[置顶] Android开发之MediaPlayerService服务详解(一)

前面一节我们分析了Binder通信相关的两个重要类：ProcessState 和 IPCThreadState。ProcessState负责打开Binder
驱动，每个进程只有一个。而 IPCThreadState负责提供与Binder通信相关的接口，每个线程有一个。下面我们通过具体
示例MediaPlayerService来分析我们应用程序中怎么通过Binder通信的。

frameworks/base/media/mediaserver/main_mediaserver.cpp

int main(int argc, char*argv[])
{
	sp<ProcessState> proc(ProcessState)::self();	// 获得ProcessState在构造函数中打开binder驱动
	sp<IServiceManager> sm = defaultServiceManager();
	MediaPlayService::instantiate();
	ProcessState::self()->startThreadPool();
	IPCThreadState::self()->joinThreadPool();
}

1）获得ServiceManager的代理BpServiceManager

sp<IServiceManager> sm = defaultServiceManager();
sp<IServiceManager> defaultServiceManager()
{
	if(gDefaultServiceManager != NULL) return gDefaultServiceManager;
	{
		AutoMutex -l(gDefaultServiceManagerLock);
		if(gDefaultServiceManager == NULL)
			gDefaultServiceManager = interface_cast<IServiceManager>(
				ProcessState::self()->getContextObject(NULL));
	}
	return gDefaultServiceManager;
}

这里又是一个单例模式，每个进程只需要一个BpServiceManager代理，通过interface_cast获得。
首先看看ProcessState::self()->getContextObject(NULL)

sp<IBinder> ProcessState::getContextObject(const sp<IBinder>& caller)
{
	return getStrongProxyForHandle(0);
}

sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
	sp<IBinder> result;
	AutoMutex _l(mLock);
	handle_entry *e = lookupHandleLocked(handle);
	if( e != NULL) {
		IBinder* b = e->binder;
		if(b == NULL || !e->refs->attemptIncWeak(this)) {
			b = new BpBinder(handle);
			e->binder = b;
			if(b) e->refs = b->getWeakRefs();
			result = b;
		}else{
			result.force_set(b);
			e->refs->decWeak(this);
		}
	}
	return result;
}
struct handle_entry{
	IBinder* binder;
	RefBase::weakref_type* refs;
}

ProcessState::handle_entry* ProcessState::lookupHandleLocked()从数组mHandleToObject里面根据handle索引，查找
一个handle_entry结构体。然后根据传入的句柄handle这里为0，表示ServiceManager，new一个BpBinder
所以现在相当于：
gDefaultServiceManager = interface_cast<IServiceManager>(new BpBinder(0));

现在我们看看interface_cast是什么？

frameworks/base/include/binder/IInterface.h
template<typename INTERFACE>
inline sp<INTERFACE> interface_cast(const sp<IBinder>& obj)
{
	return INTERFACE::asInterface(obj);
}
等价于：
inline sp<IServiceManager> interface_cast(const sp<IBinder>& obj)
{
	return IServiceManager::asInterface(obj);
}
继续我们跟到IServiceManager里面去：
frameworks/base/include/binder/IServiceManager.h
class IServiceManager:public IInterface
{
public:
	DECLARE_META_INTERFACE(ServiceManager);// MLGB的又是宏！！！
	virtual status_t addService(const String16& name, const sp<IBinder>& service) = 0;
	virtual sp<IBinder> getService(const String16& name) const = 0;
}
#define DECLARE_META_INTERFACE(INTERFACE)				\
	static const android::String16 descriptor;			\
	static android::sp<I##INTERFACE> asInterface(			\
			const android::sp<android::IBinder>& obj);	\
	virtual const android::String16& getInterfaceDescriptor() const;\
	I##INTERFACE();							\
	virtual !I##INTERFACE();
替换之后就是：
	static const android::String16 descriptor;
	static android::sp<IServiceManager> asInterface(
			const android::sp<android::IBinder>& obj);
	virtual const android::String16& getInterfaceDescriptor() const;
	IServiceManager();
	virtual !IServiceManager();
都是一些函数声明，既然有声明的地方，肯定有实现的地方了。
#define IMPLEMENT_META_INTERFACE(INTERFACE, NAME)                       \
    const android::String16 I##INTERFACE::descriptor(NAME);             \
    const android::String16&                                            \
            I##INTERFACE::getInterfaceDescriptor() const {              \
        return I##INTERFACE::descriptor;                                \
    }                                                                   \
    android::sp<I##INTERFACE> I##INTERFACE::asInterface(                \
            const android::sp<android::IBinder>& obj)                   \
    {                                                                   \
        android::sp<I##INTERFACE> intr;                                 \
        if (obj != NULL) {                                              \
            intr = static_cast<I##INTERFACE*>(                          \
                obj->queryLocalInterface(                               \
                        I##INTERFACE::descriptor).get());               \
            if (intr == NULL) {                                         \
                intr = new Bp##INTERFACE(obj);                          \
            }                                                           \
        }                                                               \
        return intr;                                                    \
    }                                                                   \
    I##INTERFACE::I##INTERFACE() { }                                    \
    I##INTERFACE::~I##INTERFACE() { }
继续替换：
{
    const android::String16 IServiceManager::descriptor(NAME);
    const android::String16&
           IServiceManager::getInterfaceDescriptor() const {
        return IServiceManager::descriptor;
    }
    android::sp<IServiceManager> IServiceManager::asInterface(
            const android::sp<android::IBinder>& obj)   // 参数为new BpBinder(0)
    {
        android::sp<IServiceManager> intr;
        if (obj != NULL) {
            intr = static_cast<IServiceManager*>(
                obj->queryLocalInterface(
                        IServiceManager::descriptor).get());
            if (intr == NULL) {
                intr = new BpServiceManager(obj);     // 原来在这里new 了一个BpServiceManager对象
            }
        }
        return intr;
    }
    IServiceManager::IServiceManager() { }
    IServiceManager::~IServiceManager() { }
}

