Android Camera 采用C/S架构，client 与server两个独立的线程之间（CameraService）使用Binder通信。
一 CameraService的注册。
1.手机开机后，会走init.rc流程，init.rc会启动MediaServer Service。
service media /system/bin/mediaserver
class main
user root ####
# google default ####
# user media ####
group audio camera inet net_bt net_bt_admin net_bw_acct drmrpc mediadrm media sdcard_r system net_bt_stack ####
# google default ####
# group audio camera inet net_bt net_bt_admin net_bw_acct drmrpc mediadrm ####
ioprio rt 4
2.MediaServer的main函数位于frameworks/base/media/mediaserver/main_mediaserver.cpp中。
在Main_MediaServer.cpp的main函数中，CameraService完成了注册
1 int main(int argc __unused, char** argv)
2 {
3 signal(SIGPIPE, SIG_IGN);
4 char value[PROPERTY_VALUE_MAX];
5 bool doLog = (property_get("ro.test_harness", value, "0") > 0) && (atoi(value) == 1);
6 pid_t childPid;
7 ..............................
8 sp<ProcessState> proc(ProcessState::self());
9 sp<IServiceManager> sm = defaultServiceManager();
10 ALOGI("ServiceManager: %p", sm.get());
11 AudioFlinger::instantiate();
12 MediaPlayerService::instantiate();
13 #ifdef MTK_AOSP_ENHANCEMENT
14 MemoryDumper::instantiate();
15 #endif
16 CameraService::instantiate();
17 ......................................
3. instantiate的实现在CameraService的父类中，
namespace android {
template<typename SERVICE>
class BinderService
{
public:
static status_t publish(bool allowIsolated = false) {
sp<IServiceManager> sm(defaultServiceManager());
return sm->addService(
String16(SERVICE::getServiceName()),
new SERVICE(), allowIsolated);
}
static void publishAndJoinThreadPool(bool allowIsolated = false) {
publish(allowIsolated);
joinThreadPool();
}
static void instantiate() { publish(); }
static status_t shutdown() { return NO_ERROR; }
private:
static void joinThreadPool() {
sp<ProcessState> ps(ProcessState::self());
ps->startThreadPool();
ps->giveThreadPoolName();
IPCThreadState::self()->joinThreadPool();
}
};
}; // namespace android
可以发现在publish(）函数中，CameraService完成服务的注册 。SERVICE是个模板，这里是注册CameraService，所以可用CameraService代替
return sm->addService(String16(CameraService::getServiceName()), new CameraService());
这样，Camera就在ServiceManager完成服务注册，提供给client随时使用。
二 client如何连上server端，并打开camera模块
1.Client如何连接到server端。
我们从Camera.open()开始往framework进行分析，调用frameworks\base\core\java\android\hardware\Camera.java类的open方法 。
public static Camera open() {
............................................
return new Camera(cameraId);
.............................................
}
这里调用了Camera的构造函数,在构造函数中调用了cameraInitVersion
private int cameraInitVersion(int cameraId, int halVersion) {
..................................................
return native_setup(new WeakReference<Camera>(this), cameraId, halVersion, packageName);
}
此后进入JNI层android_hardware_camera.cpp
// connect to camera service
static jint android_hardware_Camera_native_setup(JNIEnv *env, jobject thiz,
jobject weak_this, jint cameraId, jint halVersion, jstring clientPackageName)
{
.......................
camera = Camera::connect(cameraId, clientName,
Camera::USE_CALLING_UID);
......................
sp<JNICameraContext> context = new JNICameraContext(env, weak_this, clazz, camera);
...........................
camera->setListener(context);
..........................
