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：
p2.png
IBookManager.AIDL
// IAIDLServer.aidl
package com.nancyyihao.aidlserver;
import com.nancyyihao.aidlserver.Book;
// Declare any non-default types here with import statements
interface 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;
}
@Override
public int describeContents() {
return 0;
}
@Override
public void writeToParcel(Parcel dest, int flags) {
dest.writeString(bookName);
dest.writeInt(bookId);
}
public static final Parcelable.Creator<Book> CREATOR = new Creator<Book>(){
@Override
public Book createFromParcel(Parcel source) {
return new Book(source);
}
@Override
public 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<>();
@Override
public IBinder onBind(Intent intent) {
return mBinder;
}
@Override
public void onCreate() {
super.onCreate();
// to distinguish with client module, we set the book id different from client module
mBookList.add(new Book(3,"Android"));
mBookList.add(new Book(4,"iOS"));
}
private Binder mBinder = new IBookManager.Stub() {
@Override
public List<Book> getBookList() throws RemoteException {
return mBookList;
}
@Override
public 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() {
@Override
public 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();
}
}
@Override
public void onServiceDisconnected(ComponentName name) {
}
};
@Override
protected 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 name
Log.e("trace", "bindService");
bindService(intent, mConnection, BIND_AUTO_CREATE);
}
@Override
protected 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 statements
public 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 Indentifier
public 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
}
@Override
public android.os.IBinder asBinder() {
return this;
}
@Override
public 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;
}
@Override
public android.os.IBinder asBinder() {
return mRemote;
}
public java.lang.String getInterfaceDescriptor() {
return DESCRIPTOR;
}
@Override
public 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;
}
@Override
public 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。
@Override
public IBinder onBind(Intent intent) {
return mBinder;
}
private Binder mBinder = new IBookManager.Stub() {
@Override
public List<Book> getBookList() throws RemoteException {
return mBookList;
}
@Override
public 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() {
@Override
public 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;
}
@Override
public android.os.IBinder asBinder() {
return mRemote;
}
public java.lang.String getInterfaceDescriptor() {
return DESCRIPTOR;
}
@Override
public 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;
}
@Override
public 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。
@Override
public 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。
@Override
public 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() {
@Override
public List<Book> getBookList() throws RemoteException {
return mBookList;
}
@Override
public 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》
