Question

在我的程序中，我希望允许用户背靠背捕获多个图像，但是一旦我点击捕获按钮获取空指针异常（NPE）。我创建了一个自定义相机。< / p>

如果我将从现有代码中删除闪光灯代码，那么它对我来说工作正常（是的，然后我能够在不面对任何NPE的情况下背靠背捕获多个图像），但每当我使用Flash Light代码时 NPE

在行号：324

mediaFile = new File(mediaStorageDir.getPath() + File.separator
        + "IMG_" + timeStamp + ".jpg");

在行号：301

    pictureFile = getOutputMediaFile();

Java代码

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_camera);

    mCamera = getCameraInstance();

    mCameraPreview = new PreviewSurface(this, mCamera);
    FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview);
    preview.addView(mCameraPreview);


    Bundle extras = getIntent().getExtras();
    if (extras != null) {
        String value = extras.getString("folder_name");
        Log.d(CameraLauncherActivity.LOG_TAG, "folder_name :: " + value);    
    }


    Button captureButton = (Button) findViewById(R.id.btnCapture);
    Log.d(CameraLauncherActivity.LOG_TAG, "captureButton :: " + captureButton);
    captureButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            mCamera.takePicture(null, null, mPicture);
            Log.d(CameraLauncherActivity.LOG_TAG, "mCamera.takePicture :: " + mCamera);                                         
        }
    });

    Button viewButton = (Button) findViewById(R.id.btnView);
    Log.d(CameraLauncherActivity.LOG_TAG, "SingleAngelActivityButton :: " + viewButton);
    viewButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {             

            Intent intentNewEvent = new Intent(CameraLauncherActivity.this, UploadActivity.class);
            String event_id = customFolder;
            intentNewEvent.putExtra("event_id", event_id);
            startActivity(intentNewEvent);        
        }
    });

    cd = new ConnectionDetector(getApplicationContext());

    // Check if Internet present
    if (!cd.isConnectingToInternet()) {
        // Internet Connection is not present
        alert.showAlertDialog(CameraLauncherActivity.this, "Internet Connection Error",
                "Please connect to working Internet connection", false);
        // stop executing code by return
        return;
    }


   hasFlash = getApplicationContext().getPackageManager()
            .hasSystemFeature(PackageManager.FEATURE_CAMERA_FLASH);

   // Check if Internet present
    if (!hasFlash) {
        // device doesn't support flash
        alert.showAlertDialog(CameraLauncherActivity.this, "Sorry",
                "your Device doesn't support Flash Light !", false);
        // stop executing code by return
        return;
    }

     // displaying button image
   toggleButtonImage();

   // flash switch button
    btnSwitch = (ImageButton) findViewById(R.id.btnFlash);

    btnSwitch.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            if (isFlashOn) {
                // turn off flash
                turnOffFlash();
            } else {
                // turn on flash
                turnOnFlash();
            }
        }
    });              

// Get event id, angel id
Intent i = getIntent();
event_id = i.getStringExtra("event_id");
angel_id = i.getStringExtra("angel_id");

// calling background thread
new LoadSingleTrack().execute();
}

 /**
 * Background Async Task to get single angel information
 * */
class LoadSingleTrack extends AsyncTask<String, String, String> {

    /**
     * Before starting background thread Show Progress Dialog
     * */
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        pDialog = new ProgressDialog(CameraLauncherActivity.this);
        pDialog.setMessage("Initializing Camera...");
        pDialog.setIndeterminate(false);
        pDialog.setCancelable(false);
        pDialog.show();
    }

    /**
     * getting angel json and parsing
     * */
    protected String doInBackground(String... args) {
        // Building Parameters
        List<NameValuePair> params = new ArrayList<NameValuePair>();

        // post event id, angel id as GET parameters
        params.add(new BasicNameValuePair("event", event_id));
        params.add(new BasicNameValuePair("angel", angel_id));

        // getting JSON string from URL
        String json = jsonParser.makeHttpRequest(URL_angel, "GET",
                params);

        // Check your log cat for JSON reponse
        Log.d("Single Track JSON: ", json);

        try {
            JSONObject jObj = new JSONObject(json);
            if(jObj != null){
                angel_name = jObj.getString(TAG_ANGEL);
                event_name = jObj.getString(TAG_EVENT);
            }           

        } catch (JSONException e) {
            e.printStackTrace();
        }

        return null;
    }

    /**
     * After completing background task Dismiss the progress dialog
     * **/
    protected void onPostExecute(String file_url) {
        // dismiss the dialog after getting angel information
        pDialog.dismiss();

        // updating UI from Background Thread
        runOnUiThread(new Runnable() {
            public void run() {

                TextView txt_angel_name = (TextView) findViewById(R.id.angel_title);
                String timeStamp = new SimpleDateFormat("dd-MM-yyyy").format(new Date());

                txt_angel_name.setText(event_name + " Event_"+ angel_name + "-" + timeStamp);
                customFolder = txt_angel_name.getText().toString();

                // Change Activity Title with angel title
                setTitle(angel_name);

                // folder name
                mediaStorageDir = new File(
                        Environment
                                .getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES),
                                "/CaptureImages/"+ customFolder + "/");

                if (!mediaStorageDir.exists()) {
                    if (!mediaStorageDir.mkdirs()) {
                        Log.d("App", "failed to create directory");                  
                    }
                }                   
            }
        });
    }
}

}

logcat的： -

12-04 23:23:04.610: E/AndroidRuntime(798): FATAL EXCEPTION: main
12-04 23:23:04.610: E/AndroidRuntime(798): java.lang.NullPointerException
12-04 23:23:04.610: E/AndroidRuntime(798):  at com.example.camera.CameraLauncherActivity.getOutputMediaFile(CameraLauncherActivity.java:324)
12-04 23:23:04.610: E/AndroidRuntime(798):  at com.example.camera.CameraLauncherActivity$1.onPictureTaken(CameraLauncherActivity.java:301)
12-04 23:23:04.610: E/AndroidRuntime(798):  at android.hardware.Camera$EventHandler.handleMessage(Camera.java:773)
12-04 23:23:04.610: E/AndroidRuntime(798):  at android.os.Handler.dispatchMessage(Handler.java:99)
12-04 23:23:04.610: E/AndroidRuntime(798):  at android.os.Looper.loop(Looper.java:137)
12-04 23:23:04.610: E/AndroidRuntime(798):  at android.app.ActivityThread.main(ActivityThread.java:5103)
12-04 23:23:04.610: E/AndroidRuntime(798):  at java.lang.reflect.Method.invokeNative(Native Method)
12-04 23:23:04.610: E/AndroidRuntime(798):  at java.lang.reflect.Method.invoke(Method.java:525)
12-04 23:23:04.610: E/AndroidRuntime(798):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
12-04 23:23:04.610: E/AndroidRuntime(798):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
12-04 23:23:04.610: E/AndroidRuntime(798):  at dalvik.system.NativeStart.main(Native Method)
12-04 23:23:07.570: I/Process(798): Sending signal. PID: 798 SIG: 9

Answer 1

我无法弄清楚代码的各个部分的确切执行顺序，但是该行唯一可以生成NPE的是mediaStorageDir是null。由于在mediaStorageDir实例异步执行后初始化LoadSingleTrack，我只能假设它在第324行执行后正在执行。解决这种无序的代码执行顺序，问题就会消失。

正如旁注：从runOnUiThread的{{1}}致电onPostExecute是没有意义的。调用AsyncTask时，您已经在UI线程上。

空指针异常 - 带闪光灯的自定义相机

1 个答案: