Camera的架构与Android系统的整体架构保持一致,如下图所示,本文主要从以下四个方面对其进行说明。

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


一、Framework:Camera.java

Camera是应用层软件直接使用的类,涵盖了启动、预览、拍摄及关闭等操作摄像头的全部接口。Camera.java在Android源码中的路径为:framework/base/core/java/android/hardware。为了说明整个Camera系统的架构,这里暂不横向分析Camera.java的功能,下面从open()方法着手:

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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()方法需要注意以下几点:

  • getNumberOfCameras为native方法,实现在android_hardware_Camera.cpp中;
  • CameraInfo是Camera定义的静态内部类,包含facing、orientation、canDisableShutterSound;
  • getCameraInfo内部调用native方法_getCameraInfo获取摄像头信息;
  • open()默认启动的是后置摄像头(CAMERA_FACING_BACK)。
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/** 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构造器的核心实现在cameraInitNormal中,cameraInitNormal调用cameraInitVersion,并传入参数cameraId和CAMERA_HAL_API_VERSION_NORMAL_CONNECT,后者代表HAL的版本。

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private int cameraInitVersion(int cameraId, int halVersion) {
……
String packageName = ActivityThread.currentPackageName();
return native_setup(new WeakReference<Camera>(this), cameraId, halVersion, packageName);
}

cameraInitNormal调用本地方法native_setup(),由此进入到android_hardware_Camera.cpp中,native_setup()的签名如下:

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private native final int native_setup(Object camera_this, 
int cameraId, int halVersion, String packageName);

二、Android Runtime:android_hardware_Camera.cpp

native_setup()被动态注册到JNI,通过JNI调用android_hardware_Camera_native_setup()方法。

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static JNINativeMethod camMethods[] = {
……
{ "native_setup", "(Ljava/lang/Object;ILjava/lang/String;)V",
(void*)android_hardware_Camera_native_setup }
……
};

JNI的重点是android_hardware_Camera_native_setup()方法的实现:

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// 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()方法通过调用Camera::connect()方法请求连接CameraService服务。入参中:

  • clientName是通过将clientPackageName从jstring转换为String16格式得到;
  • Camera::USE_CALLING_UID是定义在Camera.h中的枚举类型,其值为ICameraService::USE_CALLING_UID(同样为枚举类型,值为-1)。

Camera::connect()位于Camera.cpp中,由此进入到Library层。

三、Library:Camera Client和Camera Service

如上述架构图中所示,ICameraService.h、ICameraClient.h和ICamera.h三个类定义了Camera的接口和架构,ICameraService.cpp和Camera.cpp两个文件用于Camera架构的实现,Camera的具体功能在下层调用硬件相关的接口来实现。Camera.h是Camera系统对上层的接口。

具体的,Camera类继承模板类CameraBase,Camera::connect()调用了CameraBase.cpp中的connect()方法。

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sp<Camera> Camera::connect(int cameraId, const String16& clientPackageName,
int clientUid) {
return CameraBaseT::connect(cameraId, clientPackageName, clientUid);
}

CameraBase实际上又继承了IBinder的DeathRecipient内部类,DeathRecipient虚拟继承自RefBase。RefBase是Android中的引用计数基础类,其中定义了incStrong、decStrong、incWeak和decWeak等涉及sp/wp的指针操作函数,当然这扯远了。

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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);
……
}

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class DeathRecipient : public virtual RefBase
{
public:
virtual void binderDied(const wp<IBinder>& who) = 0;
};

回到Camera::connect()的实现上,其中,new TCam(cameraId)生成BnCameraClient对象,BnCameraClient定义在ICameraClient.h文件中,继承自模板类BnInterface。getCameraService()方法返回CameraService的服务代理BpCameraService,BpCameraService同样继承自模板类BnInterface。然后通过Binder通信发送CONNECT命令,当BnCameraService收到CONNECT命令后调用CameraService的connect()成员函数来做相应的处理。

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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;
}
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class BnCameraClient: public BnInterface<ICameraClient>
{
public:
virtual status_t onTransact( uint32_t code,
const Parcel& data,
Parcel* reply,
uint32_t flags = 0);
};
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class BpCameraService: public BpInterface<ICameraService>
{
public:
BpCameraService(const sp<IBinder>& impl)
: BpInterface<ICameraService>(impl)
{
}
……
}

