我写的Android位置定位不能定位

Question

我写了一个位置定项目，在模拟器上可以定位，但装到手机上，就不能定位了，请问是怎么回事？我已打开GPS，配置文件件里也配置了代码如下

import android.location.Location;
import android.location.LocationManager;
import android.os.Bundle;
import android.widget.TextView;

public class Main extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        LocationManager locationManager;
        String context = Context.LOCATION_SERVICE;
        locationManager = (LocationManager) getSystemService(context);
        
        String provider = LocationManager.GPS_PROVIDER;
        String provider2 = LocationManager.NETWORK_PROVIDER;
        Location location = locationManager.getLastKnownLocation(provider);
        updateWithNewLocation(location);
    }
    private void updateWithNewLocation(Location location){
        String latLongString;
        TextView myLocationText;
        myLocationText =(TextView) findViewById(R.id.tv);
        
        if(location != null){
            double lat = location.getLatitude();
            double lng = location.getLongitude();
            latLongString = "Lat: "+lat+"\nLong: "+lng;
        }else{
            latLongString = "No location found";
        }
        myLocationText.setText("Your Current Position is: \n"+ latLongString);
    }
}

 配置文件：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.action"
      android:versionCode="1"
      android:versionName="1.0">
    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".Main"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

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

 main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView  
    android:id="@+id/tv"
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="@string/hello"
    />
</LinearLayout>