binder是一個非常好的跨進程通信工具,Android對其進行了各種封裝,雖然我們用起來簡單,但是理解起來卻比較困難。
1.自己設計一個跨進程通信機制
在理解binder之前呢,首先我們想一下,如果我們自己設計一個跨進程通信的機制,那我們會怎麼設計呢?
如果是我的話,我可能會按照下圖設計。
圖中左邊是客戶端,右邊是服務端,客戶端想要調用服務端的call函數,首先我們需要先將函數名稱以及參數值進行序列化,然後再使用linux系統所提供的跨進程通信方式,例如socket或者是管道,將這些序列化過後的數據傳遞給服務端,然後服務端拿到這些數據之後,首先進行反序列化,然後再調用相應的函數,將返回值返回給客戶端。
其實和我們使用網絡訪問服務器的結構很像。
接着我們來看下binder的通信流程。
2.跨進程通信流程
1.生成AIDL文件
首先我們新建一個AIDL文件,
interface IFile {
Bitmap getBitmap(String path);
}
然後build過後,系統會自動幫我們生成一個IFile.java文件
public interface IFile extends android.os.IInterface {
/**
*繼承binder,實現IFile接口,運行於服務端
*/
public static abstract class Stub extends android.os.Binder implements com.android.hdemo.IFile {
private static final java.lang.String DESCRIPTOR = "com.android.hdemo.IFile";
/**
* Construct the stub at attach it to the interface.
*/
public Stub() {
this.attachInterface(this, DESCRIPTOR);
}
/**
* Cast an IBinder object into an com.android.hdemo.IFile interface,
* generating a proxy if needed.
*/
public static com.android.hdemo.IFile asInterface(android.os.IBinder obj) {
if ((obj == null)) {
return null;
}
android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
if (((iin != null) && (iin instanceof com.android.hdemo.IFile))) {
return ((com.android.hdemo.IFile) iin);
}
return new com.android.hdemo.IFile.Stub.Proxy(obj);
}
@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_getBitmap: {
data.enforceInterface(DESCRIPTOR);
java.lang.String _arg0;
_arg0 = data.readString();
android.graphics.Bitmap _result = this.getBitmap(_arg0);
reply.writeNoException();
if ((_result != null)) {
reply.writeInt(1);
_result.writeToParcel(reply, android.os.Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
} else {
reply.writeInt(0);
}
return true;
}
}
return super.onTransact(code, data, reply, flags);
}
//實現IFile接口,運行於客戶端
private static class Proxy implements com.android.hdemo.IFile {
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 android.graphics.Bitmap getBitmap(java.lang.String path) throws android.os.RemoteException {
android.os.Parcel _data = android.os.Parcel.obtain();
android.os.Parcel _reply = android.os.Parcel.obtain();
android.graphics.Bitmap _result;
try {
_data.writeInterfaceToken(DESCRIPTOR);
_data.writeString(path);
mRemote.transact(Stub.TRANSACTION_getBitmap, _data, _reply, 0);
_reply.readException();
if ((0 != _reply.readInt())) {
_result = android.graphics.Bitmap.CREATOR.createFromParcel(_reply);
} else {
_result = null;
}
} finally {
_reply.recycle();
_data.recycle();
}
return _result;
}
}
static final int TRANSACTION_getBitmap = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
}
public android.graphics.Bitmap getBitmap(java.lang.String path) throws android.os.RemoteException;
}
生成的代碼結構並不複雜,IFile是一個接口,繼承了android.os.IInterface,並加入了我們自定義的接口方法getBitmap,IFile中包含一個靜態抽象類Stub,Stub又包含一個靜態內部類Proxy。Stub繼承binder,實現IFile接口,運行於服務端,Proxy實現IFile接口,運行於客戶端。
接着我們來看一下從客戶端到服務端的整個流程
2.從客戶端到服務端
客戶端
首先來看下我們是如何來獲取服務的,從註釋1處可以看出,客戶端拿到binder對象之後,調用asInterface方法將其轉換成本地的IFile對象。
Intent intent = new Intent(this,TestService.class);
this.bindService(intent, new ServiceConnection() {
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
try {
//1
IFile iFile = IFile.Stub.asInterface(service);
iFile.getBitmap("");
}catch (Exception e){
}
}
@Override
public void onServiceDisconnected(ComponentName name) {
}
}, Service.BIND_AUTO_CREATE);
而asInterface是系統生成的代碼,從下面註釋1處可以看出,如果客戶端和服務端在同一個進程,則直接將binder強轉成本地接口對象,否則返回Proxy對象。如註釋2所示,Proxy的getFile方法會調用mRemote.transact方法,mRemote是一個binder對象,其真正的實現是BinderProxy。
public static com.android.hdemo.IFile asInterface(android.os.IBinder obj) {
if ((obj == null)) {
return null;
}
//1.如果客戶端和服務端在同一個進程,則直接調用,否則使用Proxy
android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
if (((iin != null) && (iin instanceof com.android.hdemo.IFile))) {
return ((com.android.hdemo.IFile) iin);
}
return new com.android.hdemo.IFile.Stub.Proxy(obj);
}
//Proxy
private static class Proxy implements com.android.hdemo.IFile {
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 android.graphics.Bitmap getBitmap(java.lang.String path) throws android.os.RemoteException {
android.os.Parcel _data = android.os.Parcel.obtain();
android.os.Parcel _reply = android.os.Parcel.obtain();
android.graphics.Bitmap _result;
try {
_data.writeInterfaceToken(DESCRIPTOR);
_data.writeString(path);
//2.調用binder的transact方法
mRemote.transact(Stub.TRANSACTION_getBitmap, _data, _reply, 0);
_reply.readException();
if ((0 != _reply.readInt())) {
_result = android.graphics.Bitmap.CREATOR.createFromParcel(_reply);
} else {
_result = null;
}
} finally {
_reply.recycle();
_data.recycle();
}
return _result;
}
}
BinderProxy的transact方法會調用transactNative方法,最終會調用native層的BpBinder的transact方法,然後由BpBinder和binder驅動進行交互。
//BinderProxy
public boolean transact(int code, Parcel data, Parcel reply, int flags) throws RemoteException {
......