注:connect()函数在BpCameraService和BnCameraService的父类ICameraService中声明为纯虚函数,在BpCameraService和CameraService中分别给出了实现,BpCameraService作为代理类,提供接口给客户端,真正实现在BnCameraService的子类CameraService中。

在BpCameraService中,connect()函数实现如下:

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// 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()函数向IPCThreadState实例发送消息,通知其有消息要发送给binder driver
if (readExceptionCode(reply)) return -EPROTO;
status_t status = reply.readInt32();
if (reply.readInt32() != 0) {
device = interface_cast<ICamera>(reply.readStrongBinder()); // client端读出server返回的bind
}
return status;
}

首先将传递过来的Camera对象cameraClient转换成IBinder类型,将调用的参数写到Parcel(可理解为Binder通信的管道)中,通过BpBinder的transact()函数发送消息,然后由BnCameraService去响应该连接,最后就是等待服务端返回,如果成功则生成一个BpCamera实例。

真正的服务端响应实现在BnCameraService的onTransact()函数中,其负责解包收到的Parcel并执行client端的请求的方法。

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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对象生成Camera客户代理BpCameraClient实例
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); // 将生成的BpCameraClient对象作为参数传递到CameraService的connect()函数中
reply->writeNoException();
reply->writeInt32(status); // 将BpCamera对象以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;
……
}
}

主要的处理包括:

  1. 通过data中Camera的Binder对象生成Camera客户代理BpCameraClient实例;
  2. 将生成的BpCameraClient对象作为参数传递到CameraService(/frameworks/av/services/camera /libcameraservice/CameraService.cpp)的connect()函数中,该函数会返回一个BpCamera实例;
  3. 将在上述实例对象以IBinder的形式打包到Parcel中返回。

最后,BpCamera实例是通过CameraService::connect()函数返回的。CameraService::connect()实现的核心是调用connectHelperLocked()函数根据HAL不同API的版本创建不同的client实例(早期版本中好像没有connectHelperLocked()这个函数,但功能基本相似)。

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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;
}

可见,在CAMERA_DEVICE_API_VERSION_2_0之前使用CameraClient进行实例化,之后则采用Camera2Client进行实例化。以CameraClient为例,其initialize()函数如下:

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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;
}

上述函数中,主要注意以下流程:

  1. 加粗的代码CameraHardwareInterface新建了了一个Camera硬件接口,当然,camera_device_name为摄像头设备名;
  2. mHardware->initialize(&module->common)调用底层硬件的初始化方法;
  3. mHardware->setCallbacks将CamerService处的回调函数注册到HAL处。

CameraHardwareInterface定义了Camera的硬件抽象特征,由此进入到HAL。

四、HAL:CameraHardwareInterface

CameraHardwareInterface的作用在于链接Camera Server和V4L2,通过实现CameraHardwareInterface可以屏蔽不同的driver对Camera Server的影响。CameraHardwareInterface同样虚拟继承自RefBase。

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class CameraHardwareInterface : public virtual RefBase {
public:
CameraHardwareInterface(const char *name)
{
mDevice = 0;
mName = name;
}
……
}

CameraHardwareInterface中包含了控制通道和数据通道,控制通道用于处理预览和视频获取的开始/停止、拍摄照片、自动对焦等功能,数据通道通过回调函数来获得预览、视频录制、自动对焦等数据。当需要支持新的硬件时就需要继承于CameraHardwareInterface ,来实现对应的功能。CameraHardwareInterface提供的public方法如下:

在前一节中,initialize()函数调用了mHardware->initialize和mHardware->setCallbacks,下面来看下CameraHardwareInterface.h对其的实现。

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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,通过cameraModule->open_legacy打开摄像头模组,initHalPreviewWindow()用于初始化Preview的相关流opspreview_stream_ops,初始化hal的预览窗口。

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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;
}

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/** 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分别消息回调,数据回调,时间戳回调,以及内存相关操作的回调。

以上通过简略分析应用层调用Camera.open()之后在Framework、ART、Library以及HAL层的响应,来说明Android中Camera系统的整体架构,希望对读者能有一定的帮助,后续将在理解Camera整体架构的基础,探索更加高效的Preview方式,敬请期待!