我花了一些时间调试，但无法理解为什么会出现null。当我立即运行应用程序时，它崩溃了，并指向这一行：

mSupportedPreviewSizes = mCamera.getParameters().getSupportedPreviewSizes();

并说对象引用为null，我不明白为什么mCamera返回null。
我的清单文件具有适当的权限：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.CAMERA" />
<uses-feature android:name="android.hardware.camera" />
<uses-feature android:name="android.hardware.camera.autofocus" />
<uses-feature android:name="android.hardware.camera.front" android:required="false" />

示例：

import java.io.File;
        import java.io.FileNotFoundException;
        import java.io.FileOutputStream;
        import java.io.IOException;
        import java.text.SimpleDateFormat;
        import java.util.Date;

        import android.app.Activity;
        import android.content.Context;
        import android.content.pm.PackageManager;
        import android.hardware.Camera;
        import android.hardware.Camera.CameraInfo;
        import android.hardware.Camera.PictureCallback;
        import android.os.Bundle;
        import android.view.View;
        import android.view.View.OnClickListener;
        import android.view.WindowManager;
        import android.widget.Button;
        import android.widget.LinearLayout;
        import android.widget.Toast;

public class AndroidCameraExample extends Activity {
    private Camera mCamera;
    private CameraPreview mPreview;
    private PictureCallback mPicture;
    private Button capture, switchCamera;
    private Context myContext;
    private LinearLayout cameraPreview;
    private boolean cameraFront = false;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
        myContext = this;
        initialize();
    }

    private int findFrontFacingCamera() {
        int cameraId = -1;
        // Search for the front facing camera
        int numberOfCameras = Camera.getNumberOfCameras();
        for (int i = 0; i < numberOfCameras; i++) {
            CameraInfo info = new CameraInfo();
            Camera.getCameraInfo(i, info);
            if (info.facing == CameraInfo.CAMERA_FACING_FRONT) {
                cameraId = i;
                cameraFront = true;
                break;
            }
        }
        return cameraId;
    }

    private int findBackFacingCamera() {
        int cameraId = -1;
        //Search for the back facing camera
        //get the number of cameras
        int numberOfCameras = Camera.getNumberOfCameras();
        //for every camera check
        for (int i = 0; i < numberOfCameras; i++) {
            CameraInfo info = new CameraInfo();
            Camera.getCameraInfo(i, info);
            if (info.facing == CameraInfo.CAMERA_FACING_BACK) {
                cameraId = i;
                cameraFront = false;
                break;
            }
        }
        return cameraId;
    }

    public void onResume() {
        super.onResume();
        if (!hasCamera(myContext)) {
            Toast toast = Toast.makeText(myContext, "Sorry, your phone does not have a camera!", Toast.LENGTH_LONG);
            toast.show();
            finish();
        }
        if (mCamera == null) {
            //if the front facing camera does not exist
            if (findFrontFacingCamera() < 0) {
                Toast.makeText(this, "No front facing camera found.", Toast.LENGTH_LONG).show();
                switchCamera.setVisibility(View.GONE);
            }
            mCamera = Camera.open(findBackFacingCamera());
            mPicture = getPictureCallback();
            mPreview.refreshCamera(mCamera);
        }
    }

    public void initialize() {
        cameraPreview = (LinearLayout) findViewById(R.id.camera_preview);
        mPreview = new CameraPreview(myContext, mCamera);
        cameraPreview.addView(mPreview);

        capture = (Button) findViewById(R.id.button_capture);
        capture.setOnClickListener(captrureListener);

        switchCamera = (Button) findViewById(R.id.button_ChangeCamera);
        switchCamera.setOnClickListener(switchCameraListener);
    }

    OnClickListener switchCameraListener = new OnClickListener() {
        @Override
        public void onClick(View v) {
            //get the number of cameras
            int camerasNumber = Camera.getNumberOfCameras();
            if (camerasNumber > 1) {
                //release the old camera instance
                //switch camera, from the front and the back and vice versa

                releaseCamera();
                chooseCamera();
            } else {
                Toast toast = Toast.makeText(myContext, "Sorry, your phone has only one camera!", Toast.LENGTH_LONG);
                toast.show();
            }
        }
    };

    public void chooseCamera() {
        //if the camera preview is the front
        if (cameraFront) {
            int cameraId = findBackFacingCamera();
            if (cameraId >= 0) {
                //open the backFacingCamera
                //set a picture callback
                //refresh the preview

                mCamera = Camera.open(cameraId);
                mPicture = getPictureCallback();
                mPreview.refreshCamera(mCamera);
            }
        } else {
            int cameraId = findFrontFacingCamera();
            if (cameraId >= 0) {
                //open the backFacingCamera
                //set a picture callback
                //refresh the preview

                mCamera = Camera.open(cameraId);
                mPicture = getPictureCallback();
                mPreview.refreshCamera(mCamera);
            }
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        //when on Pause, release camera in order to be used from other applications
        releaseCamera();
    }

    private boolean hasCamera(Context context) {
        //check if the device has camera
        if (context.getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA)) {
            return true;
        } else {
            return false;
        }
    }

    private PictureCallback getPictureCallback() {
        PictureCallback picture = new PictureCallback() {

