我使用Android工作室创建的应用程序不断崩溃，我不知道为什么。它是一个天气应用程序，我正在关注谷歌开发Android应用程序课程，这里是logcat：
07-20 18:19:45.740 16410-16434/com.alexander.sunshine E/ActivityThread﹕ Failed to find provider info for com.alexander.android.sunshine.app
07-20 18:19:45.748 16410-16434/com.alexander.sunshine E/AndroidRuntime﹕ FATAL EXCEPTION: AsyncTask #1
Process: com.alexander.sunshine, PID: 16410
java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:304)
at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
at java.util.concurrent.FutureTask.run(FutureTask.java:242)
at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'boolean android.database.Cursor.moveToFirst()' on a null object reference
at com.alexander.sunshine.app.FetchWeatherTask.addLocation(FetchWeatherTask.java:109)
at com.alexander.sunshine.app.FetchWeatherTask.getWeatherDataFromJson(FetchWeatherTask.java:214)
at com.alexander.sunshine.app.FetchWeatherTask.doInBackground(FetchWeatherTask.java:414)
at com.alexander.sunshine.app.FetchWeatherTask.doInBackground(FetchWeatherTask.java:34)
at android.os.AsyncTask$2.call(AsyncTask.java:292)
at java.util.concurrent.FutureTask.run(FutureTask.java:237)
at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)
这是FetchWeatherTask.java：
package com.alexander.sunshine.app;
import android.content.ContentUris;
import android.content.ContentValues;
import android.content.Context;
import android.content.SharedPreferences;
import android.database.Cursor;
import android.database.DatabaseUtils;
import android.net.Uri;
import android.os.AsyncTask;
import android.preference.PreferenceManager;
import android.text.format.Time;
import android.util.Log;
import android.widget.ArrayAdapter;
import com.alexander.sunshine.R;
import com.alexander.sunshine.app.data.WeatherContract;
import com.alexander.sunshine.app.data.WeatherContract.WeatherEntry;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Vector;
public class FetchWeatherTask extends AsyncTask<String, Void, String[]> {
private final String LOG_TAG = FetchWeatherTask.class.getSimpleName();
private ArrayAdapter<String> mForecastAdapter;
private final Context mContext;
public FetchWeatherTask(Context context, ArrayAdapter<String> forecastAdapter) {
mContext = context;
mForecastAdapter = forecastAdapter;
}
private boolean DEBUG = true;
/* 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.
Date date = new Date(time);
SimpleDateFormat format = new SimpleDateFormat("E, MMM d");
return format.format(date).toString();
}
/**
* Prepare the weather high/lows for presentation.
*/
private String formatHighLows(double high, double low) {
// Data is fetched in Celsius by default.
// If user prefers to see in Fahrenheit, convert the values here.
// We do this rather than fetching in Fahrenheit so that the user can
// change this option without us having to re-fetch the data once
// we start storing the values in a database.
SharedPreferences sharedPrefs =
PreferenceManager.getDefaultSharedPreferences(mContext);
String unitType = sharedPrefs.getString(
mContext.getString(R.string.pref_units_key),
mContext.getString(R.string.pref_units_metric));
if (unitType.equals(mContext.getString(R.string.pref_units_imperial))) {
high = (high * 1.8) + 32;
low = (low * 1.8) + 32;
} else if (!unitType.equals(mContext.getString(R.string.pref_units_metric))) {
Log.d(LOG_TAG, "Unit type not found: " + unitType);
}
// 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;
}
/**
* Helper method to handle insertion of a new location in the weather database.
*
* @param locationSetting The location string used to request updates from the server.
* @param cityName A human-readable city name, e.g "Mountain View"
* @param lat the latitude of the city
* @param lon the longitude of the city
* @return the row ID of the added location.
