我开始学习android和当前项目中的服务，即使应用程序关闭，我也希望在后台跟踪位置，但是当我从主活动调用服务的意图时整个应用程序崩溃。我已经包含了这项服务：
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.location.Location;
import android.location.LocationManager;
import android.os.Bundle;
import android.os.IBinder;
import android.util.Log;
import android.widget.Toast;
/**
* Created by Gurchani on 4/21/2017.
*/
public class service extends Service {
private static final String TAG = "BOOMBOOMTESTGPS";
private LocationManager mLocationManager = null;
private static final int LOCATION_INTERVAL = 1000;
private static final float LOCATION_DISTANCE = 10f;
private localDatabase database;
SQLiteDatabase db;
private double UserLong;
private double UserLat;
private class LocationListener implements android.location.LocationListener
{
Location mLastLocation;
public LocationListener(String provider)
{
Log.e(TAG, "LocationListener " + provider);
mLastLocation = new Location(provider);
makeToast();
}
@Override
public void onLocationChanged(Location location)
{
Log.e(TAG, "onLocationChanged: " + location);
mLastLocation.set(location);
UserLat = mLastLocation.getLatitude();
UserLong = mLastLocation.getLongitude();
makeToast();
doSQLQuery();
}
@Override
public void onProviderDisabled(String provider)
{
Log.e(TAG, "onProviderDisabled: " + provider);
}
@Override
public void onProviderEnabled(String provider)
{
Log.e(TAG, "onProviderEnabled: " + provider);
}
@Override
public void onStatusChanged(String provider, int status, Bundle extras)
{
Log.e(TAG, "onStatusChanged: " + provider);
}
}
private void makeToast() {
Toast.makeText(this,"its on baby", Toast.LENGTH_LONG).show();
}
private int doSQLQuery() {
double distance = 0.1;
// Define a projection that specifies which columns from the database
// you will actually use after this query.
String[] projection = {
FeederClass.FeedEntry.barId,
};
// Filter results WHERE "title" = 'My Title'
Cursor c = dis(String.valueOf(Math.cos((double) distance / (double) 6380)), Math.cos(deg2rad(UserLat)), Math.sin(deg2rad(UserLat)), Math.cos(deg2rad(UserLong)), Math.sin(deg2rad(UserLong)));
Toast.makeText(this,"Your message.", Toast.LENGTH_LONG).show();
return c.getCount();
}
LocationListener[] mLocationListeners = new LocationListener[] {
new LocationListener(LocationManager.GPS_PROVIDER),
new LocationListener(LocationManager.NETWORK_PROVIDER)
};
@Override
public IBinder onBind(Intent arg0)
{
return null;
}
@Override
public int onStartCommand(Intent intent, int flags, int startId)
{
Log.e(TAG, "onStartCommand");
super.onStartCommand(intent, flags, startId);
return START_STICKY;
}
@Override
public void onCreate()
{
Log.e(TAG, "onCreate");
initializeLocationManager();
db = database.getReadableDatabase();
try {
mLocationManager.requestLocationUpdates(
LocationManager.NETWORK_PROVIDER, LOCATION_INTERVAL, LOCATION_DISTANCE,
mLocationListeners[1]);
} catch (java.lang.SecurityException ex) {
Log.i(TAG, "fail to request location update, ignore", ex);
} catch (IllegalArgumentException ex) {
Log.d(TAG, "network provider does not exist, " + ex.getMessage());
}
try {
mLocationManager.requestLocationUpdates(
LocationManager.GPS_PROVIDER, LOCATION_INTERVAL, LOCATION_DISTANCE,
mLocationListeners[0]);
} catch (java.lang.SecurityException ex) {
Log.i(TAG, "fail to request location update, ignore", ex);
} catch (IllegalArgumentException ex) {
Log.d(TAG, "gps provider does not exist " + ex.getMessage());
}
}
@Override
public void onDestroy()
{
Log.e(TAG, "onDestroy");
super.onDestroy();
if (mLocationManager != null) {
for (int i = 0; i < mLocationListeners.length; i++) {
try {
mLocationManager.removeUpdates(mLocationListeners[i]);
} catch (Exception ex) {
Log.i(TAG, "fail to remove location listners, ignore", ex);
}
}
}
}
private void initializeLocationManager() {
Log.e(TAG, "initializeLocationManager");
if (mLocationManager == null) {
mLocationManager = (LocationManager) getApplicationContext().getSystemService(Context.LOCATION_SERVICE);
}
}
public Cursor dis(String dis, double cos_lat_rad, double sin_lat_rad, double cos_lon_rad, double sin_lon_rad) {
Cursor cursor = db.rawQuery("SELECT barID ,(" + sin_lat_rad + "*\"SinLat\"+" + cos_lat_rad + "*\"CosLat\"*(" + sin_lon_rad + "*\"SinLong\"+" + cos_lon_rad + "*\"CosLong\")) AS \"distance_acos\" FROM BarDetails WHERE ("+sin_lat_rad+" * \"SinLat\" +"+ cos_lat_rad +"* \"CosLat\" * (+"+sin_lon_rad +"* \"SinLong\" + "+cos_lon_rad +"* \"CosLong\")) >"+dis+ " ORDER BY \"distance_acos\" DESC ", null);
return cursor;
}
public static double deg2rad(double deg) {
return (deg * Math.PI / 180.0);
}
}
我在我的主要活动中就是这样称呼这项服务：
Intent intent = new Intent(MainActivity.this , service.class);
startService(intent);
在我的清单文件中，我包括以下内容：
<service android:name=".service" android:process=":my_service" />
这是我的logcat
04-22 21:11:00.960 18698-18698/com.example.android.find:my_service E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.find:my_service, PID: 18698
java.lang.RuntimeException: Unable to instantiate service com.example.android.findbar.service: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
at android.app.ActivityThread.handleCreateService(ActivityThread.java:2962)
at android.app.ActivityThread.access$1800(ActivityThread.java:178)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1553)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:194)
at android.app.ActivityThread.main(ActivityThread.java:5631)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:959)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:754)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
at android.content.ContextWrapper.getResources(ContextWrapper.java:86)
at android.widget.Toast.<init>(Toast.java:106)
at android.widget.Toast.makeText(Toast.java:263)
at com.example.android.findbar.service.makeToast(service.java:71)
at com.example.android.findbar.service.access$000(service.java:19)
at com.example.android.findbar.service$LocationListener.<init>(service.java:37)
at com.example.android.findbar.service.<init>(service.java:90)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1606)
at android.app.ActivityThread.handleCreateService(ActivityThread.java:2959)
at android.app.ActivityThread.access$1800(ActivityThread.java:178)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1553)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:194)
at android.app.ActivityThread.main(ActivityThread.java:5631)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:959)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:754)
答案 0 :(得分：0)
步骤1：摆脱makeToast()。
步骤2：等待初始化服务的字段（例如mLocationListeners），直到服务上调用onCreate()。
目前，您尝试在字段初始值设定项中显示Toast，但由于Service尚未设置，因此无效。