyihao.aidlserver.Book> _result = this.getBookList();reply.writeNoException();reply.writeTypedList(_result);return true;}case TRANSACTION_addBook: {data.enforceInterface(DESCRIPTOR);com.nancyyihao.aidlserver.Book _arg0;if ((0 != data.readInt())) {_arg0 = com.nancyyihao.aidlserver.Book.CREATOR.createFromParcel(data);} else {_arg0 = null;}this.addBook(_arg0);reply.writeNoException();return true;}}return super.onTransact(code, data, reply, flags);}private static class Proxy implements com.nancyyihao.aidlserver.IBookManager {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 java.util.List<com.nancyyihao.aidlserver.Book> getBookList() throws android.os.RemoteException {android.os.Parcel _data = android.os.Parcel.obtain();android.os.Parcel _reply = android.os.Parcel.obtain();java.util.List<com.nancyyihao.aidlserver.Book> _result;try {_data.writeInterfaceToken(DESCRIPTOR);mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0);_reply.readException();_result = _reply.createTypedArrayList(com.nancyyihao.aidlserver.Book.CREATOR);} finally {_reply.recycle();_data.recycle();}return _result;}@Overridepublic void addBook(com.nancyyihao.aidlserver.Book book) throws android.os.RemoteException {android.os.Parcel _data = android.os.Parcel.obtain();android.os.Parcel _reply = android.os.Parcel.obtain();try {_data.writeInterfaceToken(DESCRIPTOR);if ((book != null)) {_data.writeInt(1);book.writeToParcel(_data, 0);} else {_data.writeInt(0);}mRemote.transact(Stub.TRANSACTION_addBook, _data, _reply, 0);_reply.readException();} finally {_reply.recycle();_data.recycle();}}}static final int TRANSACTION_getBookList = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);static final int TRANSACTION_addBook = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);}public java.util.List<com.nancyyihao.aidlserver.Book> getBookList() throws android.os.RemoteException;public void addBook(com.nancyyihao.aidlserver.Book book) throws android.os.RemoteException; }

Client先調用bindService啟動服務，會調用BookManagerService的onCreate方法，接著調用onBind方法，該方法會返回遠程的Binder---mBinder，該Binder包含getBookList和AddBook兩個方法的具體實現。

@Overridepublic IBinder onBind(Intent intent) {return mBinder;}private Binder mBinder = new IBookManager.Stub() {@Overridepublic List<Book> getBookList() throws RemoteException {return mBookList;}@Overridepublic void addBook(Book book) throws RemoteException {mBookList.add(book);}} ;

當Client和server成功建立連接時，就會調用Client的onServiceConnected(name, service)方法把遠程的mBinder回調給Client，此時的service就是遠程的mBinder對象

private ServiceConnection mConnection = new ServiceConnection() {@Overridepublic void onServiceConnected(ComponentName name, IBinder service) {IBookManager bookManager = IBookManager.Stub.asInterface(service);Log.e("trace", "onServiceConnected");try {List<Book> bookList = bookManager.getBookList();} catch (Exception e) {e.printStackTrace();}}

Client端通過asInterface方法把mBinder轉成AIDL接口，如果是本進程內的Binder就直接返回，否則返回內部代理類proxy

public static com.nancyyihao.aidlserver.IBookManager asInterface(android.os.IBinder obj) {if ((obj == null)) {return null;}android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);if (((iin != null) && (iin instanceof com.nancyyihao.aidlserver.IBookManager))) {return ((com.nancyyihao.aidlserver.IBookManager) iin); // local Binder}return new com.nancyyihao.aidlserver.IBookManager.Stub.Proxy(obj); // remote Binder}

接著執行

try {List<Book> bookList = bookManager.getBookList();} catch (Exception e) {e.printStackTrace();}

