一、在android中开发人员可以做那些工作？ 应用程序开发：利用android提供的强大的sdk，开发出各种各样新颖的应用。 系统开发：在android中Google实现了与硬件无关的所有代码，但是与硬件密切相关的硬件抽象层却没有也无法提供，对于移动设备不同的设备提供商底层硬件是千变万化的，不可能提供统一的硬件驱动以及接口实现，只能提供标准的接口，因此硬件提供商需要自个儿开发设备驱动，并去实现android框架提供的接口。二、android框架中Camera系统源码分析 在每个android手机中都有一个Camera应用程序用来实现拍照功能，不同硬件提供商可能会对这个应用程序进行改变来适合自己的UI风格，这里仅仅分析android原生Camera应用以及框架（Android 4.0） 原生Camera应用代码在Camera.java(android4.0\packages\apps\camera\src\com\android\camera)，这个应该算是Camera系统最上层，应用层的实现。 下面是Camera类部分代码
public class Camera extends ActivityBase implements FocusManager.Listener,View.OnTouchListener, ShutterButton.OnShutterButtonListener,SurfaceHolder.Callback, ModePicker.OnModeChangeListener,FaceDetectionListener, CameraPreference.OnPreferenceChangedListener,LocationManager.Listener, ShutterButton.OnShutterButtonLongPressListener 从上面可以看出，Camera在继承了很多监听接口，用来监听各种事件（对焦事件、用户触摸事件等）。这个应用时继承ActivityBase，可以重载OnCreate、OnResume等接口，在这些接口中完成相关初始化的工作，基本就是初始化各种监听对象，，以及获取相机参数等相关。 比较关键的在doOnResume这个函数中： @Overrideprotected void doOnResume() {if (mOpenCameraFail || mCameraDisabled) return;mPausing = false;mJpegPictureCallbackTime = 0;mZoomValue = 0;// Start the preview if it is not started.if (mCameraState == PREVIEW_STOPPED) {try {mCameraDevice = Util.openCamera(this, mCameraId);initializeCapabilities();resetExposureCompensation();startPreview();if (mFirstTimeInitialized) startFaceDetection();} catch (CameraHardwareException e) {Util.showErrorAndFinish(this, R.string.cannot_connect_camera);return;} catch (CameraDisabledException e) {Util.showErrorAndFinish(this, R.string.camera_disabled);return;}}if (mSurfaceHolder != null) {// If first time initialization is not finished, put it in the// message queue.if (!mFirstTimeInitialized) {mHandler.sendEmptyMessage(FIRST_TIME_INIT);} else {initializeSecondTime();}}keepScreenOnAwhile();if (mCameraState == IDLE) {mOnResumeTime = SystemClock.uptimeMillis();mHandler.sendEmptyMessageDelayed(CHECK_DISPLAY_ROTATION, 100);}}在这个函数中看到通过这个函数获得Camera底层对象mCameraDevice = Util.openCamera(this, mCameraId)，这里使用Util这个类，这个类的实现在Util.java (android4.0\packages\apps\camera\src\com\android\camera)中，找到OpenCamera这个函数实现：public static android.hardware.Camera openCamera(Activity activity, int cameraId)throws CameraHardwareException, CameraDisabledException {// Check if device policy has disabled the camera.DevicePolicyManager dpm = (DevicePolicyManager) activity.getSystemService(Context.DEVICE_POLICY_SERVICE);if (dpm.getCameraDisabled(null)) {throw new CameraDisabledException();}try {return CameraHolder.instance().open(cameraId);} catch (CameraHardwareException e) {// In eng build, we throw the exception so that test tool// can detect it and report itif ("eng".equals(Build.TYPE)) {throw new RuntimeException("openCamera failed", e);} else {throw e;}}}从这个函数可以看出，android系统中对下层Camera管理，是通过一个单例模式CameraHolder来管理的，定位到这个类的实现CameraHolder.java (android4.0\packages\apps\camera\src\com\android\camera)通过调用open函数获取一个Camera硬件设备对象，因为Camera设备是独享设备，不能同时被两个进程占用，而整个android系统是一个多进程环境，因此需要加入一些进程间互斥同步的方法。定位到这个类的open函数： public synchronized android.hardware.Camera open(int cameraId)throws CameraHardwareException {Assert(mUsers == 0);if (mCameraDevice != null && mCameraId != cameraId) {mCameraDevice.release();mCameraDevice = null;mCameraId = -1;}if (mCameraDevice == null) {try {Log.v(TAG, "open camera " + cameraId);mCameraDevice = android.hardware.Camera.open(cameraId);mCameraId = cameraId;} catch (RuntimeException e) {Log.e(TAG, "fail to connect Camera", e);throw new CameraHardwareException(e);}mParameters = mCameraDevice.getParameters();} else {try {mCameraDevice.reconnect();} catch (IOException e) {Log.e(TAG, "reconnect failed.");throw new