Android 在doInBackground方法之后无法返回数据

这是我的密码。在其中，onPostExecute（）方法得到的结果为null，但我从doInBackground（）方法返回值。 doInBackground（）正在调用getWeatherDataFromJson（）方法。我在日志（请参阅代码）中获得返回数组resultStrs，但在onPostExecute（）中，结果为null

public class ForecastFragment extends Fragment {

    public ForecastFragment(){

    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);
    }


    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        // Inflate the menu; this adds items to the action bar if it is present.
        inflater.inflate(R.menu.forecastfragment, menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_refresh) {
            FetchWeatherTask weatherTask = new FetchWeatherTask();
            weatherTask.execute("600001");
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    private ArrayAdapter<String> mForecastAdapter;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                              Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);


        FetchWeatherTask weatherTask = new FetchWeatherTask();
        weatherTask.execute("600001");

        // Will contain the raw JSON response as a string.
        String[] forecastArray = {
                "Today - Sunny - 88/64",
                "Tomorrow - Foggy - 78/64",
                "Wed - Cloudy - 72/64",
                "Thurs - Asteroids - 75/55",
                "Fri - Heavy Rain - 67/44",
                "Sat - Sunny - 88/76",
                "Today - Light Rain - 81/69",
        };
        List<String> weekForecast = new ArrayList<String>(
                Arrays.asList(forecastArray)
        );

        mForecastAdapter =
                new ArrayAdapter<String>(
                        // the current context
                        getActivity(),
                        //id of list item layout
                        R.layout.list_item_forecast,
                        // id of text view to populate
                        R.id.list_item_forecast_textview,
                        // array data
                        weekForecast);

        ListView listView = (ListView) rootView.findViewById(R.id.listview_forecast);
        listView.setAdapter(mForecastAdapter);

        return rootView;
    }

    public class FetchWeatherTask extends AsyncTask<String, Void, String[]> {

        private final String LOG_TAG = FetchWeatherTask.class.getSimpleName();


        /* The date/time conversion code is going to be moved outside the asynctask later,
  * so for convenience we're breaking it out into its own method now.
  */
        private String getReadableDateString(long time){
            // Because the API returns a unix timestamp (measured in seconds),
            // it must be converted to milliseconds in order to be converted to valid date.
            SimpleDateFormat shortenedDateFormat = new SimpleDateFormat("EEE MMM dd");
            return shortenedDateFormat.format(time);
        }

        /**
         * Prepare the weather high/lows for presentation.
         */
        private String formatHighLows(double high, double low) {
            // For presentation, assume the user doesn't care about tenths of a degree.
            long roundedHigh = Math.round(high);
            long roundedLow = Math.round(low);

            String highLowStr = roundedHigh + "/" + roundedLow;
            return highLowStr;
        }

        /**
         * Take the String representing the complete forecast in JSON Format and
         * pull out the data we need to construct the Strings needed for the wireframes.
         *
         * Fortunately parsing is easy:  constructor takes the JSON string and converts it
         * into an Object hierarchy for us.
         */
        private String[] getWeatherDataFromJson(String forecastJsonStr, int numDays)
                throws JSONException {

            // These are the names of the JSON objects that need to be extracted.
            final String OWM_LIST = "list";
            final String OWM_WEATHER = "weather";
            final String OWM_TEMPERATURE = "temp";
            final String OWM_MAX = "max";
            final String OWM_MIN = "min";
            final String OWM_DESCRIPTION = "main";

            JSONObject forecastJson = new JSONObject(forecastJsonStr);
            JSONArray weatherArray = forecastJson.getJSONArray(OWM_LIST);

            // OWM returns daily forecasts based upon the local time of the city that is being
            // asked for, which means that we need to know the GMT offset to translate this data
            // properly.

            // Since this data is also sent in-order and the first day is always the
            // current day, we're going to take advantage of that to get a nice
            // normalized UTC date for all of our weather.

            Time dayTime = new Time();
            dayTime.setToNow();

            // we start at the day returned by local time. Otherwise this is a mess.
            int julianStartDay = Time.getJulianDay(System.currentTimeMillis(), dayTime.gmtoff);

            // now we work exclusively in UTC
            dayTime = new Time();

            String[] resultStrs = new String[numDays];
            for(int i = 0; i < weatherArray.length(); i++) {
                // For now, using the format "Day, description, hi/low"
                String day;
                String description;
                String highAndLow;

                // Get the JSON object representing the day
                JSONObject dayForecast = weatherArray.getJSONObject(i);

