java - 处理相机调用并预览时出现 javaNullPointer 异常

Question

我正在关注 Android 开发人员的相机指南。但是，当我尝试运行该 Activity 时，我收到一个 Java lang NullPointer 异常，我不明白它来自哪里。

这是我写下的相机 Activity 。

public class Call extends Activity {
     private Camera mCamera;
        private CameraPreview mPreview;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_call);

        // Create an instance of Camera
        mCamera = getCameraInstance();
        System.out.println(Camera.open());

        // Create our Preview view and set it as the content of our activity.
        mPreview = new CameraPreview(this, mCamera);
        FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview);
        preview.addView(mPreview);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.start_call, menu);
        return true;
    }

    /** Check if this device has a camera */
    private boolean checkCameraHardware(Context context) {
        if (context.getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA)){
            // this device has a camera
            System.out.println("Has a camera it has "+Camera.getNumberOfCameras());
            getCameraInstance();
            return true;
        } else {
            // no camera on this device
            System.out.println("Does not have a camera");
            return false;
        }
    }

    /** A safe way to get an instance of the Camera object. */
    public static Camera getCameraInstance(){
        Camera c = null;
        try {
            c = Camera.open(); // attempt to get a Camera instance
            System.out.println("Got the camera");
        }
        catch (Exception e){
            // Camera is not available (in use or does not exist)
            System.out.println("Not available");
        }
        return c; // returns null if camera is unavailable
    }

}

这是camerapreview类

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

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

        // 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) {
        // The Surface has been created, now tell the camera where to draw the preview.
        try {
            mCamera.setPreviewDisplay(holder);
            mCamera.startPreview();
        } catch (IOException e) {
            Log.d("error", "Error setting camera preview: " + e.getMessage());
        }
    }

    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) {
        // 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 {
            mCamera.setPreviewDisplay(mHolder);
            mCamera.startPreview();

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

这是我遇到的错误。

07-24 17:55:11.856: E/AndroidRuntime(796): FATAL EXCEPTION: main
07-24 17:55:11.856: E/AndroidRuntime(796): java.lang.NullPointerException
07-24 17:55:11.856: E/AndroidRuntime(796):  at com.reflap.reflap.CameraPreview.surfaceCreated(CameraPreview.java:31)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.SurfaceView.updateWindow(SurfaceView.java:543)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.SurfaceView.access$000(SurfaceView.java:81)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.SurfaceView$3.onPreDraw(SurfaceView.java:169)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:671)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1820)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4214)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.Choreographer$CallbackRecord.run(Choreographer.java:725)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.Choreographer.doCallbacks(Choreographer.java:555)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.Choreographer.doFrame(Choreographer.java:525)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:711)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.os.Handler.handleCallback(Handler.java:615)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.os.Handler.dispatchMessage(Handler.java:92)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.os.Looper.loop(Looper.java:137)
07-24 17:55:11.856: E/AndroidRuntime(796):  at android.app.ActivityThread.main(ActivityThread.java:4745)
07-24 17:55:11.856: E/AndroidRuntime(796):  at java.lang.reflect.Method.invokeNative(Native Method)
07-24 17:55:11.856: E/AndroidRuntime(796):  at java.lang.reflect.Method.invoke(Method.java:511)
07-24 17:55:11.856: E/AndroidRuntime(796):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
07-24 17:55:11.856: E/AndroidRuntime(796):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
07-24 17:55:11.856: E/AndroidRuntime(796):  at dalvik.system.NativeStart.main(Native Method)
07-24 17:55:15.045: I/Process(796): Sending signal. PID: 796 SIG: 9
07-24 17:55:15.956: E/Trace(817): error opening trace file: No such file or directory (2)

我不知道 null 从哪里来。我确实打印出了这个 System.out.println(Camera.open());它打印出 null，这很奇怪，但错误堆栈指向此，mCamera.setPreviewDisplay(holder);

我在 Android 模拟器上运行它，但我打开了相机并在有效的相机应用程序中对其进行了测试。

Answer 1

您没有代码来测试您是否拥有相机。在使用相机进行任何其他操作之前，请确保您有相机。

在这段代码中，我正在寻找唯一的相机或面向背面的相机......

public int getCameraId() { 
    Log.d(TAG, "getCameraId()");
    int cameraId = -1;
    // Search for the back facing camera (or any 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_BACK || numberOfCameras == 1) {
            Log.d(TAG, "CameraInfo.CAMERA_FACING_BACK = " 
                  + (info.facing == CameraInfo.CAMERA_FACING_BACK));
            cameraId = i;
            break;
        }
    }
    return cameraId;
}

public static Camera getCameraInstance(int cameraId) {
    Log.d(TAG, "getCameraInstance("+cameraId+")");
    Camera c = null;
    try {
        c = Camera.open(cameraId); // attempt to get a Camera instance
        Camera.Parameters cp = c.getParameters();
        Log.d(TAG, "getCameraInstance("+cameraId+"): Camera.Parameters = " 
        + cp.flatten());
    } catch (Exception e) {
        Log.d(TAG, "Camera.open("+cameraId+") exception="+e);
    }
    Log.d(TAG, "getCameraInstance("+cameraId+") = "+c);
    return c; // returns null if camera is unavailable
}

在调用 getCameraInstance 之前，调用会检查从 getCameraId 返回的 cameraId 是否 > -1。