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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。
Framework：Camera.java
Android Runtime：android_hardware_Camera.cpp
Library：Camera Client和Camera Service
HAL：CameraHardwareInterface
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：
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）。
/** 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。
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：
private native final int native_setup(Object camera_this, int cameraId, int halVersion, String packageName);
native_setup()被动态注册到JNI，通过JNI调用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的重点是android_hardware_Camera_native_setup()方法的实现：
// 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。
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。
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。
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。
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：
// 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。
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：
通过data中Camera的Binder对象生成Camera客户代理BpCameraClient实例；
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；
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）。
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：
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：
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；
mHardware->initialize(&module->common)调用底层硬件的初始化方法；
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。
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。
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。
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。
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
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