API 21中将原来的camera API弃用转而推荐使用新增的camera2 API，这是一个大的动作，因为新API换了架构，让开发者用起来更难了。
先来看看camera2包架构示意图：

这里引用了管道的概念将安卓设备和摄像头之间联通起来，系统向摄像头发送Capture请求，而摄像头会返回CameraMetadata。这一切建立在一个叫作CameraCaptureSession的会话中。

下面是camera2包中的主要类：

其中CameraManager是那个站在高处统管所有摄像投设备（CameraDevice）的管理者，而每个CameraDevice自己会负责建立CameraCaptureSession以及建立CaptureRequest。CameraCharacteristics是CameraDevice的属性描述类，非要做个对比的话，那么它与原来的CameraInfo有相似性。
类图中有着三个重要的callback，虽然这增加了阅读代码的难度，但是你必须要习惯，因为这是新包的风格。其中CameraCaptureSession.CaptureCallback将处理预览和拍照图片的工作，需要重点对待。

这些类是如何相互配合的？下面是简单的流程图。

我是用SurfaceView作为显示对象（当然还可以TextureView去显示，详见参考中的项目）
核心代码如下：

        mCameraManager = (CameraManager) this.getSystemService(Context.CAMERA_SERVICE);
        mSurfaceView = (SurfaceView)findViewById(R.id.surfaceview);
        mSurfaceHolder = mSurfaceView.getHolder();
        mSurfaceHolder.addCallback(new SurfaceHolder.Callback() {
            @Override
            public void surfaceCreated(SurfaceHolder holder) {
                initCameraAndPreview();
            }
        });

    private void initCameraAndPreview() {
        Log.d("linc","init camera and preview");
        HandlerThread handlerThread = new HandlerThread("Camera2");
        handlerThread.start();
        mHandler = new Handler(handlerThread.getLooper());
        try {
            mCameraId = ""+CameraCharacteristics.LENS_FACING_FRONT;
            mImageReader = ImageReader.newInstance(mSurfaceView.getWidth(), mSurfaceView.getHeight(),
                    ImageFormat.JPEG,/*maxImages*/7);
            mImageReader.setOnImageAvailableListener(mOnImageAvailableListener, mHandler);

            mCameraManager.openCamera(mCameraId, DeviceStateCallback, mHandler);
        } catch (CameraAccessException e) {
            Log.e("linc", "open camera failed." + e.getMessage());
        }
    }

private CameraDevice.StateCallback DeviceStateCallback = new CameraDevice.StateCallback() {

        @Override
        public void onOpened(CameraDevice camera) {
            Log.d("linc","DeviceStateCallback:camera was opend.");
            mCameraOpenCloseLock.release();
            mCameraDevice = camera;
            try {
                createCameraCaptureSession();
            } catch (CameraAccessException e) {
                e.printStackTrace();
            }
        }
    };

    private void createCameraCaptureSession() throws CameraAccessException {
        Log.d("linc","createCameraCaptureSession");

        mPreviewBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        mPreviewBuilder.addTarget(mSurfaceHolder.getSurface());
        mState = STATE_PREVIEW;
        mCameraDevice.createCaptureSession(
                Arrays.asList(mSurfaceHolder.getSurface(), mImageReader.getSurface()),
                mSessionPreviewStateCallback, mHandler);
    }

private CameraCaptureSession.StateCallback mSessionPreviewStateCallback = new
            CameraCaptureSession.StateCallback() {

        @Override
        public void onConfigured(CameraCaptureSession session) {
            Log.d("linc","mSessionPreviewStateCallback onConfigured");
            mSession = session;
            try {
                mPreviewBuilder.set(CaptureRequest.CONTROL_AF_MODE,
                        CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
                mPreviewBuilder.set(CaptureRequest.CONTROL_AE_MODE,
                        CaptureRequest.CONTROL_AE_MODE_ON_AUTO_FLASH);
                session.setRepeatingRequest(mPreviewBuilder.build(), mSessionCaptureCallback, mHandler);
            } catch (CameraAccessException e) {
                e.printStackTrace();
                Log.e("linc","set preview builder failed."+e.getMessage());
            }
        }
    };

private CameraCaptureSession.CaptureCallback mSessionCaptureCallback =
            new CameraCaptureSession.CaptureCallback() {

        @Override
        public void onCaptureCompleted(CameraCaptureSession session, CaptureRequest request,
                                       TotalCaptureResult result) {
//            Log.d("linc","mSessionCaptureCallback, onCaptureCompleted");
            mSession = session;
            checkState(result);
        }

        @Override
        public void onCaptureProgressed(CameraCaptureSession session, CaptureRequest request,
                                        CaptureResult partialResult) {
            Log.d("linc","mSessionCaptureCallback, onCaptureProgressed");
            mSession = session;
            checkState(partialResult);
        }

        private void checkState(CaptureResult result) {
            switch (mState) {
                case STATE_PREVIEW:
                    // NOTHING
                    break;
                case STATE_WAITING_CAPTURE:
                    int afState = result.get(CaptureResult.CONTROL_AF_STATE);

                    if (CaptureResult.CONTROL_AF_STATE_FOCUSED_LOCKED == afState ||
                            CaptureResult.CONTROL_AF_STATE_NOT_FOCUSED_LOCKED == afState
                            || CaptureResult.CONTROL_AF_STATE_PASSIVE_FOCUSED == afState
                            || CaptureResult.CONTROL_AF_STATE_PASSIVE_UNFOCUSED == afState) {
                        //do something like save picture
                    }
                    break;
            }
        }

    };

按下capture按钮：

    public void onCapture(View view) {
        try {
            Log.i("linc", "take picture");
            mState = STATE_WAITING_CAPTURE;
            mSession.setRepeatingRequest(mPreviewBuilder.build(), mSessionCaptureCallback, mHandler);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }

测试用genemotion模拟器，直接调用笔记本的摄像头。
配置图如下：

demo界面如下图：

源码：
请参考github中的两个demo项目：
https://github.com/pinguo-yuyidong/Camera2
https://github.com/googlesamples/android-Camera2Basic