android binder 机制二(client和普通server)

在讲它们之间的通信之前，我们先以MediaServer为例看看普通Server进程都在干些什么。

int main()
{
	……
	// 获得ProcessState实例
		sp<ProcessState> proc(ProcessState::self());
		// 得到ServiceManager的Binderclient实例
        sp<IServiceManager> sm = defaultServiceManager();
		……
		// 通过ServiceManager的Binderclient向系统注冊MediaPlayer服务
        MediaPlayerService::instantiate();
        ……
		// start run
        ProcessState::self()->startThreadPool();
        IPCThreadState::self()->joinThreadPool();
}

defaultServiceManager()在上一篇中已经有介绍。

MediaPlayerService::instantiate()的实现例如以下，就是addService到ServiceManager。和上一篇的getService类似，故不作介绍。

void MediaPlayerService::instantiate() {
    defaultServiceManager()->addService(
            String16("media.player"), new MediaPlayerService());
}

接下来看ProcessState::self()->startThreadPool()的实现

void ProcessState::startThreadPool()
{
    AutoMutex _l(mLock);
    if (!mThreadPoolStarted) {
        mThreadPoolStarted = true;
        spawnPooledThread(true);
    }
}

void ProcessState::spawnPooledThread(bool isMain)
{
    if (mThreadPoolStarted) {
        String8 name = makeBinderThreadName();
        ALOGV("Spawning new pooled thread, name=%s\n", name.string());
        sp<Thread> t = new PoolThread(isMain);
        t->run(name.string());
    }
}

class PoolThread : public Thread
{
public:
    PoolThread(bool isMain)
        : mIsMain(isMain)
    {
    }
    
protected:
    virtual bool threadLoop()
    {
        IPCThreadState::self()->joinThreadPool(mIsMain);
        return false;
    }
    
    const bool mIsMain;
};

实际上。这个函数只是是创建了一个新的线程，然后在线程中又创建了一个IPCThreadState。并调用了joinThreadPool函数。

void IPCThreadState::joinThreadPool(bool isMain)
{
    mOut.writeInt32(isMain ? BC_ENTER_LOOPER : BC_REGISTER_LOOPER);
    
    set_sched_policy(mMyThreadId, SP_FOREGROUND);
        
    status_t result;
    do {
        processPendingDerefs();
        // now get the next command to be processed, waiting if necessary
        result = getAndExecuteCommand();

        if (result < NO_ERROR && result != TIMED_OUT && result != -ECONNREFUSED && result != -EBADF) {
            ALOGE("getAndExecuteCommand(fd=%d) returned unexpected error %d, aborting",
                  mProcess->mDriverFD, result);
            abort();
        }
        
        // Let this thread exit the thread pool if it is no longer
        // needed and it is not the main process thread.
        if(result == TIMED_OUT && !isMain) {
            break;
        }
    } while (result != -ECONNREFUSED && result != -EBADF);
	
    mOut.writeInt32(BC_EXIT_LOOPER);
    talkWithDriver(false);
}

status_t IPCThreadState::getAndExecuteCommand()
{
    status_t result;
    int32_t cmd;

    result = talkWithDriver();
    if (result >= NO_ERROR) {
        size_t IN = mIn.dataAvail();
        if (IN < sizeof(int32_t)) return result;
        cmd = mIn.readInt32();
        result = executeCommand(cmd);

        set_sched_policy(mMyThreadId, SP_FOREGROUND);
    }

    return result;
}

我们能够看到。主线程和新创建的线程。都在做一件事，talkWithDriver读取Binder驱动，然后就executeCommand处理请求。这就是普通Server进程启动后一直都在干的事：等待client请求。处理请求。然后返回给client。

既然Server进程已经准备就绪了。那么如今就应该要轮到Client端闪亮登场，Client端将通过Binder来请求Server做一些事情。接下来看代码：

status_t MediaPlayer::setDataSource(int fd, int64_t offset, int64_t length)
{
    status_t err = UNKNOWN_ERROR;
    const sp<IMediaPlayerService>& service(getMediaPlayerService());
    if (service != 0) {
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(fd, offset, length))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

getMediaPlayerService()之前分析过。返回一个BpMediaPlayerService，这里问一个问题：为什么这个BpMediaPlayerService就能和MediaPlayerService进程进行Binder通信，而不是和别的什么Server进程？

再回想一下代码：

/*static*/const sp<IMediaPlayerService>&
IMediaDeathNotifier::getMediaPlayerService()
{
    Mutex::Autolock _l(sServiceLock);
    if (sMediaPlayerService == 0) {
        sp<IServiceManager> sm = defaultServiceManager();
        sp<IBinder> binder;
        do {
            binder = sm->getService(String16("media.player"));
            if (binder != 0) {
                break;
            }
            ALOGW("Media player service not published, waiting...");
            usleep(500000); // 0.5 s
        } while (true);

        if (sDeathNotifier == NULL) {
            sDeathNotifier = new DeathNotifier();
        }
        binder->linkToDeath(sDeathNotifier);
        sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
    }
    ALOGE_IF(sMediaPlayerService == 0, "no media player service!?
");
    return sMediaPlayerService;
}

