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。