*/
long addLocation(String locationSetting, String cityName, double lat, double lon) {
long locationId;
// First, check if the location with this city name exists in the db
Cursor locationCursor = mContext.getContentResolver().query(
WeatherContract.LocationEntry.CONTENT_URI,
new String[]{WeatherContract.LocationEntry._ID},
WeatherContract.LocationEntry.COLUMN_LOCATION_SETTING + " = ?",
new String[]{locationSetting},
null);
if (locationCursor.moveToFirst()) {
int locationIdIndex = locationCursor.getColumnIndex(WeatherContract.LocationEntry._ID);
locationId = locationCursor.getLong(locationIdIndex);
} else {
// Now that the content provider is set up, inserting rows of data is pretty simple.
// First create a ContentValues object to hold the data you want to insert.
ContentValues locationValues = new ContentValues();
// Then add the data, along with the corresponding name of the data type,
// so the content provider knows what kind of value is being inserted.
locationValues.put(WeatherContract.LocationEntry.COLUMN_CITY_NAME, cityName);
locationValues.put(WeatherContract.LocationEntry.COLUMN_LOCATION_SETTING, locationSetting);
locationValues.put(WeatherContract.LocationEntry.COLUMN_COORD_LAT, lat);
locationValues.put(WeatherContract.LocationEntry.COLUMN_COORD_LONG, lon);
// Finally, insert location data into the database.
Uri insertedUri = mContext.getContentResolver().insert(
WeatherContract.LocationEntry.CONTENT_URI,
locationValues
);
// The resulting URI contains the ID for the row. Extract the locationId from the Uri.
locationId = ContentUris.parseId(insertedUri);
}
locationCursor.close();
// Wait, that worked? Yes!
return locationId;
}
/*
Students: This code will allow the FetchWeatherTask to continue to return the strings that
the UX expects so that we can continue to test the application even once we begin using
the database.
*/
String[] convertContentValuesToUXFormat(Vector<ContentValues> cvv) {
// return strings to keep UI functional for now
String[] resultStrs = new String[cvv.size()];
for ( int i = 0; i < cvv.size(); i++ ) {
ContentValues weatherValues = cvv.elementAt(i);
String highAndLow = formatHighLows(
weatherValues.getAsDouble(WeatherEntry.COLUMN_MAX_TEMP),
weatherValues.getAsDouble(WeatherEntry.COLUMN_MIN_TEMP));
resultStrs[i] = getReadableDateString(
weatherValues.getAsLong(WeatherEntry.COLUMN_DATE)) +
" - " + weatherValues.getAsString(WeatherEntry.COLUMN_SHORT_DESC) +
" - " + highAndLow;
}
return resultStrs;
}
/**
* 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,
String locationSetting)
throws JSONException {
// Now we have a String representing the complete forecast in JSON Format.
// Fortunately parsing is easy: constructor takes the JSON string and converts it
// into an Object hierarchy for us.
// These are the names of the JSON objects that need to be extracted.
// Location information
final String OWM_CITY = "city";
final String OWM_CITY_NAME = "name";
final String OWM_COORD = "coord";
// Location coordinate
final String OWM_LATITUDE = "lat";
final String OWM_LONGITUDE = "lon";
// Weather information. Each day's forecast info is an element of the "list" array.
final String OWM_LIST = "list";
final String OWM_PRESSURE = "pressure";
final String OWM_HUMIDITY = "humidity";
final String OWM_WINDSPEED = "speed";
final String OWM_WIND_DIRECTION = "deg";
// All temperatures are children of the "temp" object.
final String OWM_TEMPERATURE = "temp";
final String OWM_MAX = "max";
final String OWM_MIN = "min";
final String OWM_WEATHER = "weather";
final String OWM_DESCRIPTION = "main";
final String OWM_WEATHER_ID = "id";
try {
JSONObject forecastJson = new JSONObject(forecastJsonStr);
JSONArray weatherArray = forecastJson.getJSONArray(OWM_LIST);
JSONObject cityJson = forecastJson.getJSONObject(OWM_CITY);
String cityName = cityJson.getString(OWM_CITY_NAME);
JSONObject cityCoord = cityJson.getJSONObject(OWM_COORD);
double cityLatitude = cityCoord.getDouble(OWM_LATITUDE);
double cityLongitude = cityCoord.getDouble(OWM_LONGITUDE);
long locationId = addLocation(locationSetting, cityName, cityLatitude, cityLongitude);
// Insert the new weather information into the database
Vector<ContentValues> cVVector = new Vector<ContentValues>(weatherArray.length());
// 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();
for(int i = 0; i < weatherArray.length(); i++) {
// These are the values that will be collected.