答案应该在binder = sm->getService(String16("media.player"))这句话里面，返回值binder将会作为BpMediaPlayerService构造函数的參数。以下来看getService

virtual sp<IBinder> getService(const String16& name) const
    {
        unsigned n;
        for (n = 0; n < 5; n++){
            sp<IBinder> svc = checkService(name);
            if (svc != NULL) return svc;
            ALOGI("Waiting for service %s...\n", String8(name).string());
            sleep(1);
        }
        return NULL;
    }

    virtual sp<IBinder> checkService( const String16& name) const
    {
        Parcel data, reply;
        data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
        data.writeString16(name);
        remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);
        return reply.readStrongBinder();
    }
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);
        }        
    }
    return BAD_TYPE;
}

unflatten_binder函数中。flat->type和flat->handle在ServiceManager中被赋值，当中flat->type的值是BINDER_TYPE_HANDLE，flat->handle的值是所查询Service相应的Handle值，中间过程涉及到binder底层驱动的代码，不在这里阐述。

这样binder = sm->getService(String16("media.player"))运行后。binder=BpBinder(Handle)，当中Handle的值为所查询Service相应的Handle值。这样。client端和service端之间的通信就建立了。

分析完getMediaPlayerService()。建立了通信的通路，接下来正式进入通信。

sp<IMediaPlayer>player(service->create(this, mAudioSessionId));

进入IMediaPlayerService.cpp看看create的实现。

virtual sp<IMediaPlayer> create(
            const sp<IMediaPlayerClient>& client, int audioSessionId) {
        Parcel data, reply;
        data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());
        data.writeStrongBinder(client->asBinder());
        data.writeInt32(audioSessionId);

        remote()->transact(CREATE, data, &reply);
        return interface_cast<IMediaPlayer>(reply.readStrongBinder());
    }

经过之前的分析，我们能够非常easy的指导remote()返回的是BpBinder(handle), transact(CREATE,data, &reply)将数据写入到Binder驱动，并唤醒Service进程。接下来我们就来看Server将作何反应。

如今我们已经知道，Server进程一直都在读取Binder驱动。然后executeCommand。以下就直接看executeCommand的实现。

status_t IPCThreadState::executeCommand(int32_t cmd)
{
    BBinder* obj;
    RefBase::weakref_type* refs;
    status_t result = NO_ERROR;
    
    switch (cmd) {
    ……
    case BR_TRANSACTION:
        {
            binder_transaction_data tr;
            result = mIn.read(&tr, sizeof(tr));
            ALOG_ASSERT(result == NO_ERROR,
                "Not enough command data for brTRANSACTION");
            if (result != NO_ERROR) break;
            
            Parcel buffer;
            buffer.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(size_t), freeBuffer, this);
            
            const pid_t origPid = mCallingPid;
            const uid_t origUid = mCallingUid;
            
            mCallingPid = tr.sender_pid;
            mCallingUid = tr.sender_euid;
            
            int curPrio = getpriority(PRIO_PROCESS, mMyThreadId);
            if (gDisableBackgroundScheduling) {
                if (curPrio > ANDROID_PRIORITY_NORMAL) {
                    // We have inherited a reduced priority from the caller, but do not
                    // want to run in that state in this process.  The driver set our
                    // priority already (though not our scheduling class), so bounce
                    // it back to the default before invoking the transaction.
                    setpriority(PRIO_PROCESS, mMyThreadId, ANDROID_PRIORITY_NORMAL);
                }
            } else {
                if (curPrio >= ANDROID_PRIORITY_BACKGROUND) {
                    // We want to use the inherited priority from the caller.
                    // Ensure this thread is in the background scheduling class,
                    // since the driver won‘t modify scheduling classes for us.
                    // The scheduling group is reset to default by the caller
                    // once this method returns after the transaction is complete.
                    set_sched_policy(mMyThreadId, SP_BACKGROUND);
                }
            }

            //ALOGI(">>>> TRANSACT from pid %d uid %d\n", mCallingPid, mCallingUid);
            
            Parcel reply;
            IF_LOG_TRANSACTIONS() {
                TextOutput::Bundle _b(alog);
                alog << "BR_TRANSACTION thr " << (void*)pthread_self()
                    << " / obj " << tr.target.ptr << " / code "
                    << TypeCode(tr.code) << ": " << indent << buffer
                    << dedent << endl
                    << "Data addr = "
                    << reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer)
                    << ", offsets addr="
                    << reinterpret_cast<const size_t*>(tr.data.ptr.offsets) << endl;
            }
            if (tr.target.ptr) {
                sp<BBinder> b((BBinder*)tr.cookie);
                const status_t error = b->transact(tr.code, buffer, &reply, tr.flags);
                if (error < NO_ERROR) reply.setError(error);