                // The date/time is returned as a long.  We need to convert that
                // into something human-readable, since most people won't read "1400356800" as
                // "this saturday".
                long dateTime;
                // Cheating to convert this to UTC time, which is what we want anyhow
                dateTime = dayTime.setJulianDay(julianStartDay+i);
                day = getReadableDateString(dateTime);

                // description is in a child array called "weather", which is 1 element long.
                JSONObject weatherObject = dayForecast.getJSONArray(OWM_WEATHER).getJSONObject(0);
                description = weatherObject.getString(OWM_DESCRIPTION);

                // Temperatures are in a child object called "temp".  Try not to name variables
                // "temp" when working with temperature.  It confuses everybody.
                JSONObject temperatureObject = dayForecast.getJSONObject(OWM_TEMPERATURE);
                double high = temperatureObject.getDouble(OWM_MAX);
                double low = temperatureObject.getDouble(OWM_MIN);

                highAndLow = formatHighLows(high, low);
                resultStrs[i] = day + " - " + description + " - " + highAndLow;
            }

                for (String s : resultStrs) {
                    Log.v(LOG_TAG, "Forecast entry: " + s);
                }
            return resultStrs;


        }

        @Override
        protected String[] doInBackground(String... params) {

                // These two need to be declared outside the try/catch
                // so that they can be closed in the finally block.
                HttpURLConnection urlConnection = null;
                BufferedReader reader = null;

                // Will contain the raw JSON response as a string.
                String forecastJsonStr = null;

                String format = "json";
                String units = "metric";
                int numDays = 7;

                try {
                    // Construct the URL for the OpenWeatherMap query
                    // Possible parameters are avaiable at OWM's forecast API page, at
                    // http://openweathermap.org/API#forecast
                    final String BASE_URL = "http://api.openweathermap.org/data/2.5/forecast/daily?";

                    final String QUERY_PARAMS = "q";
                    final String FORMAT_PARAMS = "mode";
                    final String UNIT_PARAMS = "units";
                    final String DAYS_PARAMS = "cnt";

                    Uri builtUri = Uri.parse(BASE_URL).buildUpon()
                            .appendQueryParameter(QUERY_PARAMS, params[0])
                            .appendQueryParameter(FORMAT_PARAMS, format)
                            .appendQueryParameter(UNIT_PARAMS, units)
                            .appendQueryParameter(DAYS_PARAMS, Integer.toString(numDays))
                            .build();

                    URL url = new URL(builtUri.toString());

                    Log.v(LOG_TAG, "Built Url:" +builtUri.toString());

                    // Create the request to OpenWeatherMap, and open the connection
                    urlConnection = (HttpURLConnection) url.openConnection();
                    urlConnection.setRequestMethod("GET");
                    urlConnection.connect();

                    // Read the input stream into a String
                    InputStream inputStream = urlConnection.getInputStream();
                    StringBuffer buffer = new StringBuffer();
                    if (inputStream == null) {
                        // Nothing to do.
                        //forecastJsonStr = null;
                        return null;
                    }
                    reader = new BufferedReader(new InputStreamReader(inputStream));

                    String line;
                    while ((line = reader.readLine()) != null) {
                        // Since it's JSON, adding a newline isn't necessary (it won't affect parsing)
                        // But it does make debugging a *lot* easier if you print out the completed
                        // buffer for debugging.
                        buffer.append(line + "\n");
                    }

                    if (buffer.length() == 0) {
                        // Stream was empty.  No point in parsing.
                        //forecastJsonStr = null;
                        return null;
                    }
                    forecastJsonStr = buffer.toString();
                    try {
                        return getWeatherDataFromJson(forecastJsonStr, numDays);

                    } catch (JSONException e) {
                        Log.e(LOG_TAG,e.getMessage());
                        e.printStackTrace();
                    }
                } catch (IOException e) {
                    Log.e(LOG_TAG , "Error", e);
                    // If the code didn't successfully get the weather data, there's no point in attemping
                    // to parse it.
                    return null;
                    //forecastJsonStr = null;
                    //return forecastJsonStr;
                } finally{
                    if (urlConnection != null) {
                        urlConnection.disconnect();
                        return null;
                    }
                    if (reader != null) {
                        try {
                            reader.close();
                        } catch (final IOException e) {
                            Log.e("PlaceholderFragment", "Error closing stream", e);
                        }
                    }
                }

                return null;
        }

        @Override
        protected void onPostExecute(String[] result) {
            if (result != null) {
                Log.v(LOG_TAG,"dfgihuffhuffuhgfufdhuidfhudfuhli");
                mForecastAdapter.clear();
                for (String dayForecastStr : result) {
                    mForecastAdapter.add(dayForecastStr);
                }
            }else {
                    Log.v(LOG_TAG, "errprdvdfdffd");

