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Camera的架構與Android系統的整體架構保持一致，如下圖所示，本文主要從以下四個方面對其進行說明。
Framework：Camera.java
Android Runtime：android_hardware_Camera.cpp
Library：Camera Client和Camera Service
HAL：CameraHardwareInterface
Camera是應用層軟件直接使用的類，涵蓋了啟動、預覽、拍攝及關閉等操作攝像頭的全部接口。Camera.java在Android源碼中的路徑為：framework/base/core/java/android/hardware。為了說明整個Camera系統的架構，這里暫不橫向分析Camera.java的功能，下面從open()方法著手：
public static Camera open() { int numberOfCameras = getNumberOfCameras();
CameraInfo cameraInfo = new CameraInfo(); for (int i = 0; i < numberOfCameras; i++) {
getCameraInfo(i, cameraInfo); if (cameraInfo.facing == CameraInfo.CAMERA_FACING_BACK) { return new Camera(i);
}
} return null;
}
open()方法需要注意以下幾點：
getNumberOfCameras為native方法，實現在android_hardware_Camera.cpp中；
CameraInfo是Camera定義的靜態內部類，包含facing、orientation、canDisableShutterSound；
getCameraInfo內部調用native方法_getCameraInfo獲取攝像頭信息；
open()默認啟動的是后置攝像頭（CAMERA_FACING_BACK）。
/** used by Camera#open, Camera#open(int) */Camera(int cameraId) { int err = cameraInitNormal(cameraId); if (checkInitErrors(err)) { switch(err) { case EACCESS: throw new RuntimeException("Fail to connect to camera service"); case ENODEV: throw new RuntimeException("Camera initialization failed"); default: // Should never hit this.
throw new RuntimeException("Unknown camera error");
}
}
}
Camera構造器的核心實現在cameraInitNormal中，cameraInitNormal調用cameraInitVersion，并傳入參數cameraId和CAMERA_HAL_API_VERSION_NORMAL_CONNECT，后者代表HAL的版本。
private int cameraInitVersion(int cameraId, int halVersion) {……
String packageName = ActivityThread.currentPackageName(); return native_setup(new WeakReference<Camera>(this), cameraId, halVersion, packageName); }
cameraInitNormal調用本地方法native_setup()，由此進入到android_hardware_Camera.cpp中，native_setup()的簽名如下：
private native final int native_setup(Object camera_this, int cameraId, int halVersion, String packageName);
native_setup()被動態注冊到JNI，通過JNI調用android_hardware_Camera_native_setup()方法。
static JNINativeMethod camMethods[] = {
……
{ "native_setup", "(Ljava/lang/Object;ILjava/lang/String;)V",
(void*)android_hardware_Camera_native_setup }
……
};
JNI的重點是android_hardware_Camera_native_setup()方法的實現：
// connect to camera servicestatic jint android_hardware_Camera_native_setup(JNIEnv *env, jobject thiz,
jobject weak_this, jint cameraId, jint halVersion, jstring clientPackageName)
{ // Convert jstring to String16
const char16_t *rawClientName = env->GetStringChars(clientPackageName, NULL);
jsize rawClientNameLen = env->GetStringLength(clientPackageName);
String16 clientName(rawClientName, rawClientNameLen);
env->ReleaseStringChars(clientPackageName, rawClientName);
sp<Camera> camera; if (halVersion == CAMERA_HAL_API_VERSION_NORMAL_CONNECT) { // Default path: hal version is don't care, do normal camera connect.
camera = Camera::connect(cameraId, clientName,
Camera::USE_CALLING_UID);
} else {
jint status = Camera::connectLegacy(cameraId, halVersion, clientName,
Camera::USE_CALLING_UID, camera); if (status != NO_ERROR) { return status;
}
} if (camera == NULL) { return -EACCES;
} // make sure camera hardware is alive
if (camera->getStatus() != NO_ERROR) { return NO_INIT;
}
jclass clazz = env->GetObjectClass(thiz); if (clazz == NULL) { // This should never happen
jniThrowRuntimeException(env, "Can't find android/hardware/Camera"); return INVALID_OPERATION;
} // We use a weak reference so the Camera object can be garbage collected. // The reference is only used as a proxy for callbacks.
