android - locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);返回空指针异常

Question

Followind 代码始终为位置管理器返回空指针异常。

我的 GPSTracker 类 私有(private)最终上下文 mContext；

// flag for GPS status
boolean isGPSEnabled = false;

// flag for network status
boolean isNetworkEnabled = false;

// flag for GPS status
boolean canGetLocation = false;

Location location; // location
double latitude; // latitude
double longitude; // longitude

// The minimum distance to change Updates in meters
private static final long MIN_DISTANCE_CHANGE_FOR_UPDATES = 10; // 10 meters

// The minimum time between updates in milliseconds
private static final long MIN_TIME_BW_UPDATES = 1000 * 60 * 1; // 1 minute

// Declaring a Location Manager
protected LocationManager locationManager;
//Location locManager = locationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER);
public GPSTracker(Context context) {
    this.mContext = context;
    getLocation();
}

public Location getLocation() {
    try {

        locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);
        // getting GPS status
        isGPSEnabled = locationManager
                .isProviderEnabled(LocationManager.GPS_PROVIDER);

        // getting network status
        isNetworkEnabled = locationManager
                .isProviderEnabled(LocationManager.NETWORK_PROVIDER);

        if (!isGPSEnabled && !isNetworkEnabled) {
            // no network provider is enabled
        } else {
            this.canGetLocation = true;
            // First get location from Network Provider
            if (isNetworkEnabled) {
                locationManager.requestLocationUpdates(
                        LocationManager.NETWORK_PROVIDER,
                        MIN_TIME_BW_UPDATES,
                        MIN_DISTANCE_CHANGE_FOR_UPDATES, this);
                Log.d("Network", "Network");
                if (locationManager != null) {
                    location = locationManager
                            .getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
                    if (location != null) {
                        latitude = location.getLatitude();
                        longitude = location.getLongitude();
                    }
                }
            }
            // if GPS Enabled get lat/long using GPS Services
            if (isGPSEnabled) {
                if (location == null) {
                    locationManager.requestLocationUpdates(
                            LocationManager.GPS_PROVIDER,
                            MIN_TIME_BW_UPDATES,
                            MIN_DISTANCE_CHANGE_FOR_UPDATES, this);
                    Log.d("GPS Enabled", "GPS Enabled");
                    if (locationManager != null) {
                        location = locationManager
                                .getLastKnownLocation(LocationManager.GPS_PROVIDER);
                        if (location != null) {
                            latitude = location.getLatitude();
                            longitude = location.getLongitude();
                        }
                    }
                }
            }
        }

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

    return location;
}

/**
 * Stop using GPS listener
 * Calling this function will stop using GPS in your app
 * */
public void stopUsingGPS(){
    if(locationManager != null){
        locationManager.removeUpdates(GPSTracker.this);
    }      
}

/**
 * Function to get latitude
 * */
public double getLatitude(){
    if(location != null){
        latitude = location.getLatitude();
    }

    // return latitude
    return latitude;
}

/**
 * Function to get longitude
 * */
public double getLongitude(){
    if(location != null){
        longitude = location.getLongitude();
    }

    // return longitude
    return longitude;
}

/**
 * Function to check GPS/wifi enabled
 * @return boolean
 * */
public boolean canGetLocation() {
    return this.canGetLocation;
}

/**
 * Function to show settings alert dialog
 * On pressing Settings button will lauch Settings Options
 * */
public void showSettingsAlert(){
    AlertDialog.Builder alertDialog = new AlertDialog.Builder(mContext);

    // Setting Dialog Title
    alertDialog.setTitle("GPS is settings");

    // Setting Dialog Message
    alertDialog.setMessage("GPS is not enabled. Do you want to go to settings menu?");

    // On pressing Settings button
    alertDialog.setPositiveButton("Settings", new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog,int which) {
            Intent intent = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
            mContext.startActivity(intent);
        }
    });

    // on pressing cancel button
    alertDialog.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog, int which) {
        dialog.cancel();
        }
    });

    // Showing Alert Message
    alertDialog.show();
}

@Override
public void onLocationChanged(Location location) {
    this.location = location;
}

@Override
public void onProviderDisabled(String provider) {
}

@Override
public void onProviderEnabled(String provider) {
}

@Override
public void onStatusChanged(String provider, int status, Bundle extras) {
}

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

我正在打电话

if(gps.isGPSEnabled){

            Toast.makeText(getBaseContext(),"GPS enabled", 
                    Toast.LENGTH_SHORT).show();
            baseurl = "http://my.careerfolio.net";
        }
        else{
            gps.showSettingsAlert();
        }

它总是显示设置警报而不是 toast 文本，有什么帮助吗？

我的日志。

06-20 02:54:06.336: W/System.err(909): java.lang.NullPointerException
06-20 02:54:06.416: W/System.err(909):  at android.content.ContextWrapper.getSystemService(ContextWrapper.java:519)
06-20 02:54:06.416: W/System.err(909):  at com.incv.mobile.freejscf.GPSTracker.getLocation(GPSTracker.java:50)
06-20 02:54:06.446: W/System.err(909):  at com.incv.mobile.freejscf.GPSTracker.<init>(GPSTracker.java:44)
06-20 02:54:06.446: W/System.err(909):  at com.incv.mobile.freejscf.MyApplication.<init>(MyApplication.java:34)
06-20 02:54:06.446: W/System.err(909):  at java.lang.Class.newInstanceImpl(Native Method)
06-20 02:54:06.456: W/System.err(909):  at java.lang.Class.newInstance(Class.java:1130)
06-20 02:54:06.486: W/System.err(909):  at android.app.Instrumentation.newApplication(Instrumentation.java:990)
06-20 02:54:06.486: W/System.err(909):  at android.app.Instrumentation.newApplication(Instrumentation.java:975)
06-20 02:54:06.486: W/System.err(909):  at android.app.LoadedApk.makeApplication(LoadedApk.java:504)
06-20 02:54:06.486: W/System.err(909):  at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4417)
06-20 02:54:06.486: W/System.err(909):  at android.app.ActivityThread.access$1300(ActivityThread.java:141)
06-20 02:54:06.486: W/System.err(909):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1316)
06-20 02:54:06.506: W/System.err(909):  at android.os.Handler.dispatchMessage(Handler.java:99)
06-20 02:54:06.506: W/System.err(909):  at android.os.Looper.loop(Looper.java:137)
06-20 02:54:06.516: W/System.err(909):  at android.app.ActivityThread.main(ActivityThread.java:5103)
06-20 02:54:06.516: W/System.err(909):  at java.lang.reflect.Method.invokeNative(Native Method)
06-20 02:54:06.516: W/System.err(909):  at java.lang.reflect.Method.invoke(Method.java:525)
06-20 02:54:06.546: W/System.err(909):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
06-20 02:54:06.546: W/System.err(909):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
06-20 02:54:06.546: W/System.err(909):  at dalvik.system.NativeStart.main(Native Method)

Answer 1

您正在尝试在 Location 类中获取位置管理器，而不是在 activity context 中。因此更改 Location 类实例 this 到 Activity 的上下文 mContext。所以改变这个

 locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);

到

 locationManager = (LocationManager) mContext.getSystemService(Context.LOCATION_SERVICE);

编辑
如果您使用 GPS 获取位置，则需要添加 ACCESS_FINE_LOCATION(包括 ACCESS_FINE_LOCATION 和 ACCESS_COARSE_LOCATION )。此外，如果您要获取基于网络的位置，则还需要添加 INTERNET permission。

因此，打开您的 AndroidManifest.xml 文件并进行以下更改。

<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />    
 <uses-permission android:name="android.permission.INTERNET" />