Camera的架構與Android系統(tǒng)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。

1. Framework：Camera.java
2. Android Runtime：android_hardware_Camera.cpp
3. Library：Camera Client和Camera Service
4. HAL：CameraHardwareInterface

Camera系統(tǒng)架構

一、Framework：Camera.java

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(tǒng)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(xiàn)在android_hardware_Camera.cpp中；
• CameraInfo是Camera定義的靜態(tài)內(nèi)部類，包含facing、orientation、canDisableShutterSound；
• getCameraInfo內(nèi)部調(diào)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(xiàn)在cameraInitNormal中，cameraInitNormal調(diào)用cameraInitVersion，并傳入?yún)?shù)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(diào)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);

二、Android Runtime：android_hardware_Camera.cpp

native_setup()被動態(tài)注冊到JNI，通過JNI調(diào)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(xiàn)：

// connect to camera service
static 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(diào)用Camera::connect()方法請求連接CameraService服務。入?yún)⒅校?/p>

• clientName是通過將clientPackageName從jstring轉(zhuǎn)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。

三、Library：Camera Client和Camera Service

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(xiàn)，Camera的具體功能在下層調(diào)用硬件相關的接口來實現(xiàn)。Camera.h是Camera系統(tǒng)對上層的接口。

具體的，Camera類繼承模板類CameraBase，Camera::connect()調(diào)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(nèi)部類，DeathRecipient虛擬繼承自RefBase。RefBase是Android中的引用計數(shù)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(shù)，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(xiàn)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(fā)送CONNECT命令，當BnCameraService收到CONNECT命令后調(diào)用CameraService的connect()成員函數(shù)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(shù)在BpCameraService和BnCameraService的父類ICameraService中聲明為純虛函數(shù)，在BpCameraService和CameraService中分別給出了實現(xiàn)，BpCameraService作為代理類，提供接口給客戶端，真正實現(xiàn)在BnCameraService的子類CameraService中。

在BpCameraService中，connect()函數(shù)實現(xiàn)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);
        // BpBinder的transact()函數(shù)向IPCThreadState實例發(fā)送消息，通知其有消息要發(fā)送給binder driver
        remote()->transact(BnCameraService::CONNECT, data, &reply); 
        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(zhuǎn)換成IBinder類型，將調(diào)用的參數(shù)寫到Parcel（可理解為Binder通信的管道）中，通過BpBinder的transact()函數(shù)發(fā)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(xiàn)在BnCameraService的onTransact()函數(shù)中，其負責解包收到的Parcel并執(zhí)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(shù)傳遞到CameraService的connect()函數(shù)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：

1. 通過data中Camera的Binder對象生成Camera客戶代理BpCameraClient實例；
2. 將生成的BpCameraClient對象作為參數(shù)傳遞到CameraService（/frameworks/av/services/camera /libcameraservice/CameraService.cpp）的connect()函數(shù)中，該函數(shù)會返回一個BpCamera實例；
3. 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(shù)返回的。CameraService::connect()實現(xiàn)的核心是調(diào)用connectHelperLocked()函數(shù)根據(jù)HAL不同API的版本創(chuàng)建不同的client實例（早期版本中好像沒有connectHelperLocked()這個函數(shù)，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(shù)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(shù)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；

1. mHardware->initialize(&module->common)調(diào)用底層硬件的初始化方法；
2. mHardware->setCallbacks將CamerService處的回調(diào)函數(shù)注冊到HAL處。
3. CameraHardwareInterface定義了Camera的硬件抽象特征，由此進入到HAL。

四、HAL：CameraHardwareInterface

CameraHardwareInterface的作用在于鏈接Camera Server和V4L2，通過實現(xiàn)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(shù)據(jù)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(shù)據(jù)通道通過回調(diào)函數(shù)來獲得預覽、視頻錄制、自動對焦等數(shù)據(jù)。當需要支持新的硬件時就需要繼承于CameraHardwareInterface ，來實現(xiàn)對應的功能。CameraHardwareInterface提供的public方法如下：

CameraHardwareInterface

在前一節(jié)中，initialize()函數(shù)調(diào)用了mHardware->initialize和mHardware->setCallbacks，下面來看下CameraHardwareInterface.h對其的實現(xiàn)。

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(diào)，數(shù)據(jù)回調(diào)，時間戳回調(diào)，以及內(nèi)存相關操作的回調(diào)。

以上通過簡略分析應用層調(diào)用Camera.open()之后在Framework、ART、Library以及HAL層的響應，來說明Android中Camera系統(tǒng)的整體架構，希望對讀者能有一定的幫助。