CameraHardwareException(e);}mCameraDevice.setParameters(mParameters);}++mUsers;mHandler.removeMessages(RELEASE_CAMERA);mKeepBeforeTime = 0;return mCameraDevice;}通过android.hardware.Camera.open(cameraId)调用进入下一层封装，JNI层，这一层是java代码的最下层，对下层CameraC++代码进行JNI封装，封装实现类在Camera.java (android4.0\frameworks\base\core\java\android\hardware)下面是这个类的部分实现，里面定义了不少回调函数：public class Camera {private static final String TAG = "Camera";// These match the enums in frameworks/base/include/camera/Camera.hprivate static final int CAMERA_MSG_ERROR= 0x001;private static final int CAMERA_MSG_SHUTTER= 0x002;private static final int CAMERA_MSG_FOCUS= 0x004;private static final int CAMERA_MSG_ZOOM= 0x008;private static final int CAMERA_MSG_PREVIEW_FRAME = 0x010;private static final int CAMERA_MSG_VIDEO_FRAME= 0x020;private static final int CAMERA_MSG_POSTVIEW_FRAME = 0x040;private static final int CAMERA_MSG_RAW_IMAGE= 0x080;private static final int CAMERA_MSG_COMPRESSED_IMAGE = 0x100;private static final int CAMERA_MSG_RAW_IMAGE_NOTIFY = 0x200;private static final int CAMERA_MSG_PREVIEW_METADATA = 0x400;private static final int CAMERA_MSG_ALL_MSGS= 0x4FF;private int mNativeContext; // accessed by native methodsprivate EventHandler mEventHandler;private ShutterCallback mShutterCallback;private PictureCallback mRawImageCallback;private PictureCallback mJpegCallback;private PreviewCallback mPreviewCallback;private PictureCallback mPostviewCallback;private AutoFocusCallback mAutoFocusCallback;private OnZoomChangeListener mZoomListener;private FaceDetectionListener mFaceListener;private ErrorCallback mErrorCallback;定位到Open函数： public static Camera open(int cameraId) { return new Camera(cameraId); }Open函数是一个静态方法,构造一个Camera对象： Camera(int cameraId) {mShutterCallback = null;mRawImageCallback = null;mJpegCallback = null;mPreviewCallback = null;mPostviewCallback = null;mZoomListener = null;Looper looper;if ((looper = Looper.myLooper()) != null) {mEventHandler = new EventHandler(this, looper);} else if ((looper = Looper.getMainLooper()) != null) {mEventHandler = new EventHandler(this, looper);} else {mEventHandler = null;}native_setup(new WeakReference<Camera>(this), cameraId);}在构造函数中调用native_setup方法，此方法对应于C++代码的android_hardware_Camera_native_setup方法，实现在android_hardware_Camera.cpp (android4.0\frameworks\base\core\jni)，具体代码如下：static void android_hardware_Camera_native_setup(JNIEnv *env, jobject thiz,jobject weak_this, jint cameraId){sp<Camera> camera = Camera::connect(cameraId);if (camera == NULL) {jniThrowRuntimeException(env, "Fail to connect to camera service");return;}// make sure camera hardware is aliveif (camera->getStatus() != NO_ERROR) {jniThrowRuntimeException(env, "Camera initialization failed");return;}jclass clazz = env->GetObjectClass(thiz);if (clazz == NULL) {jniThrowRuntimeException(env, "Can’t find android/hardware/Camera");return;}// 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(thiz);camera->setListener(context);// save context in opaque fieldenv->SetIntField(thiz, fields.context, (int)context.get());}在android_hardware_Camera_native_setup方法中调用了Camera对象的connect方法，这个Camera类的声明在Camera.h (android4.0\frameworks\base\include\camera）定位到connect方法：sp<Camera> Camera::connect(int cameraId){LOGV("connect");sp<Camera> c = new Camera();const sp<ICameraService>& cs = getCameraService();if (cs != 0) {c->mCamera = cs->connect(c, cameraId);}if (c->mCamera != 0) {c->mCamera->asBinder()->linkToDeath(c);c->mStatus = NO_ERROR;} else {c.clear();}return c;}这里以下的代码就比较关键了，涉及到Camera框架的实现机制，Camera系统使用的是Server-Client机制，Service和Client位于不同的进程中，进程间使用Binder机制进行通信，Service端实际实现相机相关的操作，Client端通过Binder接口调用Service对应的操作。