sp<JNICameraContext> context = new JNICameraContext(env, weak_this, clazz, camera);
context->incStrong((void*)android_hardware_Camera_native_setup);
camera->setListener(context); // save context in opaque field
env->SetLongField(thiz, fields.context, (jlong)context.get()); return NO_ERROR;
}
android_hardware_Camera_native_setup()方法通過調用Camera::connect()方法請求連接CameraService服務。入參中：
clientName是通過將clientPackageName從jstring轉換為String16格式得到；
Camera::USE_CALLING_UID是定義在Camera.h中的枚舉類型，其值為ICameraService::USE_CALLING_UID（同樣為枚舉類型，值為-1）。
Camera::connect()位于Camera.cpp中，由此進入到Library層。
如上述架構圖中所示，ICameraService.h、ICameraClient.h和ICamera.h三個類定義了Camera的接口和架構，ICameraService.cpp和Camera.cpp兩個文件用于Camera架構的實現，Camera的具體功能在下層調用硬件相關的接口來實現。Camera.h是Camera系統對上層的接口。
具體的，Camera類繼承模板類CameraBase，Camera::connect()調用了CameraBase.cpp中的connect()方法。
sp<Camera> Camera::connect(int cameraId, const String16& clientPackageName, int clientUid)
{ return CameraBaseT::connect(cameraId, clientPackageName, clientUid);
}CameraBase實際上又繼承了IBinder的DeathRecipient內部類，DeathRecipient虛擬繼承自RefBase。RefBase是Android中的引用計數基礎類，其中定義了incStrong、decStrong、incWeak和decWeak等涉及sp/wp的指針操作函數，當然這扯遠了。
template <typename TCam>struct CameraTraits {
};
template <typename TCam, typename TCamTraits = CameraTraits<TCam> >class CameraBase : public IBinder::DeathRecipient
{public:
static sp<TCam> connect(int cameraId, const String16& clientPackageName, int clientUid);
……
}
class DeathRecipient : public virtual RefBase
{public: virtual void binderDied(const wp<IBinder>& who) = 0;
};回到Camera::connect()的實現上，其中，new TCam(cameraId)生成BnCameraClient對象，BnCameraClient定義在ICameraClient.h文件中，繼承自模板類BnInterface。getCameraService()方法返回CameraService的服務代理BpCameraService，BpCameraService同樣繼承自模板類BnInterface。然后通過Binder通信發送CONNECT命令，當BnCameraService收到CONNECT命令后調用CameraService的connect()成員函數來做相應的處理。
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); // BnCameraClient
sp<TCamCallbacks> cl = c;
status_t status = NO_ERROR; const sp<ICameraService>& cs = getCameraService(); // BpCameraService 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;
}
class BnCameraClient: public BnInterface<ICameraClient>{public: virtual status_t onTransact( uint32_t code, const Parcel& data,
Parcel* reply,
uint32_t flags = 0);
};
class BpCameraService: public BpInterface<ICameraService>{public:
BpCameraService(const sp<IBinder>& impl)
: BpInterface<ICameraService>(impl)
{
}
……
}
注：connect()函數在BpCameraService和BnCameraService的父類ICameraService中聲明為純虛函數，在BpCameraService和CameraService中分別給出了實現，BpCameraService作為代理類，提供接口給客戶端，真正實現在BnCameraService的子類CameraService中。
在BpCameraService中，connect()函數實現如下：
// connect to camera service (android.hardware.Camera)
virtual status_t connect(const sp<ICameraClient>& cameraClient, int cameraId, const String16 &clientPackageName, int clientUid, /*out*/
sp<ICamera>& device)
{
Parcel data, reply;
data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());
data.writeStrongBinder(cameraClient->asBinder());
data.writeInt32(cameraId);
data.writeString16(clientPackageName);
data.writeInt32(clientUid);
remote()->transact(BnCameraService::CONNECT, data, &reply); // BpBinder的transact()函數向IPCThreadState實例發送消息，通知其有消息要發送給binder driver if (readExceptionCode(reply)) return -EPROTO;