            }

        }
    }

}

公共类ForecastFragment扩展了片段{
公众预报员（）{
}
@凌驾
创建时的公共void（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）；
设置选项菜单（真）；
}
@凌驾
创建选项菜单（菜单菜单，菜单充气机）{
//为菜单充气；这会将项目添加到操作栏（如果存在）。
充气机。充气（右菜单。艏楼碎片，菜单）；
}
@凌驾
公共布尔值onOptionsItemSelected（菜单项项）{
//处理操作栏项目单击此处。操作栏将
//自动处理Home/Up按钮上的点击，只要
//在AndroidManifest.xml中指定父活动时。
int id=item.getItemId（）；
if（id==R.id.action\u刷新）{
FetchWeatherTask weatherTask=新的FetchWeatherTask（）；
weatherTask.执行（“600001”）；
返回true；
}
返回super.onOptionsItemSelected（项目）；
}
专用阵列适配器mForecastAdapter；
@凌驾
创建视图上的公共视图（布局、充气机、视图组容器、，
Bundle savedInstanceState）{
视图根视图=充气机。充气（R.layout.fragment_main，容器，错误）；
FetchWeatherTask weatherTask=新的FetchWeatherTask（）；
weatherTask.执行（“600001”）；
//将以字符串形式包含原始JSON响应。
字符串[]forecastArray={
“今天-阳光明媚-88/64”，
“明天-多雾-78/64”，
“周三-多云-72/64”，
“周四-小行星-75/55”，
“周五-大雨-67/44”，
“周六-阳光明媚-88/76”，
“今天-小雨-81/69”，
};
List weekForecast=新建ArrayList(
Arrays.asList（forecastArray）
);
mForecastAdapter=
新阵列适配器(
//当前环境
getActivity（），
//列表项布局的id
R.layout.list\u item\u forecast，
//要填充的文本视图的id
R.id.list\u item\u forecast\u textview，
//数组数据
周预测）；
ListView ListView=（ListView）rootView.findViewById（R.id.ListView\u forecast）；
setAdapter（mForecastAdapter）；
返回rootView；
}
公共类FetchWeatherTask扩展了AsyncTask{
私有最终字符串LOG_TAG=FetchWeatherTask.class.getSimpleName（）；
/*日期/时间转换代码稍后将被移到asynctask之外，
*为了方便起见，我们现在将其分解为自己的方法。
*/
私有字符串getReadableDateString（长时间）{
//因为API返回unix时间戳（以秒为单位），
//必须将其转换为毫秒才能转换为有效日期。
SimpleDateFormat shortenedDateFormat=新的SimpleDateFormat（“EEE MMM dd”）；
返回shortenedDateFormat.format（时间）；
}
/**
*为演示做好天气高/低的准备。
*/
专用字符串格式HighLows（双高、双低）{
//对于演示，假设用户不关心十分之一的学位。
长轮高=数学轮（高）；
长轮低=数学轮（低）；
字符串highLowStr=roundedHigh+“/”+roundedLow；
返回highLowStr；
}
/**
*以JSON格式获取表示完整预测的字符串，然后
*取出我们需要的数据来构造线框所需的字符串。
*
*幸运的是，解析很容易：构造函数获取JSON字符串并转换它
*为我们创建一个对象层次结构。
*/
私有字符串[]getWeatherDataFromJson（字符串forecastJsonStr，int numDays）
抛出JSONException{
//这些是需要提取的JSON对象的名称。
最终字符串OWM_LIST=“LIST”；
最终字符串OWM_WEATHER=“WEATHER”；
最终字符串OWM_TEMPERATURE=“temp”；
最终字符串OWM_MAX=“MAX”；
最终字符串OWM_MIN=“MIN”；
最终字符串OWM_DESCRIPTION=“main”；
JSONObject forecastJson=新的JSONObject（forecastJsonStr）；
JSONArray weatherArray=forecastJson.getJSONArray（OWM_列表）；
//OWM将根据所选城市的当地时间返回每日预测
//这意味着我们需要知道GMT偏移量来转换这些数据
//对。
//因为此数据也是按顺序发送的，并且第一天始终是
//今天，我们将利用这一点来获得一个好的
//我们所有天气的标准化UTC日期。
白天时间=新时间（）；
白天；
//我们从当地时间返回的那一天开始。否则这就是一片混乱。
int julianStartDay=Time.getJulianDay（System.currentTimeMillis（），day.gmtoff）；
//现在我们只在UTC工作
白天=新时间（）；
String[]resultStrs=新字符串[numDays]；
对于（int i=0；i