try {
return transactNative(code, data, reply, flags);
} finally {
......
}
}
//Native層BpBinder
status_t BpBinder::transact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
// Once a binder has died, it will never come back to life.
if (mAlive) {
bool privateVendor = flags & FLAG_PRIVATE_VENDOR;
// don't send userspace flags to the kernel
flags = flags & ~FLAG_PRIVATE_VENDOR;
// user transactions require a given stability level
if (code >= FIRST_CALL_TRANSACTION && code <= LAST_CALL_TRANSACTION) {
using android::internal::Stability;
auto stability = Stability::get(this);
auto required = privateVendor ? Stability::VENDOR : Stability::kLocalStability;
if (CC_UNLIKELY(!Stability::check(stability, required))) {
ALOGE("Cannot do a user transaction on a %s binder in a %s context.",
Stability::stabilityString(stability).c_str(),
Stability::stabilityString(required).c_str());
return BAD_TYPE;
}
}
status_t status = IPCThreadState::self()->transact(
mHandle, code, data, reply, flags);
if (status == DEAD_OBJECT) mAlive = 0;
return status;
}
return DEAD_OBJECT;
}
服務端
binder驅動收到請求之後會調用native層BBinder的onTransact方法,
status_t BBinder::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t /*flags*/)
{
switch (code) {
case INTERFACE_TRANSACTION:
reply->writeString16(getInterfaceDescriptor());
return NO_ERROR;
case DUMP_TRANSACTION: {
int fd = data.readFileDescriptor();
int argc = data.readInt32();
Vector<String16> args;
for (int i = 0; i < argc && data.dataAvail() > 0; i++) {
args.add(data.readString16());
}
return dump(fd, args);
}
case SHELL_COMMAND_TRANSACTION: {
int in = data.readFileDescriptor();
int out = data.readFileDescriptor();
int err = data.readFileDescriptor();
int argc = data.readInt32();
Vector<String16> args;
for (int i = 0; i < argc && data.dataAvail() > 0; i++) {
args.add(data.readString16());
}
sp<IShellCallback> shellCallback = IShellCallback::asInterface(
data.readStrongBinder());
sp<IResultReceiver> resultReceiver = IResultReceiver::asInterface(
data.readStrongBinder());
// XXX can't add virtuals until binaries are updated.
//return shellCommand(in, out, err, args, resultReceiver);
(void)in;
(void)out;
(void)err;
if (resultReceiver != nullptr) {
resultReceiver->send(INVALID_OPERATION);
}
return NO_ERROR;
}
case SYSPROPS_TRANSACTION: {
report_sysprop_change();
return NO_ERROR;
}
default:
return UNKNOWN_TRANSACTION;
}
}
接着會回掉到java層的onTransact方法,如下注釋1所示,this.getBitmap(_arg0)爲Stub類的方法,從服務端獲取到圖片,然後在註釋2處,將bitmap寫入到返回值。
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_getBitmap: {
data.enforceInterface(DESCRIPTOR);
java.lang.String _arg0;
_arg0 = data.readString();
//1.服務端獲取的bitmap
android.graphics.Bitmap _result = this.getBitmap(_arg0);
reply.writeNoException();
if ((_result != null)) {
reply.writeInt(1);
//2.將bitmap寫入返回值
_result.writeToParcel(reply, android.os.Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
} else {
reply.writeInt(0);
}
return true;
}
}
return super.onTransact(code, data, reply, flags);
}
我們再來看下service的定義,當綁定到一個service之後,返回的Stub對象,實現了getBitmap方法,返回了本地的bitmap。
public class TestService extends Service {
@Nullable
@Override
public IBinder onBind(Intent intent) {
return new IFile.Stub() {
@Override
public Bitmap getBitmap(String path) throws RemoteException {
//1.返回本地bitmap
return mBitmap;
}
};
}
}
這樣從客戶端到服務端的流程就走完了,我們看下它的流程圖
3.binder的優勢
linux本身提供了很多跨進程通信的方式,例如socket,共享內存,管道之類的,那爲啥還要再弄出個binder呢,有以下幾點
方便
binder使用起來對於開發者來說非常的友好,隱藏了底層的實現細節,我們只需要關注於業務邏輯即可。
高效
binder通信過程中,將內存同時映射到內核和應用進程當中,只需要拷貝一次即可。而socket和管道均需要從應用進程拷貝到內核,再從內核拷貝到應用進程,需要兩次拷貝,共享內存不需要數據拷貝,但使用起來比較複雜。
安全
調用方的身份標記由binder機制本身在內核態中添加,調用方不能自己更改。