}
JNI函数里面，我们找到Camera C/S架构的客户端了（即红色加粗的那个Camera），它调用connect函数向服务端发送连接请求。JNICameraContext这个类是一个监听类，用于处理底层Camera回调函数传来的数据和消息
sp<Camera> Camera::connect(int cameraId, const String16& clientPackageName,
int clientUid)
{
return CameraBaseT::connect(cameraId, clientPackageName, clientUid);
}
template <typename TCam, typename TCamTraits>
sp<TCam> CameraBase<TCam, TCamTraits>::connect(int cameraId,
const String16& clientPackageName,
int clientUid)
{
ALOGV("%s: connect", __FUNCTION__);
sp<TCam> c = new TCam(cameraId);
sp<TCamCallbacks> cl = c;
status_t status = NO_ERROR;
const sp<ICameraService>& cs = getCameraService();
if (cs != 0) {
TCamConnectService fnConnectService = TCamTraits::fnConnectService;
status = (cs.get()->*fnConnectService)(cl, cameraId, clientPackageName, clientUid,
/*out*/ c->mCamera);
}
if (status == OK && c->mCamera != 0) {
c->mCamera->asBinder()->linkToDeath(c);
c->mStatus = NO_ERROR;
} else {
ALOGW("An error occurred while connecting to camera: %d", cameraId);
c.clear();
}
return c;
}
// establish binder interface to camera service
template <typename TCam, typename TCamTraits>
const sp<ICameraService>& CameraBase<TCam, TCamTraits>::getCameraService()
{
Mutex::Autolock _l(gLock);
if (gCameraService.get() == 0) {
sp<IServiceManager> sm = defaultServiceManager();
sp<IBinder> binder;
do {
binder = sm->getService(String16(kCameraServiceName));
if (binder != 0) {
break;
}
ALOGW("CameraService not published, waiting...");
usleep(kCameraServicePollDelay);
} while(true);
if (gDeathNotifier == NULL) {
gDeathNotifier = new DeathNotifier();
}
binder->linkToDeath(gDeathNotifier);
gCameraService = interface_cast<ICameraService>(binder);
}
ALOGE_IF(gCameraService == 0, "no CameraService!?");
return gCameraService;
}
此处终于获得CameraService实例了，该CameraService实例是通过binder获取的。
再来看fnConnectService是什么，
在Camera.cpp中，有
CameraTraits<Camera>::TCamConnectService CameraTraits<Camera>::fnConnectService =
&ICameraService::connect;
这样也就是说fnConnectService就是使用CameraService调用connect，从这里开始，终于从进入了服务端的流程：
status_t CameraService::connect(
const sp<ICameraClient>& cameraClient,
int cameraId,
const String16& clientPackageName,
int clientUid,
/*out*/
sp<ICamera>& device) {
.........................
status_t status = validateConnect(cameraId, /*inout*/clientUid);
..................................
if (!canConnectUnsafe(cameraId, clientPackageName,
cameraClient->asBinder(),
/*out*/clientTmp)) {
return -EBUSY;
}
status = connectHelperLocked(/*out*/client,
cameraClient,
cameraId,
clientPackageName,
clientUid,
callingPid);
device = client;
return OK;
}
status_t CameraService::connectHelperLocked(
/*out*/
sp<Client>& client,
/*in*/
const sp<ICameraClient>& cameraClient,
int cameraId,
const String16& clientPackageName,
int clientUid,
int callingPid,
int halVersion,
bool legacyMode) {
....................................
client = new CameraClient(this, cameraClient,
clientPackageName, cameraId,
facing, callingPid, clientUid, getpid(), legacyMode);
....................................
status_t status = connectFinishUnsafe(client, client->getRemote());
............................................
