Question

我无法从asynctask中的活动中获取结果。 Eclipse在doInBackground（）上显示运行时异常。当我更改构造函数时，它给出了空指针异常。请帮助我得到我的结果。

public class DataCtrl extends AsyncTask<Void, Void, String>{ int position; Context context; String strValue; String data = null; public AsyncResponse delegate=null; android.support.v4.app.FragmentTransaction myft; double lat; double longi; LocationCtrl gps; public DataCtrl(int position, android.support.v4.app.FragmentTransaction ft, Context con){ this.position = position; this.myft = ft; context = con; gps = new LocationCtrl(); } @Override protected void onPreExecute() { // TODO Auto-generated method stub } @Override protected String doInBackground(Void... params) { // TODO Auto-generated method stub double lat = 0; gps.turnGPSOn(); gps.getMyCurrentLocation(); Location loc = gps.getLastKnownLocation(context); if (loc != null) { lat = loc.getLatitude(); longi = loc.getLongitude(); } Log.i("lat", Double.toString(lat)); if(data == null) { data = GET("http://twyst.in/api/v2/data/28/77"); return data; } else { return data; } } public static String GET(String url){ InputStream inputStream = null; String result = ""; try { HttpClient httpclient = new DefaultHttpClient(); HttpResponse httpResponse = httpclient.execute(new HttpGet(url)); inputStream = httpResponse.getEntity().getContent(); if(inputStream != null) result = convertInputStreamToString(inputStream); else result = "Did not work!"; } catch (Exception e) { Log.d("InputStream", e.getLocalizedMessage()); } return result; } // convert inputstream to String private static String convertInputStreamToString(InputStream inputStream) throws IOException{ BufferedReader bufferedReader = new BufferedReader( new InputStreamReader(inputStream)); String line = ""; String result = ""; while((line = bufferedReader.readLine()) != null) result += line; inputStream.close(); return result; } Bundle bundle = new Bundle(); // onPostExecute displays the results of the AsyncTask. @Override protected void onPostExecute(String result) { setData(result, position); } public void setData(String result, int pos) { Fragment fragment = null; Bundle bundle = new Bundle(); bundle.putString("data", result); switch (pos) { case 0: Fragment forme= new ForMeActivity(); forme.setArguments(bundle); myft.replace(R.id.mainContent, forme); myft.commit(); break; case 1: Fragment my= new MyTwystActivity(); my.setArguments(bundle); myft.replace(R.id.mainContent, my); myft.commit(); break; case 2: Fragment near= new NearByActivity(); near.setArguments(bundle); myft.replace(R.id.mainContent, near); myft.commit(); break; default: break; } } } public class LocationCtrl extends Activity { private boolean gps_enabled=false; private boolean network_enabled=false; Location location; Double MyLat, MyLong; Context context; TextView textView2,textView3; Button button1; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); } /** Method to turn on GPS **/ public void turnGPSOn(){ try { @SuppressWarnings("deprecation") String provider = Settings.Secure.getString(getContentResolver(), Settings.Secure.LOCATION_PROVIDERS_ALLOWED); if(!provider.contains("gps")){ //if gps is disabled final Intent poke = new Intent(); poke.setClassName("com.android.settings", "com.android.settings.widget.SettingsAppWidgetProvider"); poke.addCategory(Intent.CATEGORY_ALTERNATIVE); poke.setData(Uri.parse("3")); sendBroadcast(poke); } } catch (Exception e) { } } // Method to turn off the GPS public void turnGPSOff(){ @SuppressWarnings("deprecation") String provider = Settings.Secure.getString(getContentResolver(), Settings.Secure.LOCATION_PROVIDERS_ALLOWED); if(provider.contains("gps")) { //if gps is enabled final Intent poke = new Intent(); poke.setClassName("com.android.settings", "com.android.settings.widget.SettingsAppWidgetProvider"); poke.addCategory(Intent.CATEGORY_ALTERNATIVE); poke.setData(Uri.parse("3")); sendBroadcast(poke); } } @Override protected void onDestroy() { // TODO Auto-generated method stub super.onDestroy(); turnGPSOff(); } void getMyCurrentLocation() { LocationManager locManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE); LocationListener locListener = new MyLocationListener(); try{gps_enabled=locManager.isProviderEnabled(LocationManager.GPS_PROVIDER);} catch(Exception ex){} try{network_enabled=locManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER);} catch(Exception ex){} //don't start listeners if no provider is enabled //if(!gps_enabled && !network_enabled) //return false; if(gps_enabled){ locManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locListener); } if(gps_enabled){ location=locManager.getLastKnownLocation(LocationManager.GPS_PROVIDER); } if(network_enabled && location==null){ locManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, locListener); } if(network_enabled && location==null) { location=locManager.getLastKnownLocation(LocationManager.NETWORK_PROVIDER); } if (location != null) { MyLat = location.getLatitude(); MyLong = location.getLongitude(); } else { Location loc= getLastKnownLocation(this); if (loc != null) { MyLat = loc.getLatitude(); MyLong = loc.getLongitude(); } } locManager.removeUpdates(locListener); // removes the periodic updates from location listener try { Geocoder geocoder; List<Address> addresses; geocoder = new Geocoder(this, Locale.getDefault()); addresses = geocoder.getFromLocation(MyLat, MyLong, 1); } catch (Exception e) { e.printStackTrace(); } textView2.setText(""+MyLat); textView3.setText(""+MyLong); } public Location getLastKnownLocation(Context context) { // TODO Auto-generated method stub Location location = null; LocationManager locationmanager = (LocationManager)context.getSystemService("location"); List<?> list = locationmanager.getAllProviders(); boolean i = false; Iterator<?> iterator = list.iterator(); do { if(!iterator.hasNext()) break; String s = (String)iterator.next(); if (i != false && !locationmanager.isProviderEnabled(s)) continue; Location location1 = locationmanager.getLastKnownLocation(s); if(location1 == null) continue; if(location != null) { float f = location.getAccuracy(); float f1 = location1.getAccuracy(); if(f >= f1) { long l = location1.getTime(); long l1 = location.getTime(); if(l - l1 <= 600000L) continue; } } location = location1; i = locationmanager.isProviderEnabled(s); } while(true); return location; }

