java NullPointerException 与 LocationManager

Question

我正在构建一个 Android 应用程序，它使用 LocationManager 从用户处获取纬度和经度，然后将这些变量输入 URL，该 URL 在 WebView 中打开。

出于某种原因，在某些 Android 设备和我的 Eclipse 模拟器上，当此 Activity 尝试打开时应用程序崩溃。我的崩溃报告中出现 NPE 错误。

我已将错误确定为以下两行代码:

double longitude = lastKnownLocation.getLongitude();
double latitude = lastKnownLocation.getLatitude();

当我将这两个变量设置为静态值(例如 1)时，应用程序可以正确运行。

下面是我的完整 page1.activity。有人可以帮忙吗？

package com.xxx.yyy;

import android.annotation.SuppressLint;
import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.graphics.Bitmap;
import android.net.Uri;
import android.os.Bundle;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.webkit.GeolocationPermissions;
import android.webkit.WebChromeClient;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.webkit.GeolocationPermissions.Callback;
import com.xxx.yyy.R;

import com.actionbarsherlock.view.MenuItem;
import com.devspark.sidenavigation.ISideNavigationCallback;
import com.devspark.sidenavigation.SideNavigationView;

public class page1 extends MainActivity implements ISideNavigationCallback, GeolocationPermissions.Callback {

    public static final String EXTRA_TITLE = "com.devspark.sidenavigation.sample.extra.MTGOBJECT";
    public static final String EXTRA_RESOURCE_ID = "com.devspark.sidenavigation.sample.extra.RESOURCE_ID";

    private ImageView icon;
    private SideNavigationView sideNavigationView;
    ProgressDialog mProgress;

    @SuppressLint("SetJavaScriptEnabled") 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_page1);
        icon = (ImageView) findViewById(android.R.id.icon);
        sideNavigationView = (SideNavigationView) findViewById(R.id.side_navigation_view);
        sideNavigationView.setMenuItems(R.menu.side_navigation_menu);
        sideNavigationView.setMenuClickCallback(this);

        if (getIntent().hasExtra(EXTRA_TITLE)) {
            String title = getIntent().getStringExtra(EXTRA_TITLE);
            int resId = getIntent().getIntExtra(EXTRA_RESOURCE_ID, 0);
            setTitle(title);
            icon.setImageResource(resId);
        }

        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        //Get webview layout
         WebView engine = (WebView) findViewById(R.id.webView1);

         //loading
         mProgress = ProgressDialog.show(this, "Loading", "Finding locations near you");



         engine.setWebChromeClient(new WebChromeClient() {
                 @Override
                public void onProgressChanged(WebView view, int progress)
                 {


                 }
         });


         engine.setWebViewClient(new FixedWebViewClient() {
                 @Override
                public void onPageStarted(WebView view, String url, Bitmap favicon)
                 {
                     findViewById(R.id.progressBar1).setVisibility(view.VISIBLE);
                 }

                 @Override
                public void onPageFinished(WebView view, String url){ 

                     findViewById(R.id.progressBar1).setVisibility(view.GONE);

                     if(mProgress.isShowing()) { 
                         mProgress.dismiss();  }

               //error handeling     
                 }
                 @Override
                public void onReceivedError(WebView webView, int errorCode, String description, String failingUrl) {
                    try {
                        webView.stopLoading();
                    } catch (Exception e) {
                    }
                    try {
                        webView.clearView();
                    } catch (Exception e) {
                    }
                    if (webView.canGoBack()) {
                        webView.goBack();
                    }
                    webView.loadUrl("file:///android_asset/error.html"); //background to message dialog, or page you want to load if requested page is offline
                    AlertDialog alertDialog = new AlertDialog.Builder(page1.this).create(); //remove these 4 rules if you load an assets page instead.
                    alertDialog.setTitle("Error");
                    alertDialog.setMessage("No internet connection was found!");
                    alertDialog.setButton("Retry", new DialogInterface.OnClickListener() {
                       @Override
                    public void onClick(DialogInterface dialog, int which) {
                           finish();
                           startActivity(getIntent());
                       }
                    });

                    alertDialog.show();
                    super.onReceivedError(webView, errorCode, description, failingUrl);
                }

         });

         LocationManager lm = (LocationManager)getSystemService(Context.LOCATION_SERVICE); 
         Location location = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);

         LocationListener locationListener = new LocationListener() {
                public void onLocationChanged(Location location) {
                  // Called when a new location is found by the network location provider.

                    double longitude = location.getLongitude();
                    double latitude = location.getLatitude();

                }

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

                public void onProviderEnabled(String provider) {}

                public void onProviderDisabled(String provider) {}
              };

            // Register the listener with the Location Manager to receive location updates
            lm.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, locationListener);

            String locationProvider = LocationManager.NETWORK_PROVIDER;
            // Or use LocationManager.GPS_PROVIDER

            Location lastKnownLocation = lm.getLastKnownLocation(locationProvider);

            double longitude = lastKnownLocation.getLongitude();
            double latitude = lastKnownLocation.getLatitude();

         engine.loadUrl("http://www.turn2clients.com/app/home/iPhone/dev/?s=&geo-radius=3&geo=on&geo-lat="+latitude+"&geo-lng="+longitude+"&categories=0&locations=0&dir-search=yes");
         //if you would like to load local assets file, replace with file:///android_asset/error.html