long dateTime;
double pressure;
int humidity;
double windSpeed;
double windDirection;
double high;
double low;
String description;
int weatherId;
// Get the JSON object representing the day
JSONObject dayForecast = weatherArray.getJSONObject(i);
// Cheating to convert this to UTC time, which is what we want anyhow
dateTime = dayTime.setJulianDay(julianStartDay+i);
pressure = dayForecast.getDouble(OWM_PRESSURE);
humidity = dayForecast.getInt(OWM_HUMIDITY);
windSpeed = dayForecast.getDouble(OWM_WINDSPEED);
windDirection = dayForecast.getDouble(OWM_WIND_DIRECTION);
// Description is in a child array called "weather", which is 1 element long.
// That element also contains a weather code.
JSONObject weatherObject =
dayForecast.getJSONArray(OWM_WEATHER).getJSONObject(0);
description = weatherObject.getString(OWM_DESCRIPTION);
weatherId = weatherObject.getInt(OWM_WEATHER_ID);
// 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);
high = temperatureObject.getDouble(OWM_MAX);
low = temperatureObject.getDouble(OWM_MIN);
ContentValues weatherValues = new ContentValues();
weatherValues.put(WeatherEntry.COLUMN_LOC_KEY, locationId);
weatherValues.put(WeatherEntry.COLUMN_DATE, dateTime);
weatherValues.put(WeatherEntry.COLUMN_HUMIDITY, humidity);
weatherValues.put(WeatherEntry.COLUMN_PRESSURE, pressure);
weatherValues.put(WeatherEntry.COLUMN_WIND_SPEED, windSpeed);
weatherValues.put(WeatherEntry.COLUMN_DEGREES, windDirection);
weatherValues.put(WeatherEntry.COLUMN_MAX_TEMP, high);
weatherValues.put(WeatherEntry.COLUMN_MIN_TEMP, low);
weatherValues.put(WeatherEntry.COLUMN_SHORT_DESC, description);
weatherValues.put(WeatherEntry.COLUMN_WEATHER_ID, weatherId);
cVVector.add(weatherValues);
}
// add to database
if ( cVVector.size() > 0 ) {
ContentValues[] cvArray = new ContentValues[cVVector.size()];
cVVector.toArray(cvArray);
mContext.getContentResolver().bulkInsert(WeatherEntry.CONTENT_URI, cvArray);
}
// Sort order: Ascending, by date.
String sortOrder = WeatherEntry.COLUMN_DATE + " ASC";
Uri weatherForLocationUri = WeatherEntry.buildWeatherLocationWithStartDate(
locationSetting, System.currentTimeMillis());
// Students: Uncomment the next lines to display what what you stored in the bulkInsert
Cursor cur = mContext.getContentResolver().query(weatherForLocationUri,
null, null, null, sortOrder);
cVVector = new Vector<ContentValues>(cur.getCount());
if ( cur.moveToFirst() ) {
do {
ContentValues cv = new ContentValues();
DatabaseUtils.cursorRowToContentValues(cur, cv);
cVVector.add(cv);
} while (cur.moveToNext());
}
Log.d(LOG_TAG, "FetchWeatherTask Complete. " + cVVector.size() + " Inserted");
String[] resultStrs = convertContentValuesToUXFormat(cVVector);
return resultStrs;
} catch (JSONException e) {
Log.e(LOG_TAG, e.getMessage(), e);
e.printStackTrace();
}
return null;
}
@Override
protected String[] doInBackground(String... params) {
// If there's no zip code, there's nothing to look up. Verify size of params.