}

logcat的：

07-10 09:04:23.760: E/AndroidRuntime(3163): FATAL EXCEPTION: AsyncTask #1 07-10 09:04:23.760: E/AndroidRuntime(3163): java.lang.RuntimeException: An error occured while executing doInBackground() 07-10 09:04:23.760: E/AndroidRuntime(3163): at android.os.AsyncTask$3.done(AsyncTask.java:200) 07-10 09:04:23.760: E/AndroidRuntime(3163): at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:274) 07-10 09:04:23.760: E/AndroidRuntime(3163): at java.util.concurrent.FutureTask.setException(FutureTask.java:125) 07-10 09:04:23.760: E/AndroidRuntime(3163): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:308) 07-10 09:04:23.760: E/AndroidRuntime(3163): at java.util.concurrent.FutureTask.run(FutureTask.java:138) 07-10 09:04:23.760: E/AndroidRuntime(3163): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1088) 07-10 09:04:23.760: E/AndroidRuntime(3163): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:581) 07-10 09:04:23.760: E/AndroidRuntime(3163): at java.lang.Thread.run(Thread.java:1019) 07-10 09:04:23.760: E/AndroidRuntime(3163): Caused by: java.lang.IllegalStateException: System services not available to Activities before onCreate() 07-10 09:04:23.760: E/AndroidRuntime(3163): at android.app.Activity.getSystemService(Activity.java:3536) 07-10 09:04:23.760: E/AndroidRuntime(3163): at com.twyst.android.LocationCtrl.getMyCurrentLocation(LocationCtrl.java:93) 07-10 09:04:23.760: E/AndroidRuntime(3163): at com.twyst.android.DataCtrl.doInBackground(DataCtrl.java:72) 07-10 09:04:23.760: E/AndroidRuntime(3163): at com.twyst.android.DataCtrl.doInBackground(DataCtrl.java:1) 07-10 09:04:23.760: E/AndroidRuntime(3163): at android.os.AsyncTask$2.call(AsyncTask.java:185) 07-10 09:04:23.760: E/AndroidRuntime(3163): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:306) 07-10 09:04:23.760: E/AndroidRuntime(3163): ... 4 more

