LocationListener onLocationChanged不会发生

Question

您好我有一个应该使用LocationListener获取当前用户位置的类

public class GpsSender {
 static Context ctx;
 static LocationManager lm;
 static Location location;
 static double longitude;
 static double latitude;
 static LocationListener locationListener;

 public GpsSender(Context ctx) {
     this.ctx = ctx;
     lm = (LocationManager) ctx.getSystemService(Context.LOCATION_SERVICE);
     locationListener = new LocationListener() {
         public void onLocationChanged(Location location) {
             Log.i("LocationListener","Location changed!");
             longitude = location.getLongitude();
             latitude = location.getLatitude();
         }

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

         @Override
         public void onProviderEnabled(String provider) {
         }

         @Override
         public void onProviderDisabled(String provider) {
         }
     };

     lm.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);
 }

 static public void printLoc() {
     Log.i("TEST", "lat:"+latitude+" lon:"+longitude);
 }
}

它适用于模拟器，我通过控制台设置lat lon，但是当我在真实设备上尝试时，lat lon始终为0.00。 onLocationChanged，甚至都不会发生。