此時的bookManager通過asInterface方法轉換后，返回的實際上是本地的proxy類

private static class Proxy implements com.nancyyihao.aidlserver.IBookManager {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 java.util.List<com.nancyyihao.aidlserver.Book> getBookList() throws android.os.RemoteException {android.os.Parcel _data = android.os.Parcel.obtain();android.os.Parcel _reply = android.os.Parcel.obtain();java.util.List<com.nancyyihao.aidlserver.Book> _result;try {_data.writeInterfaceToken(DESCRIPTOR);mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0);_reply.readException();_result = _reply.createTypedArrayList(com.nancyyihao.aidlserver.Book.CREATOR);} finally {_reply.recycle();_data.recycle();}return _result;}@Overridepublic void addBook(com.nancyyihao.aidlserver.Book book) throws android.os.RemoteException {android.os.Parcel _data = android.os.Parcel.obtain();android.os.Parcel _reply = android.os.Parcel.obtain();try {_data.writeInterfaceToken(DESCRIPTOR);if ((book != null)) {_data.writeInt(1);book.writeToParcel(_data, 0);} else {_data.writeInt(0);}mRemote.transact(Stub.TRANSACTION_addBook, _data, _reply, 0);_reply.readException();} finally {_reply.recycle();_data.recycle();}}}static final int TRANSACTION_getBookList = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);static final int TRANSACTION_addBook = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);}

通過代碼我們可以看到proxy類其實也是一個IBookManager接口，調用bookManager.getBookList()其實是調用Proxy.getBookList。

@Overridepublic java.util.List<com.nancyyihao.aidlserver.Book> getBookList() throws android.os.RemoteException {android.os.Parcel _data = android.os.Parcel.obtain();android.os.Parcel _reply = android.os.Parcel.obtain();java.util.List<com.nancyyihao.aidlserver.Book> _result;try {_data.writeInterfaceToken(DESCRIPTOR);mRemote.transact(Stub.TRANSACTION_getBookList, _data, _reply, 0);_reply.readException();_result = _reply.createTypedArrayList(com.nancyyihao.aidlserver.Book.CREATOR);} finally {_reply.recycle();_data.recycle();}return _result;}

在Proxy.getBookList方法中調用了mRemote.trasact()

public final boolean transact(int code, Parcel data, Parcel reply,int flags) throws RemoteException {if (false) Log.v("Binder", "Transact: " + code + " to " + this);if (data != null) {data.setDataPosition(0);}boolean r = onTransact(code, data, reply, flags);if (reply != null) {reply.setDataPosition(0);}return r;}

Client就是在這里和Server進行遠程通信的！把需要的參數放data中，服務端執行完后把接口寫到result里。從代碼中可以看到transact方法中調用了onTransact方法。我們再看看onTransact方法有啥東西。

@Overridepublic boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException {......case TRANSACTION_getBookList: {data.enforceInterface(DESCRIPTOR);java.util.List<com.nancyyihao.aidlserver.Book> _result = this.getBookList();reply.writeNoException();reply.writeTypedList(_result);return true;}......}

直接調用了this.getBookList方法返回結果，這個this到底是哪個對象？前面提到mRemote其實是遠程中的Binder對象，其代碼如下

private Binder mBinder = new IBookManager.Stub() {@Overridepublic List<Book> getBookList() throws RemoteException {return mBookList;}@Overridepublic void addBook(Book book) throws RemoteException {mBookList.add(book);}} ;

this.getBookList其實就是mRemote.getBookList，就是上面代碼中的mBinder.getBookList！然后把返回結果放到result中即可。至此，整個通信過程分析完畢。

總結

• Client是通過本地的Proxy類像Server發起通信

• Client通過ServerConnection接口回調收到Server遠程Binder對象

• IPC過程發生在transact方法中，該方法會掛起直到服務端返回結果，因此不能在主線程調用遠程服務。

p1.jpg

圖片和文中Demo來自《Android開發技術探索》

源碼下載

• Android-AIDL-Demo

延伸閱讀

• Binder學習指南

• Android Bander設計與實現 - 設計篇

• Android進程間通信（IPC）機制Binder簡要介紹和學習計劃

轉載于:https://www.cnblogs.com/jasonkent27/p/5860680.html

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