if (params.length == 0) {
return null;
}
String locationQuery = params[0];
// 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 = 14;
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 FORECAST_BASE_URL =
"http://api.openweathermap.org/data/2.5/forecast/daily?";
final String QUERY_PARAM = "q";
final String FORMAT_PARAM = "mode";
final String UNITS_PARAM = "units";
final String DAYS_PARAM = "cnt";
Uri builtUri = Uri.parse(FORECAST_BASE_URL).buildUpon()
.appendQueryParameter(QUERY_PARAM, params[0])
.appendQueryParameter(FORMAT_PARAM, format)
.appendQueryParameter(UNITS_PARAM, units)
.appendQueryParameter(DAYS_PARAM, Integer.toString(numDays))
.build();
URL url = new 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.
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.
return null;
}
forecastJsonStr = buffer.toString();
} 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;
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
if (reader != null) {
try {
reader.close();
} catch (final IOException e) {
Log.e(LOG_TAG, "Error closing stream", e);
}
}
}
try {
return getWeatherDataFromJson(forecastJsonStr, locationQuery);
} catch (JSONException e) {
Log.e(LOG_TAG, e.getMessage(), e);
e.printStackTrace();
}
// This will only happen if there was an error getting or parsing the forecast.
return null;
}
@Override
protected void onPostExecute(String[] result) {
if (result != null && mForecastAdapter != null) {
mForecastAdapter.clear();
for(String dayForecastStr : result) {
mForecastAdapter.add(dayForecastStr);
}
// New data is back from the server. Hooray!
}
}
}
答案 0 :(得分：1)
正如堆栈跟踪所示，您尝试在空引用上调用moveToFirst()。 locationCursor是您调用该方法的唯一对象，因此它必须为null。
query()的文档说它可以返回null，因此你应该检查该对象，并且还应检查query()的调用以试图理解为什么它返回null。
答案 1 :(得分：0)
显然，在你的addLocation方法中，你通过调用Cursor locationCursor = mContext.getContentResolver().query(...)获得的locationCursor为null，这会在你之后调用locationCursor.moveToFirst()时导致崩溃。
我建议查看this problem's accepted answer。问题可能是查询为空。
在任何情况下，在应用方法之前，应始终检查Cursor是否为null，并通过警告用户以某种方式处理该情况。
该方法可能如下所示：
long addLocation(String locationSetting, String cityName, double lat, double lon) {
long locationId;
// First, check if the location with this city name exists in the db
Cursor locationCursor = mContext.getContentResolver().query(
WeatherContract.LocationEntry.CONTENT_URI,
new String[]{WeatherContract.LocationEntry._ID},
WeatherContract.LocationEntry.COLUMN_LOCATION_SETTING + " = ?",
new String[]{locationSetting},
null);
if (locationCursor != null) {
if (locationCursor.moveToFirst()) {
int locationIdIndex = locationCursor.getColumnIndex(WeatherContract.LocationEntry._ID);
locationId = locationCursor.getLong(locationIdIndex);
locationCursor.close();
return locationId;
}
}
// Now that the content provider is set up, inserting rows of data is pretty simple.
// First create a ContentValues object to hold the data you want to insert.
ContentValues locationValues = new ContentValues();
// Then add the data, along with the corresponding name of the data type,
// so the content provider knows what kind of value is being inserted.
locationValues.put(WeatherContract.LocationEntry.COLUMN_CITY_NAME, cityName);
locationValues.put(WeatherContract.LocationEntry.COLUMN_LOCATION_SETTING, locationSetting);
locationValues.put(WeatherContract.LocationEntry.COLUMN_COORD_LAT, lat);
locationValues.put(WeatherContract.LocationEntry.COLUMN_COORD_LONG, lon);
// Finally, insert location data into the database.
Uri insertedUri = mContext.getContentResolver().insert(
WeatherContract.LocationEntry.CONTENT_URI,
locationValues
);
// The resulting URI contains the ID for the row. Extract the locationId from the Uri.
locationId = ContentUris.parseId(insertedUri);
locationCursor.close();
// Wait, that worked? Yes!
return locationId;
}