}
在connectHelpLocked，CameraService返回一个其实是它内部类的client——CameraClient。（注意此client是CameraService内部的CameraClient，不是Camera客户端），此client还赋值给了device，而这个device，就是在客户端CameraBase.cpp中c->mCamera,
status = (cs.get()->*fnConnectService)(cl, cameraId, clientPackageName, clientUid,
/*out*/ c->mCamera);
而这个c->mCamera就是每一次客户端Camera.cpp中调用一些函数（如preview/takepicture）时的mCamera，这样每一次客户端调用preview/takepicture，就直接调用的是CameraClient中的相关函数。这样就真正建立了客户端和服务端的关系。如Camera.cpp中的startPreview
// start preview mode
status_t Camera::startPreview()
{
ALOGV("startPreview");
sp <ICamera> c = mCamera;
if (c == 0) return NO_INIT;
return c->startPreview();
}
其实直接就会调用到CameraService中CameraClient.cpp中
// start preview mode
status_t CameraClient::startPreview() {
LOG1("startPreview (pid %d)", getCallingPid());
return startCameraMode(CAMERA_PREVIEW_MODE);
}
2.service端（其实就是CameraClient）的初始化和运行过程。
刚才我们知道，camera客户端与服务端的调用关系，就是camera.cpp中的函数，对应到CameraClient中的函数。下面我们再看看CameraClient的初始化过程。
在connectFinishUnsafe中，
status_t status = client->initialize(mModule);
我们再来看CameraClient类的initialize函数
status_t CameraClient::initialize(camera_module_t *module) {
int callingPid = getCallingPid();
status_t res;
mHardware = new CameraHardwareInterface(camera_device_name);
res = mHardware->initialize(&module->common);
}
CameraClient的初始化就是：先实例化Camera Hal接口 CameraHardwareInterface，CameraHardwareInterface调用initialize()进入HAL层打开Camera底层驱动
status_t initialize(hw_module_t *module)
{
ALOGI("Opening camera %s", mName.string());
camera_module_t *cameraModule = reinterpret_cast<camera_module_t *>(module);
camera_info info;
status_t res = cameraModule->get_camera_info(atoi(mName.string()), &info);
if (res != OK) return res;
int rc = OK;
if (module->module_api_version >= CAMERA_MODULE_API_VERSION_2_3 &&
info.device_version > CAMERA_DEVICE_API_VERSION_1_0) {
// Open higher version camera device as HAL1.0 device.
rc = cameraModule->open_legacy(module, mName.string(),
CAMERA_DEVICE_API_VERSION_1_0,
(hw_device_t **)&mDevice);
} else {
rc = CameraService::filterOpenErrorCode(module->methods->open(
module, mName.string(), (hw_device_t **)&mDevice));
}
if (rc != OK) {
ALOGE("Could not open camera %s: %d", mName.string(), rc);
return rc;
}
initHalPreviewWindow();
return rc;
}
hardware->initialize(&mModule->common)中mModule模块是一个结构体camera_module_t，他是怎么初始化的呢？我们发现CameraService里面有个函数
void CameraService::onFirstRef()
{
BnCameraService::onFirstRef();
if (hw_get_module(CAMERA_HARDWARE_MODULE_ID,
(const hw_module_t **)&mModule) < 0) {
LOGE("Could not load camera HAL module");
mNumberOfCameras = 0;
}
}
了解HAL层的都知道hw_get_module函数就是用来获取模块的Hal stub，这里通过CAMERA_HARDWARE_MODULE_ID 获取Camera Hal层的代理stub，并赋值给mModule，后面就可通过操作mModule完成对Camera模块的控制。那么onFirstRef()函数又是何时调用的？
onFirstRef（）属于其父类RefBase，该函数在强引用sp新增引用计数时调用。就是当 有sp包装的类初始化的时候调用，那么camera是何时调用的呢？可以发现在
客户端发起连接时候
sp Camera::connect(int cameraId)
{
LOGV("connect");
sp c = new Camera();
const sp& cs = getCameraService();
}
这个时候初始化了一个CameraService实例，且用Sp包装，这个时候sp将新增计数，相应的CameraService实例里面onFirstRef（）函数完成调用。
CameraService::connect（）即实例化CameraClient并打开驱动，返回CameraClient的时候，就表明客户端和服务端连接建立。Camera完成初始化。
总结：Camera的初始化流程简单的说就是：
->先是系统注册CameraService的服务
->AP层调用Camera.open()
->Camera.java调用JNI native_setup()
->JNI层调用 android_hardware_Camera_native_setup
-> HAL 客户端(Camera.cpp)调用connect与服务端(CameraService.cpp)连接，并得到CameraService中的CameraClient的一个实例
->服务端CameraClient的初始化，实例化Camera Hal接口 CameraHardwareInterface
->CameraHardwareInterface 打开Camera驱动，初始化完毕
最终的结果就是客户端会得到一个服务端CameraService中的CameraClient的一个实例，客户端的每一个函数操作其实最终都是调用CameraClient的函数
参考文档：
Android 4.0 Camera架构分析之Camera初始化 http://blog.csdn.net/dnfchan/article/details/7594590
Android 5.1 Camera Framework源码