status_t status = reply.readInt32(); if (reply.readInt32() != 0) {
device = interface_cast<ICamera>(reply.readStrongBinder()); // client端讀出server返回的bind
} return status;
}
首先將傳遞過來的Camera對象cameraClient轉換成IBinder類型，將調用的參數寫到Parcel（可理解為Binder通信的管道）中，通過BpBinder的transact()函數發送消息，然后由BnCameraService去響應該連接，最后就是等待服務端返回，如果成功則生成一個BpCamera實例。
真正的服務端響應實現在BnCameraService的onTransact()函數中，其負責解包收到的Parcel并執行client端的請求的方法。
status_t BnCameraService::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{ switch(code) { …… case CONNECT: {
CHECK_INTERFACE(ICameraService, data, reply);
sp<ICameraClient> cameraClient =
interface_cast<ICameraClient>(data.readStrongBinder()); // 使用Camera的Binder對象生成Camera客戶代理BpCameraClient實例
int32_t cameraId = data.readInt32(); const String16 clientName = data.readString16();
int32_t clientUid = data.readInt32();
sp<ICamera> camera;
status_t status = connect(cameraClient, cameraId,
clientName, clientUid, /*out*/camera); // 將生成的BpCameraClient對象作為參數傳遞到CameraService的connect()函數中
reply->writeNoException();
reply->writeInt32(status); // 將BpCamera對象以IBinder的形式打包到Parcel中返回 if (camera != NULL) {
reply->writeInt32(1);
reply->writeStrongBinder(camera->asBinder());
} else {
reply->writeInt32(0);
} return NO_ERROR;
} break;
……
}
}
主要的處理包括：
通過data中Camera的Binder對象生成Camera客戶代理BpCameraClient實例；
將生成的BpCameraClient對象作為參數傳遞到CameraService（/frameworks/av/services/camera /libcameraservice/CameraService.cpp）的connect()函數中，該函數會返回一個BpCamera實例；
將在上述實例對象以IBinder的形式打包到Parcel中返回。
最后，BpCamera實例是通過CameraService::connect()函數返回的。CameraService::connect()實現的核心是調用connectHelperLocked()函數根據HAL不同API的版本創建不同的client實例（早期版本中好像沒有connectHelperLocked()這個函數，但功能基本相似）。
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) { int facing = -1; int deviceVersion = getDeviceVersion(cameraId, &facing); if (halVersion < 0 || halVersion == deviceVersion) { // Default path: HAL version is unspecified by caller, create CameraClient // based on device version reported by the HAL.
switch(deviceVersion) { case CAMERA_DEVICE_API_VERSION_1_0:
client = new CameraClient(this, cameraClient,
clientPackageName, cameraId,
facing, callingPid, clientUid, getpid(), legacyMode); break; case CAMERA_DEVICE_API_VERSION_2_0: case CAMERA_DEVICE_API_VERSION_2_1: case CAMERA_DEVICE_API_VERSION_3_0: case CAMERA_DEVICE_API_VERSION_3_1: case CAMERA_DEVICE_API_VERSION_3_2:
client = new Camera2Client(this, cameraClient,
clientPackageName, cameraId,
facing, callingPid, clientUid, getpid(), legacyMode); break; case -1:
ALOGE("Invalid camera id %d", cameraId); return BAD_VALUE; default:
ALOGE("Unknown camera device HAL version: %d", deviceVersion); return INVALID_OPERATION;
}
} else { // A particular HAL version is requested by caller. Create CameraClient // based on the requested HAL version.
if (deviceVersion > CAMERA_DEVICE_API_VERSION_1_0 &&
halVersion == CAMERA_DEVICE_API_VERSION_1_0) { // Only support higher HAL version device opened as HAL1.0 device.
client = new CameraClient(this, cameraClient,
clientPackageName, cameraId,
facing, callingPid, clientUid, getpid(), legacyMode);
} else { // Other combinations (e.g. HAL3.x open as HAL2.x) are not supported yet.
