java - 当我从 android 服务调用类时出现 NullPointerException

Question

这是我的第一个 Android 应用程序。我正在尝试从我使用 foursquare API 的不同类调用服务中的函数。我早些时候在一个 Activity 中尝试过这个函数，它工作得很好，但在服务中我得到了一个 NullPointerException。这是我正在使用的代码:

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();

}

我在这一行收到错误:

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

这是我在 mFsqApp 类中调用的函数:

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

您在此处声明(不幸的是公开的)mFsqApp 变量:

public FoursquareApp mFsqApp;

但是您没有显示任何代码来为其赋值 - 因此它将具有默认值 null，当您取消引用它时会导致 NullPointerException。在取消引用它之前，您需要为其分配一个非空值，例如与

mFsqApp = new FoursquareApp();

...或使用在其他地方传递的值(例如传递给构造函数)。很难判断其中哪一个适合这种情况，但是某物必须为其分配一个非空值。您说类似的代码在 Activity 的早期工作 - 所以回顾一下该代码并查看该值来自哪里。

(然后改进您的代码以避免公共(public)变量，尽可能避免静态变量，避免捕获Exception，避免在遇到异常时继续，除非您真的 处理它等)