总结：根据句柄handle 0 创建一个new BpBinder(0)，根据这个BpBinder创建了一个BpServiceManager代理。

下面来看看BpServiceManager代理：

class BpServiceManager : public BpInterface<IServiceManager>
{
public:
	BpServiceManager(const sp<IBinder>& impl) : BpInterface<IServiceManager>(iml)
	{}
} 

这里BpInterface是一个模板类,表示这里BpServiceManager同时继承与BpInterface和IServiceManager类

template<typename INTERFACE>
class BpInterface : public INTERFACE, public BpRefBase
{
public: BpInterface(const sp<IBinder>& remote);
...
}
调用了基类BpInterface构造函数：
BpInterface<IServiceManager>::BpInterace(const sp<IBinder>& remote) : BpRefBase(remote)
{}
//这里的remote就是刚刚的new BpBinder(0)
BpRefBase::BpRefBase(const sp<IBinder>& o) : mRemote(o.get()),mRefs(NULL), mState(0)
{
}

2）添加服务 MediaPlayerService::instantiate();

frameworks/base/media/libmediaplayerservice/ibMediaPlayerService.cpp
void MediaPlayerService::instantiate()
{
	defaultServiceManager()->addService(String16("media.player"), new MediaPlayerService);
}

defaultServiceManager()返回的是刚创建的BpServiceManager，调用add函数。
BpMediaPlayService作为服务代理端，那么BnMediaPlayerService一定是实现端，MediaPlayerService继承于
BnMediaPlayerService，实现了真正的业务函数。

来看看BpServiceManager的addService()函数：

virtual status_t addService(const String16& name, const sp<IBinder>& service)
{
	Parcel data, reply;
	data.writeInterfaceToken(IServiceManager.getInterfaceDescriptor());	// android.os.IServiceManager
	data.writeString16(name);	// media.player
	data.writeStrongBinder(service);	// 也就是MediaPlayerService
	status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);
	return err == NO_ERROR ? reply.readInt32() : err;
}

这里remote()就是前面创建的BpBinder(0)对象。

status_t BpBinder::transact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
	IPCThreadState::self()->transact(mHandle, code, data, reply, flags);
}
status_t IPCThreadState::transact(int32_t handle, uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
	// 发送ADD_SERVICE_TRANSACTION请求
	writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);
	if(reply)	// 等待响应
		waitForResponse(NULL, reply);
}

