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Binder客户端实例 sp<IServiceManager> sm = defaultServiceManager(); …… // 通过ServiceManager的Binder客户端向系统注册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()的实现如下，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，这个函数不过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;}我们可以看到，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：等待客户端请求，处理请求，然后返回给客户端。
既然Server进程已经准备就绪了，那么现在就应该要轮到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？
再回顾一下代码：/*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构造函数的参数。下面来看getServicevirtual 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，其中flat->type的值是BINDER_TYPE_HANDLE，flat->handle的值是所查询Service对应的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"))执行后，binder=BpBinder(Handle)，其中Handle的值为所查询Service对应的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，我们可以很容易的指导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。现在我们已经知道，Server进程一直都在读取Binder驱动，然后executeCommand，下面就直接看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，经过包括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());}看下面的继承关系，
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，如下：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。通信完成。