 }

    public void invoke(String origin, boolean allow, boolean remember) {

    }

    final class GeoClient extends WebChromeClient {

    @Override
    public void onGeolocationPermissionsShowPrompt(String origin,
    Callback callback) {
    }

    }




 private class FixedWebViewClient extends WebViewClient {
         @Override
         public boolean shouldOverrideUrlLoading(WebView view, String url) {
                 view.loadUrl(url);

                 //start a new application/activity (in this case youtube) if url starts with 
                if (url.startsWith("mlb")){

                 return true;
                 }
                 else
                 {
                 view.loadUrl(url);

                 return true;
         }
         }

 }



    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case android.R.id.home:
            sideNavigationView.toggleMenu();
            break;
        default:
            return super.onOptionsItemSelected(item);
        }
        return true;
    }

    @Override
    public void onSideNavigationItemClick(int itemId) {
        switch (itemId) {
        case R.id.side_navigation_menu_item1:
            Intent page1 = new Intent(this, page1.class);     
            startActivity(page1);
            break;

        case R.id.side_navigation_menu_item2:
            Intent page2 = new Intent(this, page2.class);     
            startActivity(page2);
            break;

        case R.id.side_navigation_menu_item3:
            Intent page3 = new Intent(this, page3.class);     
            startActivity(page3);
            break;

        default:
            return;
        }
        finish();
    }

    @Override
    public void onBackPressed() {
        //This part will close you app if a certain url is visited, ofcourse you can replace this with any other action.
        WebView engine = (WebView) findViewById(R.id.webView1);
        String url = engine.getUrl(); 
        if (url.equals("http://closeifthisurlisvisited.com/") ||
                url.equals("http://alsocloseifthisurlisvisited.com")) {
                // exit
                super.onBackPressed();
        } else {
                // Go back when back button is pressed
                engine.goBack();
        }
        //thats it
        // hide menu if it shown with backbutton
        if (sideNavigationView.isShown()) {
            sideNavigationView.hideMenu();
        } else {
            //super.onBackPressed();
        }
    }

    /**
     * Start activity from SideNavigation.
     * 
     * @param title
     *            title of Activity
     * @param resId
     *            resource if of background image
     */
    private void invokeActivity(String title, int resId) {
        Intent intent = new Intent(this, MainActivity.class);
        intent.putExtra(EXTRA_TITLE, title);
        intent.putExtra(EXTRA_RESOURCE_ID, resId);

        // all of the other activities on top of it will be closed and this
        // Intent will be delivered to the (now on top) old activity as a
        // new Intent.
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

        startActivity(intent);
        // no animation of transition
        overridePendingTransition(0, 0);
    }

}

Answer 1

最好的方法是使用 CWAC 的位置服务 Location Poller 服务已经为我们提供了使用它，只需给出唤醒它的时间间隔

按照这样的方式进行操作，您将需要 jar 文件，您可以从[此处] https://www.dropbox.com/sh/pgxk2v9l5vl0h2j/3svyZnuwOK/CWAC-LocationPoller.jar 获取该文件。

从您的 Activity 中启动 LocationPoller 并将闹钟设置为您想要的重复时间

AlarmManager alarmMgr = (AlarmManager) getSystemService(ALARM_SERVICE);
Intent i = new Intent(this, LocationPoller.class);

Bundle bundle = new Bundle();
LocationPollerParameter parameter = new LocationPollerParameter(bundle);
parameter.setIntentToBroadcastOnCompletion(new Intent(this,
        LocationReceiver.class));
// try GPS and fall back to NETWORK_PROVIDER
parameter.setProviders(new String[] { LocationManager.GPS_PROVIDER });
parameter.setTimeout(120000);
i.putExtras(bundle);

PendingIntent pi = PendingIntent.getBroadcast(this, 0, i, 0);
alarmMgr.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP,
        SystemClock.elapsedRealtime(), 300000, pi);

创建一个接收器类位置接收器，从中获取经纬度

public class LocationReceiver extends BroadcastReceiver {
  @Override
  public void onReceive(Context context, Intent intent) {


    try {      
      Bundle b=intent.getExtras();

      LocationPollerResult locationResult = new LocationPollerResult(b);

      Location loc=locationResult.getLocation();
      String msg;

      if (loc==null) {
        loc=locationResult.getLastKnownLocation();

        if (loc==null) {
          msg=locationResult.getError();
        }
        else {
          msg="TIMEOUT, lastKnown="+loc.toString();
        }
      }
      else {
        msg=loc.toString();



        Log.i("Location Latitude", String.valueOf(loc.getLatitude()));
        Log.i("Location Longitude", String.valueOf(loc.getLongitude()));
        Log.i("Location Accuracy", String.valueOf(loc.getAccuracy()));




      }

      Log.i(getClass().getSimpleName(), "received location: " + msg);   



    }
    catch (Exception e) {
      Log.e(getClass().getName(), e.getMessage());
    }
  }

并将其添加到您的 list 中

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

   <receiver android:name="com.commonsware.cwac.locpoll.LocationPoller" />

   <service android:name="com.commonsware.cwac.locpoll.LocationPollerService" />

以及接收者声明，您将在其中获取所有内容

<receiver android:name="com.RareMediaCompany.Helios.LocationReceiver" />