            } else {
                const status_t error = the_context_object->transact(tr.code, buffer, &reply, tr.flags);
                if (error < NO_ERROR) reply.setError(error);
            }
            
            //ALOGI("<<<< TRANSACT from pid %d restore pid %d uid %d\n",
            //     mCallingPid, origPid, origUid);
            
            if ((tr.flags & TF_ONE_WAY) == 0) {
                LOG_ONEWAY("Sending reply to %d!", mCallingPid);
                sendReply(reply, 0);
            } else {
                LOG_ONEWAY("NOT sending reply to %d!", mCallingPid);
            }
            
            mCallingPid = origPid;
            mCallingUid = origUid;

            IF_LOG_TRANSACTIONS() {
                TextOutput::Bundle _b(alog);
                alog << "BC_REPLY thr " << (void*)pthread_self() << " / obj "
                    << tr.target.ptr << ": " << indent << reply << dedent << endl;
            }
            
        }
        break;
    ……
    default:
        printf("*** BAD COMMAND %d received from Binder driver\n", cmd);
        result = UNKNOWN_ERROR;
        break;
    }

    if (result != NO_ERROR) {
        mLastError = result;
    }
    
    return result;
}

看这里：

if (tr.target.ptr) {
sp<BBinder> b((BBinder*)tr.cookie);
const status_t error = b->transact(tr.code, buffer, &reply, tr.flags);
if (error < NO_ERROR) reply.setError(error);

} else {
const status_t error = the_context_object->transact(tr.code, buffer, &reply, tr.flags);
if (error < NO_ERROR) reply.setError(error);
}

这里的b实际上就是我们在addService时创建的MediaPlayerService对象，经过包含Binder驱动在内的传输和转换，就成这么个数据类型了。

void MediaPlayerService::instantiate() {
    defaultServiceManager()->addService(
            String16("media.player"), new MediaPlayerService());
}

看以下的继承关系，

classMediaPlayerService : public BnMediaPlayerService

MediaPlayerService本身没有实现transact方法，因此。b->transact(tr.code,buffer, &reply, tr.flags)是调用了BnMediaPlayerService的transact方法。

进入IMediaPlayerService.cpp中找到BnMediaPlayerService的transact方法，例如以下：

status_t BnMediaPlayerService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch (code) {
        case CREATE: {
            CHECK_INTERFACE(IMediaPlayerService, data, reply);
            sp<IMediaPlayerClient> client =
                interface_cast<IMediaPlayerClient>(data.readStrongBinder());
            int audioSessionId = data.readInt32();
            sp<IMediaPlayer> player = create(client, audioSessionId);
            reply->writeStrongBinder(player->asBinder());
            return NO_ERROR;
        } break;
	……
}

sp<IMediaPlayer> player = create(client,audioSessionId)这里调用的create方法在MediaPlayerService类中实现。进入MediaPlayerService.cpp中：

sp<IMediaPlayer> MediaPlayerService::create(const sp<IMediaPlayerClient>& client,
        int audioSessionId)
{
    pid_t pid = IPCThreadState::self()->getCallingPid();
    int32_t connId = android_atomic_inc(&mNextConnId);

    sp<Client> c = new Client(
            this, pid, connId, client, audioSessionId,
            IPCThreadState::self()->getCallingUid());

    ALOGV("Create new client(%d) from pid %d, uid %d, ", connId, pid,
         IPCThreadState::self()->getCallingUid());
    /* add by Gary. start {{----------------------------------- */
    c->setScreen(mScreen);
    /* add by Gary. end   -----------------------------------}} */
    c->setSubGate(mGlobalSubGate);  // 2012-03-12, add the global interfaces to control the subtitle gate

    wp<Client> w = c;
    {
        Mutex::Autolock lock(mLock);
        mClients.add(w);
    }
    return c;
}

到这里为止，Server处理完了事务。接下来将处理结果返回给client，看这里：

if ((tr.flags & TF_ONE_WAY) == 0) {
LOG_ONEWAY("Sending reply to %d!", mCallingPid);
sendReply(reply, 0);
} else {
LOG_ONEWAY("NOT sending reply to %d!", mCallingPid);
}

status_t IPCThreadState::sendReply(const Parcel& reply, uint32_t flags)
{
    status_t err;
    status_t statusBuffer;
    err = writeTransactionData(BC_REPLY, flags, -1, 0, reply, &statusBuffer);
    if (err < NO_ERROR) return err;
    
    return waitForResponse(NULL, NULL);
}

调用sendReply将结果写回Binder驱动，从而得以返回client进程。

通信完毕。