ALOGE("Invalid camera HAL version %x: HAL %x device can only be"
" opened as HAL %x device", halVersion, deviceVersion,
CAMERA_DEVICE_API_VERSION_1_0); return INVALID_OPERATION;
}
}
status_t status = connectFinishUnsafe(client, client->getRemote()); if (status != OK) { // this is probably not recoverable.. maybe the client can try again
return status;
}
mClient[cameraId] = client;
LOG1("CameraService::connect X (id %d, this pid is %d)", cameraId,
getpid()); return OK;
}
可見，在CAMERA_DEVICE_API_VERSION_2_0之前使用CameraClient進行實例化，之后則采用Camera2Client進行實例化。以CameraClient為例，其initialize()函數如下：
status_t CameraClient::initialize(camera_module_t *module) { int callingPid = getCallingPid();
status_t res;
LOG1("CameraClient::initialize E (pid %d, id %d)", callingPid, mCameraId); // Verify ops permissions
res = startCameraOps(); if (res != OK) { return res;
} char camera_device_name[10];
snprintf(camera_device_name, sizeof(camera_device_name), "%d", mCameraId); mHardware = new CameraHardwareInterface(camera_device_name);
res = mHardware->initialize(&module->common); if (res != OK) {
ALOGE("%s: Camera %d: unable to initialize device: %s (%d)",
__FUNCTION__, mCameraId, strerror(-res), res);
mHardware.clear(); return res;
}
mHardware->setCallbacks(notifyCallback,
dataCallback,
dataCallbackTimestamp,
(void *)(uintptr_t)mCameraId); // Enable zoom, error, focus, and metadata messages by default
enableMsgType(CAMERA_MSG_ERROR | CAMERA_MSG_ZOOM | CAMERA_MSG_FOCUS |
CAMERA_MSG_PREVIEW_METADATA | CAMERA_MSG_FOCUS_MOVE);
LOG1("CameraClient::initialize X (pid %d, id %d)", callingPid, mCameraId); return OK;
}
上述函數中，主要注意以下流程：
加粗的代碼CameraHardwareInterface新建了了一個Camera硬件接口，當然，camera_device_name為攝像頭設備名；
mHardware->initialize(&module->common)調用底層硬件的初始化方法；
mHardware->setCallbacks將CamerService處的回調函數注冊到HAL處。
CameraHardwareInterface定義了Camera的硬件抽象特征，由此進入到HAL。
CameraHardwareInterface的作用在于鏈接Camera Server和V4L2，通過實現CameraHardwareInterface可以屏蔽不同的driver對Camera Server的影響。CameraHardwareInterface同樣虛擬繼承自RefBase。
class CameraHardwareInterface : public virtual RefBase {public:
CameraHardwareInterface(const char *name)
{
mDevice = 0;
mName = name;
}
……
}
CameraHardwareInterface中包含了控制通道和數據通道，控制通道用于處理預覽和視頻獲取的開始/停止、拍攝照片、自動對焦等功能，數據通道通過回調函數來獲得預覽、視頻錄制、自動對焦等數據。當需要支持新的硬件時就需要繼承于CameraHardwareInterface ，來實現對應的功能。CameraHardwareInterface提供的public方法如下：
在前一節中，initialize()函數調用了mHardware->initialize和mHardware->setCallbacks，下面來看下CameraHardwareInterface.h對其的實現。
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;
}
在initialize()方法中，通過cameraModule->open_legacy打開攝像頭模組，initHalPreviewWindow()用于初始化Preview的相關流opspreview_stream_ops，初始化hal的預覽窗口。
void initHalPreviewWindow()
{
mHalPreviewWindow.nw.cancel_buffer = __cancel_buffer;
mHalPreviewWindow.nw.lock_buffer = __lock_buffer;
mHalPreviewWindow.nw.dequeue_buffer = __dequeue_buffer;
mHalPreviewWindow.nw.enqueue_buffer = __enqueue_buffer;
mHalPreviewWindow.nw.set_buffer_count = __set_buffer_count;
mHalPreviewWindow.nw.set_buffers_geometry = __set_buffers_geometry;
mHalPreviewWindow.nw.set_crop = __set_crop;
mHalPreviewWindow.nw.set_timestamp = __set_timestamp;
mHalPreviewWindow.nw.set_usage = __set_usage;
mHalPreviewWindow.nw.set_swap_interval = __set_swap_interval;
mHalPreviewWindow.nw.get_min_undequeued_buffer_count =
__get_min_undequeued_buffer_count;
}
/** Set the notification and data callbacks */
void setCallbacks(notify_callback notify_cb,
data_callback data_cb,
data_callback_timestamp data_cb_timestamp, void* user)
{
mNotifyCb = notify_cb;
mDataCb = data_cb;
mDataCbTimestamp = data_cb_timestamp;
mCbUser = user;
ALOGV("%s(%s)", __FUNCTION__, mName.string()); if (mDevice->ops->set_callbacks) {
mDevice->ops->set_callbacks(mDevice,
__notify_cb,
__data_cb,
__data_cb_timestamp,
__get_memory, this);
}
}
set_callbacks中，__notify_cb、__data_cb、__data_cb_timestamp和__get_memory分別消息回調，數據回調，時間戳回調，以及內存相關操作的回調。
以上通過簡略分析應用層調用Camera.open()之后在Framework、ART、Library以及HAL層的響應，來說明Android中Camera系統的整體架構
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