为什么在线程中收到java.lang.NullPointerException错误？

Question

我想开发一个用于将流视频从客户端发送到服务器的应用程序。我的客户端应用程序有一些问题：当我尝试在日志猫的客户端线程中写入从摄像机捕获的图像时，会抛出此错误：

err: java.lang.NullPointerException 
err:    ClientThread.doInBackground(ClientThread.java:38)
err:     at clientcamerasource.ClientThread.doInBackground(ClientThread.java:12)
err:     at android.os.AsyncTask$2.call(AsyncTask.java:288)
err:     at java.util.concurrent.FutureTask.run(FutureTask.java:237)
err:     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
err:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
err:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
err:     at java.lang.Thread.run(Thread.java:841)

通常找不到在mainActivity中定义的mPreview（MyCameraView的对象）。这是MainActivity：

public class MainActivity extends AppCompatActivity {
    private Camera mCamera;
    public MyCameraView mPreview;
    public TextView serverStatus;
    public static String SERVERIP = "192.168.1.9";
    private Handler handler = new Handler();
    public static final int SERVERPORT = 8080;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mCamera = getCameraInstance();
        mPreview = new MyCameraView(this, mCamera);
        FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview);
        preview.addView(mPreview);
        ClientThread myClient = new ClientThread(SERVERIP,SERVERPORT,mPreview,handler);
        myClient.execute();
    }

    public static Camera getCameraInstance()
    {
        Camera c=null;
        try{
            c=Camera.open(1);
        }catch(Exception e){
            e.printStackTrace();
        }
        return c;
    }
}

这是MyCameraView类，用于在视图上添加摄像机流：

public class MyCameraView  extends SurfaceView implements SurfaceHolder.Callback,Camera.PreviewCallback{
    private SurfaceHolder mHolder;
    private Camera mCamera;
    private int width;
    private int height;
    public ByteArrayOutputStream mFrameBuffer;
    private Context con;

    /**
     * Constructor of the MyCameraView
     * @param context
     * @param camera
     */
    public MyCameraView(Context context,Camera camera){
        super(context);
        con=context;
        mCamera=camera;
        mHolder=getHolder();
        mHolder.addCallback(this);
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }

    /**
     * set preview to the camera
     * @param holder
     */
    public void surfaceCreated(SurfaceHolder holder) {
        try {
            mCamera.setPreviewDisplay(holder);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * surface destroyed function
     * @param holder
     */
    public void surfaceDestroyed(SurfaceHolder holder) {
        mCamera.setPreviewCallback(null);
        mCamera.release();
        mCamera=null;
    }

    /**
     * surface changed function
     * @param holder
     * @param format
     * @param w
     * @param h
     */
    public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
        try{
            mCamera.stopPreview();
        } catch (Exception e){
            e.printStackTrace();
        }
        try{
            //Configration Camera Parameter(full-size)
            Camera.Parameters parameters = mCamera.getParameters();
            parameters.setPreviewSize(320,240);
            this.width=parameters.getPreviewSize().width;
            this.height=parameters.getPreviewSize().height;
            parameters.setPreviewFormat(ImageFormat.NV21);
            mCamera.setParameters(parameters);
            // mCamera.setDisplayOrientation(90);
            mCamera.setPreviewCallback(this);
            mCamera.startPreview();
        }catch(Exception e){
            e.printStackTrace();
        }
    }

    /**
     * frame call back function
     * @param data
     * @param camera
     */
    public void onPreviewFrame(byte[] data,Camera camera){
        try{
            //convert YuvImage(NV21) to JPEG Image data
            YuvImage yuvimage=new YuvImage(data,ImageFormat.NV21,this.width,this.height,null);
            System.out.println("WidthandHeight"+yuvimage.getHeight()+"::"+yuvimage.getWidth());
            ByteArrayOutputStream baos=new ByteArrayOutputStream();
            yuvimage.compressToJpeg(new Rect(0,0,this.width,this.height),100,baos);
            mFrameBuffer = baos;
        }catch(Exception e){
            Log.d("parse","error");
        }
    }
}

这是客户端线程类：

public class ClientThread extends AsyncTask<Void, Void, Void> {
    String dstAddress;
    int dstPort;
    MyCameraView myPreview;
    private Handler mHandler;
    Socket socket;

    ClientThread(String addr, int port, MyCameraView myPreview, Handler handler) {
        dstAddress = addr;
        dstPort = port;
        mHandler = handler;
        this.myPreview = myPreview;
    }

    @Override
    protected Void doInBackground(Void... arg0) {
        try {
            socket = new Socket(dstAddress, dstPort);

            try {
                System.out.println("Socket creato");
                OutputStream os = socket.getOutputStream();
                DataOutputStream dos = new DataOutputStream(os);
                dos.writeInt(4);
                dos.writeUTF("#@@#");
                dos.writeInt(myPreview.mFrameBuffer.size());
                dos.writeUTF("-@@-");
                dos.flush();
                System.out.println("SIZE: " + myPreview.mFrameBuffer.size());
                dos.write(myPreview.mFrameBuffer.toByteArray());
                dos.flush();
                Thread.sleep(1000 / 15);
            } catch (Exception e2) {
                e2.printStackTrace();
            }

        } catch (Exception e) {
            Log.d("ServerThread", "run: erro");
            }finally
        {
            //Closing the socket
            try
            {
                socket.close();
            }
            catch(Exception e)
            {
                e.printStackTrace();
            }
        }

        return null;
    }
}

为什么我的程序在客户端线程中找不到mPreview对象？