Android中异步任务代码的奇怪行为

Question

我是android和异步任务的新手。 我正在开发一个应用程序，在点击按钮时打开布局并搜索他的设备纬度和经度。 然后将这些纬度和经度发送到服务器，以便从最近到最远的地方获取分支的详细信息。 我得到一个Null指针异常无法理解AsyncTask是如何工作的AsyncTask执行永远不会被调用。以下是代码

public class FindUs extends ExpandableListActivity implements OnChildClickListener{

GeoPoint geoPoint;
public static String cities[];
LocationManager mlocManager;
double lat,lng; 
LocationListener mlocListener = new MyLocationListener();
ProgressDialog progDailog = null;

RequestTask reqTask; 
SimpleExpandableListAdapter expListAdapter;

public String city_details[][] = {   };

    @Override
public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);

    setContentView(R.layout.findus_layout);

progDailog = new ProgressDialog(FindUs.this);
    progDailog.setMessage("Loading...");

    progDailog.show();
   mlocManager = (LocationManager)getSystemService(Context.LOCATION_SERVICE);
mlocManager.requestLocationUpdates( LocationManager.GPS_PROVIDER, 0, 0, mlocListener);

try {
    reqTask = new RequestTask();
    reqTask.execute( "http://192.168.1.239:8080/StrutsExample/FindUS.slt");

    } catch (Exception e) {
        e.printStackTrace();
    }



 expListAdapter =
        new SimpleExpandableListAdapter(
            this,
            createGroupList(),// groupData describes the first-level entries
            R.layout.child_row, // Layout for the first-level entries
            new String[] { "cityBranch" },  // Key in the groupData maps to display
            new int[] { R.id.childname },       // Data under "child" key goes into this TextView
            createChildList(),  // childData describes second-level entries
            R.layout.child_row, // Layout for second-level entries
            new String[] { "cityBranchDetails", "rgb" },    // Keys in childData maps to display
            new int[] { R.id.childname, R.id.rgb }  // Data under the keys above go into these TextViews
        );

    setListAdapter( expListAdapter );


private List createGroupList() {

          ArrayList result = new ArrayList();
//getting null pointer exception when populating cities


for( int i = 0 ; i < cities.length ; ++i ) {
            HashMap m = new HashMap();
            m.put( "cityBranch",cities[i] );
            result.add( m );
          }
          return (List)result;
        }
 private List createChildList() {
        ArrayList result = new ArrayList();
        for( int i = 0 ; i < city_details.length ; ++i ) {
    // Second-level lists
          ArrayList secList = new ArrayList();
          for( int n = 0 ; n < city_details[i].length ; n += 2 ) {

            HashMap child = new HashMap();
            child.put( "cityBranchDetails", city_details[i][n] );
            child.put( "rgb", city_details[i][n+1] );

            secList.add( child );
          }
          result.add( secList );
        }
        return result;
      }


//Location Listener Class

 public class MyLocationListener implements LocationListener {

        @Override
        public void onLocationChanged(Location loc){

            double latitude = loc.getLatitude();

            double longitude = loc.getLongitude();

            String Text = "My current location is: " +  "Latitude = " + latitude +  "Longitude = " + longitude;

            Toast.makeText( FindUs.this,Text,   Toast.LENGTH_SHORT).show();

String latLongString = "";
        if (loc != null) {
            latLongString = "Lat:" + latitude + "\nLong:" + longitude;
            TextView myLocationText = (TextView)findViewById(R.id.FindUsTextView);
            myLocationText.setText("Your Current Position is:\n" + latLongString);

            progDailog.dismiss();

        } 

        Toast.makeText( FindUs.this, Text, Toast.LENGTH_SHORT).show();
        mlocManager.removeUpdates(mlocListener);

        }

        @Override

        public void onProviderDisabled(String provider){

        Toast.makeText( getApplicationContext(),"Gps Disabled, Please Enable the GPS", Toast.LENGTH_SHORT ).show();

        }

        @Override

        public void onProviderEnabled(String provider){

        Toast.makeText( getApplicationContext(),"Gps Enabled",Toast.LENGTH_SHORT).show();

