java - NullPointerException 尝试使用 Camera 和 setFaceDetectionListener() 时

Question

我正在尝试制作一个检测人脸的应用程序。下面的代码没有错误，据我所知应该可以正常工作。但是当我运行它时，我得到以下信息:

FATAL EXCEPTION: main Process: org.bytefish.videofacedetection, PID: 12075 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.hardware.Camera.setFaceDetectionListener(android.hardware.Camera$FaceDetectionListener)' on a null object reference at org.bytefish.videofacedetection. CameraActivity.surfaceCreated(CameraActivity.java:100) at android.view.SurfaceView.updateWindow(SurfaceView.java:582)
at android.view.SurfaceView$3.onPreDraw(SurfaceView.java:177)
at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:944) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2055) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858) at android.view.Choreographer.doCallbacks(Choreographer.java:670)
at android.view.Choreographer.doFrame(Choreographer.java:606)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844) at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

代码如下，全部大写注释显示第 100 行和错误

import android.app.Activity;
import android.content.Context;
import android.hardware.Camera;
import android.hardware.Camera.Face;
import android.hardware.Camera.FaceDetectionListener;
import android.hardware.SensorManager;
import android.os.Bundle;
import android.util.Log;
import android.view.OrientationEventListener;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.ViewGroup.LayoutParams;
import android.widget.Toast;

import java.util.List;


public class CameraActivity extends Activity
        implements SurfaceHolder.Callback {

    public static final String TAG = CameraActivity.class.getSimpleName();

    private Camera mCamera;

    // We need the phone orientation to correctly draw the overlay:
    private int mOrientation;
    private int mOrientationCompensation;
    private OrientationEventListener mOrientationEventListener;

    // Let's keep track of the display rotation and orientation also:
    private int mDisplayRotation;
    private int mDisplayOrientation;

    // Holds the Face Detection result:
    private Camera.Face[] mFaces;

    // The surface view for the camera data
    private SurfaceView mView;

    // Draw rectangles and other fancy stuff:
    private FaceOverlayView mFaceView;

    // Log all errors:
    private final CameraErrorCallback mErrorCallback = new CameraErrorCallback();


    /**
     * Sets the faces for the overlay view, so it can be updated
     * and the face overlays will be drawn again.
     */
    private FaceDetectionListener faceDetectionListener = new FaceDetectionListener() {
        @Override
        public void onFaceDetection(Face[] faces, Camera camera) {
            Log.d("onFaceDetection", "Number of Faces:" + faces.length);
            // Update the view now!
            mFaceView.setFaces(faces);
        }
    };

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mView = new SurfaceView(this);

        setContentView(mView);
        // Now create the OverlayView:
        mFaceView = new FaceOverlayView(this);
        addContentView(mFaceView, new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
        // Create and Start the OrientationListener:
        mOrientationEventListener = new SimpleOrientationEventListener(this);
        mOrientationEventListener.enable();
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        SurfaceHolder holder = mView.getHolder();
        holder.addCallback(this);
    }

    @Override
    protected void onPause() {
        mOrientationEventListener.disable();
        super.onPause();
    }

    @Override
    protected void onResume() {
        mOrientationEventListener.enable();
        super.onResume();
    }

    @Override
    public void surfaceCreated(SurfaceHolder surfaceHolder) {

        mCamera = Camera.open();

        mCamera.setFaceDetectionListener(faceDetectionListener);
        mCamera.startFaceDetection();
        try {
            mCamera.setPreviewDisplay(surfaceHolder);
        } catch (Exception e) {
            Log.e(TAG, "Could not preview the image.", e);
        }
    }

    @Override
    public void surfaceChanged(SurfaceHolder surfaceHolder, int format, int width, int height) {
        // We have no surface, return immediately:
        if (surfaceHolder.getSurface() == null) {
            return;
        }
        // Try to stop the current preview:
        try {
            mCamera.stopPreview();
        } catch (Exception e) {
            // Ignore...
        }

        configureCamera(width, height);
        setDisplayOrientation();
        setErrorCallback();

        // Everything is configured! Finally start the camera preview again:
        mCamera.startPreview();
    }

    private void setErrorCallback() {
        mCamera.setErrorCallback(mErrorCallback);
    }

    private void setDisplayOrientation() {
        // Now set the display orientation:
        mDisplayRotation = Util.getDisplayRotation(CameraActivity.this);
        mDisplayOrientation = Util.getDisplayOrientation(mDisplayRotation, 0);

        mCamera.setDisplayOrientation(mDisplayOrientation);

        if (mFaceView != null) {
            mFaceView.setDisplayOrientation(mDisplayOrientation);
        }
    }

    private void configureCamera(int width, int height) {
        Camera.Parameters parameters = mCamera.getParameters();
        // Set the PreviewSize and AutoFocus:
        setOptimalPreviewSize(parameters, width, height);
        setAutoFocus(parameters);
        // And set the parameters:
        mCamera.setParameters(parameters);
    }

    private void setOptimalPreviewSize(Camera.Parameters cameraParameters, int width, int height) {
        List<Camera.Size> previewSizes = cameraParameters.getSupportedPreviewSizes();
        float targetRatio = (float) width / height;
        Camera.Size previewSize = Util.getOptimalPreviewSize(this, previewSizes, targetRatio);
        cameraParameters.setPreviewSize(previewSize.width, previewSize.height);
    }

    private void setAutoFocus(Camera.Parameters cameraParameters) {
        cameraParameters.setFocusMode(Camera.Parameters.FOCUS_MODE_CONTINUOUS_VIDEO);
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder surfaceHolder) {
        mCamera.setPreviewCallback(null);
        mCamera.setFaceDetectionListener(null);
        mCamera.setErrorCallback(null);
        mCamera.release();
        mCamera = null;
    }

    /**
     * We need to react on OrientationEvents to rotate the screen and
     * update the views.
     */
    private class SimpleOrientationEventListener extends OrientationEventListener {

        public SimpleOrientationEventListener(Context context) {
            super(context, SensorManager.SENSOR_DELAY_NORMAL);
        }

        @Override
        public void onOrientationChanged(int orientation) {
            // We keep the last known orientation. So if the user first orient
            // the camera then point the camera to floor or sky, we still have
            // the correct orientation.
            if (orientation == ORIENTATION_UNKNOWN) return;
            mOrientation = Util.roundOrientation(orientation, mOrientation);
            // When the screen is unlocked, display rotation may change. Always
            // calculate the up-to-date orientationCompensation.
            int orientationCompensation = mOrientation
                    + Util.getDisplayRotation(CameraActivity.this);
            if (mOrientationCompensation != orientationCompensation) {
                mOrientationCompensation = orientationCompensation;
                mFaceView.setOrientation(mOrientationCompensation);
            }
        }
    }
}

当我尝试在 android studio 中打开它时，它打开时没有任何错误，但当我尝试运行它时就死机了。

我是 android 的新手，所以任何提示都将不胜感激。非常感谢您的帮助和时间。

Answer 1

好的，看起来你没有正确获取相机对象

此代码将正确初始化前置摄像头 第 1 部分

cameraId = findFrontFacingCamera();
if (cameraId < 0) {
    Toast.makeText(this, "No  camera found.",
        Toast.LENGTH_LONG).show();
} else {
    mcamera = Camera.open(cameraId);
}

第二部分

 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) {
        Log.d(DEBUG_TAG, "Camera found");
        cameraId = i;
        break;
      }
    }
    return cameraId;
  }

请尝试让我知道。