Android IPC 通讯机制源码分析【中】
客户端如何设置这个句柄在下面会介绍。
CameraSerivce服务的注册(Main_mediaservice.c）
int main(int argc, char** argv)
{
sp<ProcessState> proc(ProcessState::self());
sp<IServiceManager> sm = defaultServiceManager();
LOGI("ServiceManager: %p", sm.get());
AudioFlinger::instantiate(); //Audio 服务
MediaPlayerService::instantiate(); //mediaPlayer服务
CameraService::instantiate(); //Camera 服务
ProcessState::self()->startThreadPool(); //为进程开启缓冲池
IPCThreadState::self()->joinThreadPool(); //将进程加入到缓冲池
}
CameraService.cpp
void CameraService::instantiate() {
defaultServiceManager()->addService(
String16("media.camera"), new CameraService());
}
创建CameraService服务对象并添加到ServiceManager进程中。
client获取remote IServiceManager IBinder接口：
sp<IServiceManager> defaultServiceManager()
{
if (gDefaultServiceManager != NULL) return gDefaultServiceManager;
{
AutoMutex _l(gDefaultServiceManagerLock);
if (gDefaultServiceManager == NULL) {
gDefaultServiceManager = interface_cast<IServiceManager>(
ProcessState::self()->getContextObject(NULL));
}
}
return gDefaultServiceManager;
}
任何一个进程在第一次调用defaultServiceManager的时候gDefaultServiceManager值为Null，所以该进程会通过ProcessState::self得到ProcessState实例。ProcessState将打开Binder驱动。
ProcessState.cpp
sp<ProcessState> ProcessState::self()
{
if (gProcess != NULL) return gProcess;
AutoMutex _l(gProcessMutex);
if (gProcess == NULL) gProcess = new ProcessState;
return gProcess;
}
ProcessState::ProcessState()
: mDriverFD(open_driver()) //打开/dev/binder驱动
...........................
{
}
sp<IBinder> ProcessState::getContextObject(const sp<IBinder>& caller)
{
if (supportsProcesses()) {
return getStrongProxyForHandle(0);
} else {
return getContextObject(String16("default"), caller);
}
}
Android是支持Binder驱动的所以程序会调用getStrongProxyForHandle。这里handle为0，正好与Service_manager中的BINDER_SERVICE_MANAGER一致。
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; //第一次调用该函数b为Null
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;
}
第一次调用的时候b为Null所以会为b生成一BpBinder对象：
BpBinder::BpBinder(int32_t handle)
: mHandle(handle)
, mAlive(1)
, mObitsSent(0)
, mObituaries(NULL)
{
LOGV("Creating BpBinder %p handle %d\n", this, mHandle);
extendObjectLifetime(OBJECT_LIFETIME_WEAK);
IPCThreadState::self()->incWeakHandle(handle);
}
void IPCThreadState::incWeakHandle(int32_t handle)
{
LOG_REMOTEREFS("IPCThreadState::incWeakHandle(%d)\n", handle);
mOut.writeInt32(BC_INCREFS);
mOut.writeInt32(handle);
}
getContextObject返回了一个BpBinder对象。
interface_cast<IServiceManager>(
ProcessState::self()->getContextObject(NULL));
template<typename INTERFACE>
inline sp<INTERFACE> interface_cast(const sp<IBinder>& obj)
{
return INTERFACE::asInterface(obj);
}
将这个宏扩展后最终得到的是：
sp<IServiceManager> IServiceManager::asInterface(const sp<IBinder>& obj)
{
sp<IServiceManager> intr;
if (obj != NULL) {
intr = static_cast<IServiceManager*>(
obj->queryLocalInterface(
IServiceManager::descriptor).get());
if (intr == NULL) {
intr = new BpServiceManager(obj);
}
}
return intr;
}
返回一个BpServiceManager对象,这里obj就是前面我们创建的BpBInder对象。