MainActivity.java

public class MainActivity extends ActionBarActivity implements OnItemClickListener, AsyncResponse { private DrawerLayout drawerLayout; private ListView listView; private String[] menus; private ActionBarDrawerToggle drawerListener; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // dataCtrl.delegate = this; menus = getResources().getStringArray(R.array.menus); drawerLayout = (DrawerLayout) findViewById(R.id.drawerLayout); drawerListener = new ActionBarDrawerToggle(this, drawerLayout, R.drawable.ic_drawer, R.string.drawer_open, R.string.drawer_close) { @Override public void onDrawerOpened(View drawerView) { } @Override public void onDrawerClosed(View drawerView) { } }; listView = (ListView) findViewById(R.id.drawerList); listView.setAdapter(new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, menus)); listView.setOnItemClickListener(this); drawerLayout.setDrawerListener(drawerListener); getSupportActionBar().setHomeButtonEnabled(true); getSupportActionBar().setDisplayHomeAsUpEnabled(true); updateDisplay(0); } @Override public void onConfigurationChanged(Configuration newConfig) { // TODO Auto-generated method stub super.onConfigurationChanged(newConfig); drawerListener.onConfigurationChanged(newConfig); } @Override public boolean onOptionsItemSelected(MenuItem item) { if(drawerListener.onOptionsItemSelected(item)) { return true; } return super.onOptionsItemSelected(item); } @Override protected void onPostCreate(Bundle savedInstanceState) { // TODO Auto-generated method stub super.onPostCreate(savedInstanceState); drawerListener.syncState(); } @Override public void onItemClick(AdapterView<?> parent, View view, int position, long id) { Toast.makeText(this, menus[position], Toast.LENGTH_LONG).show(); updateDisplay(position); } public void updateDisplay(int position) { selectItem(position); android.support.v4.app.FragmentManager myfm = getSupportFragmentManager(); android.support.v4.app.FragmentTransaction myft = myfm.beginTransaction(); new DataCtrl(position, myft, this).execute(); } public void selectItem(int position) { listView.setItemChecked(position, true); setTitle(menus[position]); //selectItem(pos); drawerLayout.closeDrawer(listView); } public void setTitle (String title) { getSupportActionBar().setTitle(title); } @Override public void getData(String output, int position) { // TODO Auto-generated method stub }

}

Answer 1

问题是您甚至在活动中的onCreate()方法之前尝试访问某些系统级服务。

请从您的Activity中删除所有构造函数，并将构造函数内的处理移动到onCreate()方法。

编辑1：

因此，您正在使用活动的构造函数来访问活动方法，例如turnGPSOn()等，但这些方法需要系统服务来获取位置。因此，当您创建活动类的对象时，您会收到异常，因为当您的活动尚未创建但活动构造函数中的代码尝试调用时系统相关服务。

解决方案：您应该将每个GPS相关任务移动到您的活动中。例如。应在您的活动的onCreate（）方法内调用GPS方法。所以基本上，你的代码中有很多不正确的东西。我建议你用下面的例子重写。

简单示例：

public class MainActivity extends ActionBarActivity implements LocationListener {
    TextView tv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        /*
         * Turn on GPS Here. Basically everything that need to do before your
         * network connection should go here.
         */

        // Call your async task
        new GPSTask().execute();

    }

    private class GPSTask extends AsyncTask<Void, Void, Void> {

        @Override
        protected Void doInBackground(Void... params) {
            /** Make network request here */
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            super.onPostExecute(result);

            /**
             * Update Fragments here. 
             * To be able to access variables from the MainActivity. 
             * Declare them as instance variables;
             */

            tv.setText("onPostExecute");
        }

    }
}

Answer 2

在AsyncTask中，要获得Activity，您需要从该活动的context获取该活动，而不是创建该活动的构造函数。

由于活动构造函数中没有Context，因此它仅在Activity.onCreate()方法及更高版本中可用。

所以在DataCtrl构造函数更改

gps = new LocationCtrl();

到

gps = (Activity)context;

Answer 3

我会做这样的事情（仅用于查找您的位置）

在您的主要活动中：

public LocationClient mLocationClient;
ProgressDialog pd;
public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.your_layout_file, container, false);
        mLocationClient = new LocationClient(getActivity(), this, this);
        rootView.findViewById(R.id.locButton).setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        pd = ProgressDialog.show(getActivity(), "",
                                "Finding you!!!", false);
                        getAddress(v);

                    }

                });

getAddress方法：

public void getAddress(View v) {
    // In Gingerbread and later, use Geocoder.isPresent() to see if a
    // geocoder is available.
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD
            && !Geocoder.isPresent()) {
        // No geocoder is present. Issue an error message
        Toast.makeText(getActivity(), R.string.no_geocoder_available,
                Toast.LENGTH_LONG).show();
        return;
    }

    if (servicesConnected()) {
        // Turn the indefinite activity indicator on
        // Start the background task
        GetAddressTask getAddressTask = new GetAddressTask(getActivity(),
                mLocationClient, pd);
        getAddressTask.delegate = this;
        getAddressTask.execute(currentLocation);

    } else {
        Toast.makeText(getActivity(), "servicesConnected() == false",
                Toast.LENGTH_SHORT).show();
    }
}

异步类GetAddressTask：

public AsyncResponse delegate = null;
ProgressDialog pd;

protected String doInBackground() {
   // Show user progress        
   pd.show();       
   // Get the current location
   Location currentLocation = mLocationClient.getLastLocation();
   Lat = currentLocation.getLatitude();
   Lng = currentLocation.getLongitude();

   // Now do whatever else!
}

protected void onPostExecute(String  result) {
    delegate.processFinished(result);
    // Do other stuff
    // Kill Progress Dialog;
    pd.hide();
}

运行时异常在后台执行

3 个答案: