當我從android服務調用類時出現NullPointerException

Question

這是我的第一個Android應用程序。 我正在嘗試使用Foursquare API從其他類調用服務中的函數。 我在一個活動中較早地嘗試了此功能，它工作得很好，但是在服務中卻收到了NullPointerException。 這是我正在使用的代碼：

public class GuideMeService extends Service implements LocationListener {

LocationManager locationManager;
Geocoder geocoder;
// private CalendarContentResolver Calendar;
public FoursquareApp mFsqApp;
public ArrayList<FsqVenue> mNearbyList;
private static String TAG = "Service Class";
double lat;
double lng;
public static final String[] FIELDS = { CalendarContract.Calendars.NAME,
        CalendarContract.Calendars.CALENDAR_DISPLAY_NAME,
        CalendarContract.Calendars.CALENDAR_COLOR,
        CalendarContract.Calendars.VISIBLE };

public static ArrayList<String> Events = new ArrayList<String>();
public static final Uri CALENDAR_URI = Uri
        .parse("content://com.android.calendar/calendars");
public static final Uri EVENTS_URI = Uri
        .parse("content://com.android.calendar/events");
private static final long MINIMUM_DISTANCE_CHANGE_FOR_UPDATES = 1; // in Meters
private static final long MINIMUM_TIME_BETWEEN_UPDATES = 1000; // in  Milliseconds
public static String query = "";

Set<String> calendars = new HashSet<String>();

@Override
public IBinder onBind(Intent arg0) {
    return null;
}

@Override
public void onStart(Intent intent, int startId) {
    super.onStart(intent, startId); 
    Log.d(TAG, "GuideMe Servise started");
    //this.stopSelf();
    locationManager = (LocationManager)this.getSystemService(LOCATION_SERVICE);
    locationManager.requestLocationUpdates(
            LocationManager.GPS_PROVIDER,
            MINIMUM_TIME_BETWEEN_UPDATES,
            MINIMUM_DISTANCE_CHANGE_FOR_UPDATES,
            new MyLocationListener()
            );

    Events = readCalendarEvent(getApplicationContext());
    for(int i = 0 ; i < Events.size() ; i++){
        query += Events.toArray()[i].toString() + " ";
    }
    Thread thread = new Thread()
    {
          @Override
          public void run() {

                try {
                        Location location = locationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER);
                        mNearbyList = mFsqApp.SearchBykeyword(location.getLatitude(), location.getLongitude(), query);
                    }catch (Exception e) {
                        e.printStackTrace();
                    }
          }
  };

thread.start();

}

我在這條線上得到錯誤：

mNearbyList = mFsqApp.SearchBykeyword(location.getLatitude(), location.getLongitude(), query);

這是我在mFsqApp類中調用的函數：

public ArrayList<FsqVenue> SearchBykeyword(double latitude, double longitude, String query) throws Exception {
    ArrayList<FsqVenue> venueList = new ArrayList<FsqVenue>();
    try {
        String ll   = String.valueOf(latitude) + "," + String.valueOf(longitude);
        URL url     = new URL(API_URL + "/venues/search?ll=" + ll + "&query=" + query + "&radius=" + 50 + "&oauth_token=" + mAccessToken + "&v=20120610");

        Log.d(TAG, "Opening URL " + url.toString());

        HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();

        urlConnection.setRequestMethod("GET");
        urlConnection.setDoInput(true);
        //urlConnection.setDoOutput(true);

        urlConnection.connect();
        String response     = streamToString(urlConnection.getInputStream());
        JSONObject jsonObj  = (JSONObject) new JSONTokener(response).nextValue();

        JSONArray groups    = (JSONArray) jsonObj.getJSONObject("response").getJSONArray("groups");

        int length          = groups.length();
        if (length > 0) {
            for (int i = 0; i < length; i++) {
                JSONObject group    = (JSONObject) groups.get(i);
                JSONArray items     = (JSONArray) group.getJSONArray("items");

                int ilength         = items.length();

                for (int j = 0; j < ilength; j++) {
                    JSONObject item = (JSONObject) items.get(j);

                    FsqVenue venue  = new FsqVenue();

                    venue.id        = item.getString("id");
                    venue.name      = item.getString("name");

                    JSONObject location = (JSONObject) item.getJSONObject("location");

                    Location loc    = new Location(LocationManager.GPS_PROVIDER);

                    loc.setLatitude(Double.valueOf(location.getString("lat")));
                    loc.setLongitude(Double.valueOf(location.getString("lng")));

                    venue.location  = loc;
                    //venue.address = location.getString("address");
                    venue.distance  = location.getInt("distance");
                    //venue.herenow = item.getJSONObject("hereNow").getInt("count");
                    venue.type      = group.getString("type");

                    venueList.add(venue);
                }
            }
        }
    } catch (Exception ex) {
        throw ex;
    }
    return venueList;
}

更新：

07-08 21:26:18.580: W/System.err(24365): java.lang.NullPointerException
07-08 21:26:18.580: W/System.err(24365):    at com.android.guideme.GuideMeService$1.run(GuideMeService.java:86)

Answer 1

您在這里聲明了（不幸的是公開的） mFsqApp變量：

public FoursquareApp mFsqApp;

但您沒有顯示任何代碼來為其分配值-因此它將具有默認值null ，當您取消引用它時會導致NullPointerException 。 在取消引用之前，您需要為其分配一個非空值，例如

mFsqApp = new FoursquareApp();

...或使用傳入其他地方的值（例如，傳遞給構造函數）。 這很難說哪種情況在這種情況下適當的事 ，但有一個非空值分配給它。 您說類似的代碼在活動的早期就起作用了-因此，請回顧該代碼，看看該值從何而來。

（然后改進代碼以避免公共變量，在可能的情況下避免靜態變量，避免捕獲Exception ，除非在真正處理異常之前，否則避免繼續面對異常等）