java.lang.NullPointerException錯誤

Question

我正在使用OpenCV library開發一個簡單的AR應用程序，對於本機部分我做得很好，並且可以在genymotion模擬器中成功運行。 但是，當我希望部署到我的device (SONY C2105 - Android 4.2.2 same as emulator version)它只是失敗了。

錯誤日志：

03-21 16:25:05.118: E/AndroidRuntime(26424): FATAL EXCEPTION: main
03-21 16:25:05.118: E/AndroidRuntime(26424): Process: com.example.detectimage, PID: 26424
03-21 16:25:05.118: E/AndroidRuntime(26424): java.lang.NullPointerException
03-21 16:25:05.118: E/AndroidRuntime(26424):    at com.example.detectimage.CameraPreview.surfaceCreated(CameraPreview.java:68)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.SurfaceView.updateWindow(SurfaceView.java:572)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.SurfaceView.access$000(SurfaceView.java:86)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.SurfaceView$3.onPreDraw(SurfaceView.java:175)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:847)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1871)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5670)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.Choreographer.doCallbacks(Choreographer.java:574)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.Choreographer.doFrame(Choreographer.java:544)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.os.Handler.handleCallback(Handler.java:733)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.os.Handler.dispatchMessage(Handler.java:95)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.os.Looper.loop(Looper.java:136)
03-21 16:25:05.118: E/AndroidRuntime(26424):    at android.app.ActivityThread.main(ActivityThread.java:5017)

CameraPreview.java：

public class CameraPreview extends SurfaceView implements
    SurfaceHolder.Callback {
private static final String TAG = "CameraPreview";

private Camera mCamera;
private SurfaceHolder mHolder;
private int mFrameWidth;
private int mFrameHeight;
private Handler mHandler;



public CameraPreview(Context context, Handler handler) {
    super(context);
    mHolder = getHolder();
    mHolder.addCallback(this);
    mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    mHandler = handler;
    Log.i(TAG, "Instantiated new " + this.getClass());
}


public void surfaceChanged(SurfaceHolder _holder, int format, int width,
        int height) {
    Log.i(TAG, "surfaceCreated");
    if (mCamera != null) {
        Camera.Parameters params = mCamera.getParameters();
        List<Camera.Size> sizes = params.getSupportedPreviewSizes();
        mFrameWidth = width;
        mFrameHeight = height;

        // selecting optimal camera preview size
        {
            double minDiff = Double.MAX_VALUE;
            for (Camera.Size size : sizes) {
                if (Math.abs(size.height - height) < minDiff) {
                    mFrameWidth = size.width;
                    mFrameHeight = size.height;
                    minDiff = Math.abs(size.height - height);
                }
            }
        }

        params.setPreviewSize(mFrameWidth, mFrameHeight);
        mCamera.setParameters(params);
        mCamera.startPreview();
    }
}

public void surfaceCreated(SurfaceHolder holder) {
    Log.i(TAG, "surfaceCreated");
    mCamera = Camera.open();
    try {
        mCamera.setPreviewDisplay(holder);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    mCamera.setOneShotPreviewCallback(mCallback);
}

private PreviewCallback mCallback = new PreviewCallback() {
    public void onPreviewFrame(byte[] data, Camera camera) {
        Message msg = new Message();
        msg.arg1 = detectImage(mFrameWidth, mFrameHeight, data); 
        mHandler.sendMessage(msg); 
    }
};

public void restartPreviewCallback(){
    this.requestLayout();
    this.invalidate();
    if (mCamera != null) {

        mCamera.setOneShotPreviewCallback(mCallback);
        mCamera.startPreview();
    }
}


public void surfaceDestroyed(SurfaceHolder holder) {
    Log.i(TAG, "surfaceDestroyed");
    if (mCamera != null) {
        synchronized (this) {
            mCamera.stopPreview();
            mCamera.setPreviewCallback(null);
            mCamera.release();
            mCamera = null;
        }
    }
}

public native int detectImage(int width, int height, byte[] data);

static {
    System.loadLibrary("opencv_java");
    System.loadLibrary("native_sample");
}
}

Answer 1

首先檢查清單文件中的以下權限

<uses-permission android:name="android.permission.CAMERA" />
 <uses-feature android:name="android.hardware.camera" />

然后檢查您的手機是否支持相機，

/** Check if this device has a camera */
private boolean checkCameraHardware(Context context) {
    if (context.getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA)){
        // this device has a camera
        return true;
    } else {
        // no camera on this device
        return false;
    }
}

如果為true，則在CameraPreview構造函數中獲取相機實例，然后在代碼中將vale分配給mCamera實例。

mCamera = getCameraInstance（）;

//需要一些小的延遲才能獲得實例

public static Camera getCameraInstance(){
    Camera c = null;
    try {
        c = Camera.open(); // attempt to get a Camera instance
    }
    catch (Exception e){
        // Camera is not available (in use or does not exist)
    }
    return c; // returns null if camera is unavailable
}



public void surfaceCreated(SurfaceHolder holder) {

Log.i(TAG, "surfaceCreated");
   if(mCamera == null)
           mCamera = getCameraInstance();
    try {
        mCamera.setPreviewDisplay(holder);
        mCamera.startPreview();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    mCamera.setOneShotPreviewCallback(mCallback);

    }