status_t IPCThreadState::writeTransactionData(int32_t cmd, uint32_t binderFlags, int32_t handle,
		uint32_t code, const Parcel& data, status_t *statusBuffer)
{
	// cmd	BC_TRANSACTION	应用程序向BINDER发送的命令
	binder_transaction_data tr;
	tr.target.handle = handle;	// 0
	tr.code = code;			// ADD_SERVICE_TRANSACTION
	tr.flags = binderFlags;
	// 把命令和数据一起发送到 Parcel mOut中
	mOut.writeInt32(cmd);
	mOut.write(&tr, sizeof(tr));
}
status_t IPCThreadState::waitForResponse(Parcel* reply, status_t *acquireResult)
{
	int32_t cmd;

	while(1)
		talkWithDriver();
		cmd = mIn.readInt32();
		switch(cmd) {
			case BR_TRANSACTION_COMPLETE:
				...
				break;
		}
	{
	return err;
}
status_t IPCThreadState::talkWithDriver(bool doReceive)
{
	binder_write_read bwr;

	bwr.write_size = outAvail;
	bwr.write_buf = (long unsigned int)mOut.data();	// 写入mOut的数据
	bwr.read_size = mIn.dataCapacity;
	bwr.read_buffer = (long unsigned int)mIn.data();
	ioctl(mProcess->mDriverFD, BINDER_WRITE_READm &bwr);	// 把mOut写到Binder，并读取mIn数据
}

3）IPCThreadState::joinThreadPool(), ProcessState::self()->startThreadPool()
进入线程循环talkWithDriver 等待客户端Client请求，从Binder读取命令请求进行处理。

到现在为止MediaPlayerService的服务端已经向服务总管ServiceManager注册了，下面我们看看客户端是如何获得服务的代理并和服务端通信的。
我们以MediaPlayer的业务函数decode解析播放一个URL为例

sp<IMemory> MediaPlayer::decode(const char*url, uint32_t *pSampleRate, ...)
{
	sp<IMemory> p;
	const sp<IMediaPlayerService>& service = getMediaPlayerService();	// 获得BpMediaPlayerSerivce代理
	if(service != 0)
		p = service->decode(url, ....);
	return p;
}

这里我们主要分析getMediaPlayerService，客户端是如何向ServiceManager总管查询服务并获得代理的。

sp<IMediaPlayerService>& IMediaDeathNotifier::getMediaPlayerService()
{
	sp<IServiceManager> sm = defaultServiceManager(); // 生成一个BpServiceManager代理对象
	sp<IBinder> binder;
	do {
		binder = sm->getService(String16("media.player"));
		if(binder != 0)
			break;
		usleep(500000)
	} while(true);
	sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
	return sMediaPlayerService;
}

1）首先获得BpServiceManager的代理，然后调用getService()函数向服务总管ServiceManager查询服务。
frameworks/base/libs/binder/IServiceManager.cpp

class BpServiceManager : public BpInterface<IServiceManager>
{
public:
	virtual sp<IBinder> getService(const String16& name) const
	{
		for(n = 0; n < 5; n++) {
			sp<IBinder> svc = checkService(name);	// 调用checkService函数
			if(svc != NULL) return svc;
			sleep(1);
		}
		return NULL;
	}
	virtual sp<IBinder> checkService(const String16& name) const
	{
		Parcel data, reply;
		data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
		// 首先调用data.writeInt32(IPCThreadState::self()->getStrictModePolicy())
		// 然后再写入android.os.IServiceManager
		data.writeString16(name);	// 写入 media.player
		remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);
		return reply.readStrongBinder();
	}
}

这里首先将请求打包成Parcel各式，然后调用remote()->transact()函数，前面我们分析过BpServiceManager::remote()返回
的就是前面new BpBinder(0)对应句柄为ServiceManager。继续去BpBinder中寻找实现代码：
frameworks/base/libs/binder/BpBinder.cpp

status_t BpBinder::transact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
	IPCThreadState::self()->transact(mHandle, code, data, reply, flags);
}

最后调用的IPCThreadState的transact()函数，IPCThreadState是专门提供通过Binder进程间通信的接口的。

status_t IPCTheadState::transact(int32_t handle, uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
	// 填充binder_transaction_data 结构体，写入到mOut中去
	writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);

	// 调用talkWithDriver() 将mOut写到Binder驱动，并从Binder驱动读取mIn数据
	waitForResponse(reply);
}

