Android-Binder 简析

前言

对于Android来说，Binder的重要性怎么说都不为过。不管是我们的四大组件Activity、Service、BroadcastReceiver、ContentProvider，还是经常在应用中使用到的各种ServiceManager，其背后都是Binder在支撑。然而Binder机制又不是三言两语能够描述得清楚的，因此本文通过对一个简单的AIDL Demo进行分析，让读者对Binder有个初步的认识，要想深入了解Binder背后的原理，可以参考最后的延伸阅读。

Demo

首先我们通过AIDL新建一个跨进程通信的Demo，然后在代码中简单分析Binder的运行过程。

Server Module

我们先新建一个提供接口的AIDL服务端module，服务端主要提供AddBook和getBookList两个功能，其目录如下：

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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的代码拷贝一份(AIDL包名不能变)，然后新建一个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

把代码写好后，build一下，就能看到自动生成了一个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先调用bindService启动服务，会调用BookManagerService的onCreate方法，接着调用onBind方法，该方法会返回远程的Binder---mBinder，该Binder包含getBookList和AddBook两个方法的具体实现。

    @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);}} ;

当Client和server成功建立连接时，就会调用Client的onServiceConnected(name, service)方法把远程的mBinder回调给Client，此时的service就是远程的mBinder对象

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端通过asInterface方法把mBinder转成AIDL接口，如果是本进程内的Binder就直接返回，否则返回内部代理类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}

接着执行

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

此时的bookManager通过asInterface方法转换后，返回的实际上是本地的proxy类

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);}

通过代码我们可以看到proxy类其实也是一个IBookManager接口，调用bookManager.getBookList()其实是调用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方法中调用了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就是在这里和Server进行远程通信的！把需要的参数放data中，服务端执行完后把接口写到result里。从代码中可以看到transact方法中调用了onTransact方法。我们再看看onTransact方法有啥东西。

  @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;}......}

直接调用了this.getBookList方法返回结果，这个this到底是哪个对象？前面提到mRemote其实是远程中的Binder对象，其代码如下

 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其实就是mRemote.getBookList，就是上面代码中的mBinder.getBookList！然后把返回结果放到result中即可。至此，整个通信过程分析完毕。

总结

Client是通过本地的Proxy类像Server发起通信
Client通过ServerConnection接口回调收到Server远程Binder对象
IPC过程发生在transact方法中，该方法会挂起直到服务端返回结果，因此不能在主线程调用远程服务。

p1.jpg

图片和文中Demo来自《Android开发技术探索》

源码下载

Android-AIDL-Demo