        }

        @Override

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

        }
        }



// now the AsyncTask class

class RequestTask extends AsyncTask<String, String, String>{

            @Override
            protected String doInBackground(String... uri) {
                HttpClient httpclient = new DefaultHttpClient();
                HttpResponse response;
                String responseString = null;

                try {
                    HttpPost requestDetails = new HttpPost(uri[0]);
                    SharedPreferences findUsSetDetailsSharedPreference = getSharedPreferences("gpsdetails",MODE_PRIVATE);
                    String latitude = findUsSetDetailsSharedPreference.getString("latitude", "");
                    String longitude = findUsSetDetailsSharedPreference.getString("longitude", "");
                    ArrayList<NameValuePair> postParameters = new ArrayList<NameValuePair>();
                    postParameters.add(new BasicNameValuePair("latitude", latitude));
                    postParameters.add(new BasicNameValuePair("longitude", longitude));
                    requestDetails.setEntity(new UrlEncodedFormEntity(postParameters, HTTP.UTF_8));

                    response = httpclient.execute(requestDetails);
                    StatusLine statusLine = response.getStatusLine();
                    if(statusLine.getStatusCode() == HttpStatus.SC_OK){
                        ByteArrayOutputStream out = new ByteArrayOutputStream();
                        response.getEntity().writeTo(out);
                        out.close();
                        responseString = out.toString();
                        System.out.println("------------------------->"+responseString);
                    } else{
                        //Closes the connection.
                        response.getEntity().getContent().close();
                        throw new IOException(statusLine.getReasonPhrase());
                    }
                } catch (ClientProtocolException e) {
                    //TODO Handle problems..
                } catch (IOException e) {
                    //TODO Handle problems..
                }

                String []latandLong = responseString.split("\t");



/********************************************************

below we can see I am populating cities String array variables of FindUs
***********************************************************************************/

                FindUs.cities = new String[latandLong.length];
                for(int i=0 ; i<latandLong.length;i++)
                {
                    FindUs.cities[i] = latandLong[i];
                }           
                System.out.println(cities.length);
                System.out.println(cities);

                return responseString;
            }


 @Override
        protected void onDestroy() {
        super.onDestroy();
        if (mlocManager != null) {
            mlocManager.removeUpdates(mlocListener);
            mlocManager = null;
        }
        }

        @Override
        public void onConfigurationChanged(final Configuration newConfig)
        {
            // Ignore orientation change to keep activity from restarting
            super.onConfigurationChanged(newConfig);
        }



}


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

            }
        }

但它永远不会得到reqTask.execute（）;已执行但未填写变量。 期待你的回复。 感谢。

下面的

是堆栈跟踪

     01-04 19:29:42.759: E/AndroidRuntime(305): FATAL EXCEPTION: main
    01-04 19:29:42.759: E/AndroidRuntime(305): java.lang.RuntimeException: Unable to start activity 
    ComponentInfo{com.stylingandroid.IntelligentLayout/com.stylingandroid.IntelligentLayout.FindUs}: java.lang.NullPointerException

    01-04 19:29:42.759: E/AndroidRuntime(305):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at android.os.Handler.dispatchMessage(Handler.java:99)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at android.os.Looper.loop(Looper.java:123)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at android.app.ActivityThread.main(ActivityThread.java:4627)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at java.lang.reflect.Method.invokeNative(Native Method)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at java.lang.reflect.Method.invoke(Method.java:521)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)

    01-04 19:29:42.759: E/AndroidRuntime(305):  at dalvik.system.NativeStart.main(Native Method)

    01-04 19:29:42.759: E/AndroidRuntime(305): Caused by: java.lang.NullPointerException

01-04 19:29:42.759: E/AndroidRuntime(305):  at 
`com.stylingandroid.IntelligentLayout.FindUs.createGroupList(FindUs.java:353)
01-04 19:29:42.759: E/AndroidRuntime(305):  at` 

    com.stylingandroid.IntelligentLayout.FindUs.onCreate(FindUs.java:200)
01-04 19:29:42.759: E/AndroidRuntime(305):  at 

`android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
01-04 19:29:42.759: E/AndroidRuntime(305):  at `


    android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
01-04 19:29:42.759: E/AndroidRuntime(305):  ... 11 more

异常总是发生在用于填充城市的函数createGroupList中。

Answer 1

第二个代码块何时执行？由于代码片段太多，我在查看正在发生的事情时遇到了一些麻烦，但是如果您的AsyncTask需要在继续之前完成，那么您应该从任务中添加一个回调，以便在任务时运行其余的代码已完成。 AsyncTask将您带到onPostExecute()中的主线程，因此您可以使用那里的数据。

Answer 2

很难说没有在一个连续的块中看到你的代码，但看起来你在asynctask完成之前创建了你的适配器（它引用了城市）。因此它为null，这就是为什么在执行asynctask之前你得到一个空指针。

为什么不尝试在asynctask的onPostExecute方法中初始化并将适配器分配给listview ...

Answer 3

AsyncTask在另一个线程中执行。在调用execute（）之后，只要进入下一行代码，就不能指望它完成。任何依赖于任务完成的事情都需要在任务的onPostExecute()方法中发生。