继续分析代码，上面函数调用getCameraService方法，获得CameraService的引用，ICameraService有两个子类，BnCameraService和BpCameraService，这两个子类同时也继承了IBinder接口，这两个子类分别实现了Binder通信的两端，Bnxxx实现ICameraService的具体功能，Bpxxx利用Binder的通信功能封装ICameraService方法，具体如下：class ICameraService : public IInterface{public:enum {GET_NUMBER_OF_CAMERAS = IBinder::FIRST_CALL_TRANSACTION,GET_CAMERA_INFO,CONNECT};public:DECLARE_META_INTERFACE(CameraService);virtual int32_tgetNumberOfCameras() = 0;virtual status_tgetCameraInfo(int cameraId,struct CameraInfo* cameraInfo) = 0;virtual sp<ICamera>connect(const sp<ICameraClient>& cameraClient,int cameraId) = 0;};// —————————————————————————-class BnCameraService: public BnInterface<ICameraService>{public:virtual status_t onTransact( uint32_t code,const Parcel& data,Parcel* reply,uint32_t flags = 0);};}; // naclass BpCameraService: public BpInterface<ICameraService>{public:BpCameraService(const sp<IBinder>& impl): BpInterface<ICameraService>(impl){}// get number of cameras availablevirtual int32_t getNumberOfCameras(){Parcel data, reply;data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());remote()->transact(BnCameraService::GET_NUMBER_OF_CAMERAS, data, &reply);return reply.readInt32();}// get information about a cameravirtual status_t getCameraInfo(int cameraId,struct CameraInfo* cameraInfo) {Parcel data, reply;data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());data.writeInt32(cameraId);remote()->transact(BnCameraService::GET_CAMERA_INFO, data, &reply);cameraInfo->facing = reply.readInt32();cameraInfo->orientation = reply.readInt32();return reply.readInt32();}// connect to camera servicevirtual sp<ICamera> connect(const sp<ICameraClient>& cameraClient, int cameraId){Parcel data, reply;data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());data.writeStrongBinder(cameraClient->asBinder());data.writeInt32(cameraId);remote()->transact(BnCameraService::CONNECT, data, &reply);return interface_cast<ICamera>(reply.readStrongBinder());}};IMPLEMENT_META_INTERFACE(CameraService, "android.hardware.ICameraService");// ———————————————————————-status_t BnCameraService::onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){switch(code) {case GET_NUMBER_OF_CAMERAS: {CHECK_INTERFACE(ICameraService, data, reply);reply->writeInt32(getNumberOfCameras());return NO_ERROR;} break;case GET_CAMERA_INFO: {CHECK_INTERFACE(ICameraService, data, reply);CameraInfo cameraInfo;memset(&cameraInfo, 0, sizeof(cameraInfo));status_t result = getCameraInfo(data.readInt32(), &cameraInfo);reply->writeInt32(cameraInfo.facing);reply->writeInt32(cameraInfo.orientation);reply->writeInt32(result);return NO_ERROR;} break;case CONNECT: {CHECK_INTERFACE(ICameraService, data, reply);sp<ICameraClient> cameraClient = interface_cast<ICameraClient>(data.readStrongBinder());sp<ICamera> camera = connect(cameraClient, data.readInt32());reply->writeStrongBinder(camera->asBinder());return NO_ERROR;} break;default:return BBinder::onTransact(code, data, reply, flags);}}// —————————————————————————-}; // namespace android下面继续分析sp<Camera> Camera::connect(int cameraId)这个方法，，定位到getCameraService这个方法const sp<ICameraService>& Camera::getCameraService(){Mutex::Autolock _l(mLock);if (mCameraService.get() == 0) {sp<IServiceManager> sm = defaultServiceManager();sp<IBinder> binder;do {binder = sm->getService(String16("media.camera"));if (binder != 0)break;LOGW("CameraService not published, waiting…");usleep(500000); // 0.5 s} while(true);if (mDeathNotifier == NULL) {mDeathNotifier = new DeathNotifier();}binder->linkToDeath(mDeathNotifier);mCameraService = interface_cast<ICameraService>(binder);}LOGE_IF(mCameraService==0, "no CameraService!?");return mCameraService;}定位到mCameraService = interface_cast<ICameraService>(binder); mCameraService是一个ICamerService类型，更加具体具体一点来讲应该是BpCameraService，因为在这个类中实现了ICameraService的方法。友谊之花、爱情之树、以及遗憾之泪！
