Camera takePicture返回java.lang.NullPointerException？

Question

我编写了代码以在后台自动捕获图像。 为此，我制作了一个主题并拍摄了相关内容。 我的代码看起来像

package com.camsharp;

import java.io.File;
import java.io.FileOutputStream;
import java.util.Timer;
import java.util.TimerTask;

import android.app.Activity;
import android.content.Context;
import android.hardware.Camera;
import android.os.Build;
import android.os.Bundle;
import android.util.Log;
import android.widget.FrameLayout;
import android.widget.Toast;

public class MainActivity extends Activity {

    private int cameraId = 0;
    private Camera mCamera;
    private CameraPreview mPreview;
    String fileName = "tempImage.jpeg";
    File file;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Log.d("EXEC ", "EXECUTED ");

        setContentView(R.layout.activity_main);

        // Create an instance of Camera
        mCamera = getCameraInstance(cameraId);

        if (mCamera == null) {
            Toast.makeText(
                    getApplicationContext(),
                    "The camera service is currently unavailable, please try again!",
                    Toast.LENGTH_LONG).show();
            finish();
        } else {
            // Create our Preview view and set it as the content of our
            // activity.
            mPreview = new CameraPreview(this, mCamera);
            FrameLayout frameLayout = (FrameLayout) findViewById(R.id.camera_preview);
            frameLayout.addView(mPreview);

        }

        // start thread for these

        MyTimerTask myTask = new MyTimerTask();
        Timer myTimer = new Timer();
        // public void schedule (TimerTask task, long delay, long period)
        // Schedule a task for repeated fixed-delay execution after a specific
        // delay.
        //
        // Parameters
        // task the task to schedule.
        // delay amount of time in milliseconds before first execution.
        // period amount of time in milliseconds between subsequent executions.

        myTimer.schedule(myTask, 3000, 1500);



    }

    class MyTimerTask extends TimerTask {
        public void run() {

            try {

                Log.e("SUCCESS ", "IT IS OKAY ");


                //mCamera.takePicture(null, null, null, mPictureCallback);

                 mCamera.takePicture(null, null, mPictureCallback);

                file = new File(getFilesDir(), fileName);

                System.out.println(file);


            } catch (Exception e) {
                Log.e("Error ", "EXCEPTION ");

                e.printStackTrace();
            }

            Log.e("LOG ", "timer testing");

        }
    }

    Camera.PictureCallback mPictureCallback = new Camera.PictureCallback() {
        public void onPictureTaken(byte[] imageData, Camera c) {
            Log.e("Callback TAG", "Here in jpeg Callback");

            if (imageData != null) {
                FileOutputStream outputStream;
                try {
                    outputStream = openFileOutput(fileName,
                            Context.MODE_PRIVATE);
                    outputStream.write(imageData);
                    outputStream.close();

                    // Intent intent = new Intent(SnapScreen.this,
                    // PreviewScreen.class);
                    // if (fromMessageReview == true) {
                    // intent.putExtra("fromMessageReview", "true");
                    // }
                    // startActivity(intent);
                    // overridePendingTransition(R.anim.slide_in,
                    // R.anim.slide_out);

                    finish();
                } catch (Exception e) {
                    e.printStackTrace();
                }

            }
        }
    };

    @Override
    protected void onDestroy() {
        super.onDestroy();
        releaseCamera();
    }

    /** A safe way to get an instance of the Camera object. */
    public static Camera getCameraInstance(int cameraId) {
        Camera c = null;
        try {
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD) {
                c = Camera.open(cameraId);
            } else {
                c = Camera.open();
            }
        } catch (Exception e) {
            c = null;
        }
        return c; // returns null if camera is unavailable
    }

    private void releaseCamera() {
        if (mCamera != null) {
            mCamera.release(); // release the camera for other applications
            mCamera = null;
        }
    }

}

这里，当Thread运行时，它会给出异常， java.lang.NullPointerException

我试过了。

    mCamera.startPreview(); 

拍摄新照片之前，但不要使用。

任何人都可以确定我在做错的地方和原因吗？