NullPointerException for ProgressDialog in AsyncTask [duplicate]

问题

I have an AsyncTask class within another class in a separate file from MainActivity

GetWarehouseSales.java

public class GetWarehouseSales extends Activity {

    private String TAG = MainActivity.class.getSimpleName();
    private ListView lv;
    private ProgressDialog pDialog;
    //URL to get JSON details
    private static String url = "http://192.168.0.1/mycc/retrieve_ws.php";
    ArrayList<HashMap<String,String>> sales_details;

    public GetWarehouseSales(){
    }

    public void executeGWS(){
        new RetrieveWarehouseSalesTask().execute();
    }


    public class RetrieveWarehouseSalesTask extends AsyncTask<Void,Void,Void>{

        public RetrieveWarehouseSalesTask(){

        }

        @Override
        protected void onPreExecute(){
            super.onPreExecute();
            pDialog = new ProgressDialog(GetWarehouseSales.this);
            pDialog.setMessage("Getting you the best warehouse sales...");
            pDialog.setCancelable(false);
            pDialog.show();
        }

        @Override
        protected Void doInBackground(Void... arg0){
            HttpHandler sh = new HttpHandler();
            //making a request to URL and getting response
            String jsonStr = sh.makeServiceCall(url);
            Log.e(TAG, "Response from url: " + jsonStr);

            if(jsonStr != null){
                try{
                    JSONObject jsonObj = new JSONObject(jsonStr);
                    //Getting JSON Array Node
                    JSONArray sales = jsonObj.getJSONArray("Result");
                    //looping through all results
                    for(int i = 0; i<sales.length();i++){
                        JSONObject s = sales.getJSONObject(i);
                        String title = s.getString("title");
                        String description = s.getString("description");

                        HashMap<String,String> salesDetails = new HashMap<>();

                        //adding each child node to HashMap key =>value
                        salesDetails.put("title",title);
                        salesDetails.put("description",description);

                        //adding to array list
                        sales_details.add(salesDetails);
                    }
                    Log.d("TAG",sales_details.toString());
                }catch(final JSONException e){
                    Log.e(TAG, "JSON parsing error: " + e.getMessage());
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            Toast.makeText(getApplicationContext(),"JSON parsing error: " + e.getMessage(),Toast.LENGTH_LONG).show();
                        }
                    });
                }
            }else{
                Log.e(TAG,"Couldn't get json from server");
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(getApplicationContext(), "Check logcat", Toast.LENGTH_LONG ).show();
                    }
                });
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void result){
            super.onPostExecute(result);
            if(pDialog.isShowing()){
                pDialog.dismiss();
            }

            //update parsed JSON data into ListView
            ListAdapter adapter = new SimpleAdapter(GetWarehouseSales.this, sales_details,R.layout.item_listview, new String[]{
                    "title","description"}, new int[]{R.id.text,R.id.description});

            lv.setAdapter(adapter);
        }
    }


}

MainActivity.java

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        GetWarehouseSales gws = new GetWarehouseSales();
        gws.executeGWS();

    }

The error:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference

The error point to this line:

pDialog = new ProgressDialog(GetWarehouseSales.this);

I would like to execute the AsyncTask RetrieveWarehouseSalesTask which can be found in GetWarehouseSales.java in MainActivity.java but getting NPE errors.

回答1:

try passing the MainActivity Context..

 @Override
protected void onCreate(Bundle savedInstanceState) {
    GetWarehouseSales gws = new GetWarehouseSales(MainActivity.this);
    gws.executeGWS();

}

Retreive the context:

private Activity activityContext;


 public GetWarehouseSales(Activity context){
     this.activityContext=context;
}

Now initialize your ProgressDialog using that context

 @Override
    protected void onPreExecute(){
        super.onPreExecute();
        pDialog = new ProgressDialog(activityContext);
        pDialog.setMessage("Getting you the best warehouse sales...");
        pDialog.setCancelable(false);
        pDialog.show();
    }

or ..try using

 pDialog = new ProgressDialog(getApplicationContext());

回答2:

This is the reason of the error:

    GetWarehouseSales gws = new GetWarehouseSales();
    gws.executeGWS();

You create an instance of activity, and when the AsyncTask is executing, try to get access to the Context.

Looks like the simpliest fix is to place RetrieveWarehouseSalesTask into MainActivity, something like:

class MainActivity ... {
    ...
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ...

        new RetrieveWarehouseSalesTask().execute();
    }

    public class RetrieveWarehouseSalesTask extends AsyncTask<Void,Void,Void>{
    ...
}

回答3:

hope this will help you..

@Override
protected void onCreate(Bundle savedInstanceState) {
    GetWarehouseSales gws = new GetWarehouseSales(this);
    gws.executeGWS();

}

Retreive the context:

private Context activityContext;

 public GetWarehouseSales(Context context){
     this.activityContext=context;
}

Now initialize your ProgressDialog using that context

@Override
    protected void onPreExecute(){
        super.onPreExecute();
        pDialog = new ProgressDialog(activityContext);
        pDialog.setMessage("Getting you the best warehouse sales...");
        pDialog.setCancelable(false);
        pDialog.show();
    }

来源：https://stackoverflow.com/questions/41414917/nullpointerexception-for-progressdialog-in-asynctask

标签

java

android

android-asynctask