            @Override
            public void onPictureTaken(byte[] data, Camera camera) {
                //make a new picture file
                File pictureFile = getOutputMediaFile();

                if (pictureFile == null) {
                    return;
                }
                try {
                    //write the file
                    FileOutputStream fos = new FileOutputStream(pictureFile);
                    fos.write(data);
                    fos.close();
                    Toast toast = Toast.makeText(myContext, "Picture saved: " + pictureFile.getName(), Toast.LENGTH_LONG);
                    toast.show();

                } catch (FileNotFoundException e) {
                } catch (IOException e) {
                }

                //refresh camera to continue preview
                mPreview.refreshCamera(mCamera);
            }
        };
        return picture;
    }

    OnClickListener captrureListener = new OnClickListener() {
        @Override
        public void onClick(View v) {
            mCamera.takePicture(null, null, mPicture);
        }
    };

    //make picture and save to a folder
    private static File getOutputMediaFile() {
        //make a new file directory inside the "sdcard" folder
        File mediaStorageDir = new File("/sdcard/", "JCG Camera");

        //if this "JCGCamera folder does not exist
        if (!mediaStorageDir.exists()) {
            //if you cannot make this folder return
            if (!mediaStorageDir.mkdirs()) {
                return null;
            }
        }

        //take the current timeStamp
        String timeStamp = new SimpleDateFormat("yyyyMMdd_HHmmss").format(new Date());
        File mediaFile;
        //and make a media file:
        mediaFile = new File(mediaStorageDir.getPath() + File.separator + "IMG_" + timeStamp + ".jpg");

        return mediaFile;
    }

    private void releaseCamera() {
        // stop and release camera
        if (mCamera != null) {
            mCamera.release();
            mCamera = null;
        }
    }
}

相机预览：

import java.io.IOException;
import java.util.List;

import android.content.Context;
import android.hardware.Camera;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {

    private static final String TAG = "CameraPreview";

    private Context mContext;
    private SurfaceHolder mHolder;
    private Camera mCamera;
    private List<Camera.Size> mSupportedPreviewSizes;
    private Camera.Size mPreviewSize;

    public CameraPreview(Context context, Camera camera) {
        super(context);
        mContext = context;
        mCamera = camera;

            // supported preview sizes
            mSupportedPreviewSizes = mCamera.getParameters().getSupportedPreviewSizes();
            for (Camera.Size str : mSupportedPreviewSizes)
                Log.e(TAG, str.width + "/" + str.height);

        // Install a SurfaceHolder.Callback so we get notified when the
        // underlying surface is created and destroyed.
        mHolder = getHolder();
        mHolder.addCallback(this);
        // deprecated setting, but required on Android versions prior to 3.0
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }

    public void surfaceCreated(SurfaceHolder holder) {
        // empty. surfaceChanged will take care of stuff
    }

    public void surfaceDestroyed(SurfaceHolder holder) {
        // empty. Take care of releasing the Camera preview in your activity.
    }

    public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
        Log.e(TAG, "surfaceChanged => w=" + w + ", h=" + h);
        // If your preview can change or rotate, take care of those events here.
        // Make sure to stop the preview before resizing or reformatting it.
        if (mHolder.getSurface() == null){
            // preview surface does not exist
            return;
        }

        // stop preview before making changes
        try {
            mCamera.stopPreview();
        } catch (Exception e){
            // ignore: tried to stop a non-existent preview
        }

        // set preview size and make any resize, rotate or reformatting changes here
        // start preview with new settings
        try {
            Camera.Parameters parameters = mCamera.getParameters();
            parameters.setPreviewSize(mPreviewSize.width, mPreviewSize.height);
            mCamera.setParameters(parameters);
            mCamera.setDisplayOrientation(90);
            mCamera.setPreviewDisplay(mHolder);
            mCamera.startPreview();