首先通过writeTransactionData函数来填充mOut结构体，mOut里面内容为：
 mOut.writeInt32(BC_TRANSACTION);
 mOut.write(&tr, sizeof(tr));
这里binder_transaction_data tr内容为：
 tr.target.handle = 0; // 表面是发往ServiceManager的
 tr.code = CHECK_SERVICE_TRANSACTION;
 tr.flags = 0;
tr.data内容为：
 data.writeInt32(IPCThreadState::self()->getStrictModePolicy() | STRICT_MODE_PENALTY_GATHER);
 data.writeString16("android.os.IServiceManager");
 data.writeString16("media.player");
根据前面Android开发之ServiceManager一章中我们分析，svcmgr_handler处理从句柄为0的Binder的请求：
strict_policy = bio_get_string32();
s = bio_get_string16(); // 就是上面的android.os.IServiceManager
s = bio_get_string16(); // 就是上面的 media.player
根据media.player遍历全局链表svclist找到相应的服务，调用bio_put_ref(reply, ptr) 返回目标Binder实体。

这个waitForResponse()函数是关键：

status_t IPCThreadState::waitForResponse(Parcel* reply)
{
	while(1) {
		talkWithDriver();	// 输入mOut 输出mIn
		cmd = mIn.readInt32();
		switch(cmd) {
		case BR_REPLY:
		{
			binder_transaction_data tr;
			mIn.read(&tr, sizeof(tr));
			if(reply) {
			reply->ipcSetDataReference(reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
			tr.data.size, reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
			tr.offsets_size/sizeof(sizt_t), freeBuffer, this);
			} else {
			err = *static_cast<const status_t*>(tr.data.ptr.buffer);
			freeBuffer(NULL, reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
			tr.data.size, reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
			tr.offsets_size/sizeof(sizt_t), freeBuffer, this)
			}
		}
		}
	}
}
最后返回的是：return reply.readStrongBinder();进入到Parcel的readStrongBinder()函数
sp<IBinder> Parcel::readStrongBinder() const
{
	sp<IBinder> val;
	unflatten_binder(ProcessState::self(), *this, &val);
	return val;
}
status_t unflatten_binder(const sp<ProcessState>& proc, const Parcel& in, sp<IBinder>* out)
{
	const flat_binder_object* flat = in.readObject(false);
	if(flat) {
		switch(flat->type) {
			case BINDER_TYPE_BINDER:
				*out = static_cast<IBinder*>(flat->cookie);
				return finish_unflatten_binder(NULL, *flat, in);
			case BINDER_TYPE_HANDLE:
				*out = proc->getStrongProxyForHandle(flat->handle);
				return finish_unflatten_binder(static_cast<BpBinder*>(out->get()), *flat, in);
		}
	}
}

这里flat->type是BINDER_TYPE_HANDLE，所以调用ProcessState::getStrongProxyForHandle()函数

sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
	sp<IBinder> result;

	handle_entry* e = lookupHandleLocked(handle);
	if(e != NULL) {
		IBinder* b = e->binder;
		if(b == NULL || !e->refs->attemptIncWeak(this)) {
			b = new BpBinder(handle);
			e->binder = b;
			if( b ) e->refs = e->getWeakRefs();
			result = b;
		} else {
			result.force_set(b);
			e->refs->decWeak(this);
		}
	}
	return result;
}

这里的handle就是ServiceManager内维护的MediaPlayerService对应的Binder句柄，这个ProcessState根据这个句柄
new 了一个BpBinder,并将其保存起来，这样下次需要从ServiceManager请求获取到相同句柄的时候就可以直接返回了。
最后根据这个返回的BpBinder获得MediaPlayerService的代理：
sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
根据前面ServiceManager一样，最后调用的是IMediaPlayerService的asInterface()宏函数

android::sp<IMediaPlayerService> IMediaPlayerService::asInterface(const android::sp<android::IBinder>& obj)
{
	android::sp<IMediaPlayerService> intr;
	if(obj != NULL ) {
		intr = static_cast<IMediaPlayerService>(
			obj->queryLocalInterface(IMediaPlayerService::descriptor).get);
			if (intr == NULL) {
				intr = new BpMediaPlayerService(obj);
			}
	}
	return intr;
}

这样我就获得了一个代理BpMediaPlayerService对象，它的remote()为BpBinder(handle)，这个handle就是向服务总共ServiceManager
查询到的MediaPlayerService对应的Binder句柄。

下一章我们分析，客户端如何通过这个BpServiceManager代理对象调用服务端MediaPlayerService的业务函数的？