        } catch (Exception e){
            Log.d(TAG, "Error starting camera preview: " + e.getMessage());
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        final int width = resolveSize(getSuggestedMinimumWidth(), widthMeasureSpec);
        final int height = resolveSize(getSuggestedMinimumHeight(), heightMeasureSpec);

        if (mSupportedPreviewSizes != null) {
            mPreviewSize = getOptimalPreviewSize(mSupportedPreviewSizes, width, height);
        }

        float ratio;
        if(mPreviewSize.height >= mPreviewSize.width)
            ratio = (float) mPreviewSize.height / (float) mPreviewSize.width;
        else
            ratio = (float) mPreviewSize.width / (float) mPreviewSize.height;

        // One of these methods should be used, second method squishes preview slightly
        setMeasuredDimension(width, (int) (width * ratio));
//        setMeasuredDimension((int) (width * ratio), height);
    }

    private Camera.Size getOptimalPreviewSize(List<Camera.Size> sizes, int w, int h) {
        final double ASPECT_TOLERANCE = 0.1;
        double targetRatio = (double) h / w;

        if (sizes == null)
            return null;

        Camera.Size optimalSize = null;
        double minDiff = Double.MAX_VALUE;

        int targetHeight = h;

        for (Camera.Size size : sizes) {
            double ratio = (double) size.height / size.width;
            if (Math.abs(ratio - targetRatio) > ASPECT_TOLERANCE)
                continue;

            if (Math.abs(size.height - targetHeight) < minDiff) {
                optimalSize = size;
                minDiff = Math.abs(size.height - targetHeight);
            }
        }

        if (optimalSize == null) {
            minDiff = Double.MAX_VALUE;
            for (Camera.Size size : sizes) {
                if (Math.abs(size.height - targetHeight) < minDiff) {
                    optimalSize = size;
                    minDiff = Math.abs(size.height - targetHeight);
                }
            }
        }

        return optimalSize;
    }

    public void refreshCamera(Camera camera) {
        if (mHolder.getSurface() == null) {
            // preview surface does not exist
            return;
        }
        // stop preview before making changes
        try {
            mCamera.stopPreview();
        } catch (Exception e) {
            // ignore: tried to stop a non-existent preview
        }
        // set preview size and make any resize, rotate or
        // reformatting changes here
        // start preview with new settings
        setCamera(camera);
        try {
            Camera.Parameters parameters = mCamera.getParameters();
            parameters.setPreviewSize(mPreviewSize.width, mPreviewSize.height);
            mCamera.setParameters(parameters);
            mCamera.setDisplayOrientation(90);
            mCamera.setPreviewDisplay(mHolder);
            mCamera.startPreview();
        } catch (Exception e) {
            Log.d(VIEW_LOG_TAG, "Error starting camera preview: " + e.getMessage());
        }
    }

    public void setCamera(Camera camera) {
        //method to set a camera instance
        mCamera = camera;
    }
}

As I am using a tutorial, if I use this CameraPreview.java instead (keeping the same Example class) it works, but I needed to make some modifications to the tutorial for the aspect ratio reasons:

import java.io.IOException;

import android.content.Context;
import android.hardware.Camera;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {
    private SurfaceHolder mHolder;
    private Camera mCamera;

    public CameraPreview(Context context, Camera camera) {
        super(context);
        mCamera = camera;
        mHolder = getHolder();
        mHolder.addCallback(this);
        // deprecated setting, but required on Android versions prior to 3.0
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }

    public void surfaceCreated(SurfaceHolder holder) {
        try {
            // create the surface and start camera preview
            if (mCamera == null) {
                mCamera.setPreviewDisplay(holder);
                mCamera.startPreview();
            }
        } catch (IOException e) {
            Log.d(VIEW_LOG_TAG, "Error setting camera preview: " + e.getMessage());
        }
    }

    public void refreshCamera(Camera camera) {
        if (mHolder.getSurface() == null) {
            // preview surface does not exist
            return;
        }
        // stop preview before making changes
        try {
            mCamera.stopPreview();
        } catch (Exception e) {
            // ignore: tried to stop a non-existent preview
        }
        // set preview size and make any resize, rotate or
        // reformatting changes here
        // start preview with new settings
        setCamera(camera);
        try {
            mCamera.setPreviewDisplay(mHolder);
            mCamera.startPreview();
        } catch (Exception e) {
            Log.d(VIEW_LOG_TAG, "Error starting camera preview: " + e.getMessage());
        }
    }

    public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
        // If your preview can change or rotate, take care of those events here.
        // Make sure to stop the preview before resizing or reformatting it.
        refreshCamera(mCamera);
    }

    public void setCamera(Camera camera) {
        //method to set a camera instance
        mCamera = camera;
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        // TODO Auto-generated method stub
        // mCamera.release();

    }
}

更新：此外，我试图把chooseCamera();的权利之前的mPreview = new CameraPreview(myContext, mCamera);，但它没有工作。