android - 尝试在空对象引用上调用虚拟方法 'android.content.Context android.support.v4.app.FragmentActivity.getApplicationContext()'

Question

以下是我的 fragment 代码，我得到上面的空异常错误

 /**
  * Created by USer on 09-04-2016.
  */
 public class FeaturedFragment extends android.support.v4.app.Fragment {
     private List url;
     private SliderLayout sliderLayout;
     private GridView gridView;
     private int firstVisiblepos;
     AppnextAPI api;

     @Nullable
     @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
         View rootView = inflater.inflate(
                 R.layout.featured, container, false);
         url = new ArrayList<String>();
         url.add("http://192.168.5.51/Wallpapers/Autumn.jpg");
         url.add("http://192.168.5.51/Wallpapers/Tree.jpg");
         url.add("http://192.168.5.51/Wallpapers/Flower.jpg");
         url.add("http://192.168.5.51/Wallpapers/Lion.jpg");
         api = new AppnextAPI(getActivity(), "my id");
         sliderLayout = (SliderLayout) rootView.findViewById(R.id.slider);
         gridView = (GridView) rootView.findViewById(R.id.gridView);
         ThemeShowcase theme = new ThemeShowcase();
         theme.execute("getThemes");
         for (int i = 0; i < url.size(); i++) {
             TextSliderView textSliderView = new TextSliderView(getActivity().getApplicationContext());
             textSliderView.image((String) url.get(i));
             sliderLayout.addSlider(textSliderView);
         }
         sliderLayout.setDuration(4000);
         sliderLayout.setPresetTransformer(SliderLayout.Transformer.Stack);
         return rootView;
     }


     private class ThemeShowcase extends AsyncTask<String, Integer, ArrayList<ThemeModel>> {

         private final String NAMESPACE = "http://tempuri.org/";
         private final String URL = "http://192.168.5.51/WebApplication/WebService.asmx";
         private final String SOAP_ACTION = "http://tempuri.org/";
         private String responseString;
         private ArrayList model = null;
         private ThemeModel dummy;

         @Override
         protected void onPreExecute() {
             super.onPreExecute();
             model = new ArrayList<ThemeModel>();
         }

         @Override
         protected ArrayList<ThemeModel> doInBackground(String... params) {
             try {
                 SoapObject request = new SoapObject(NAMESPACE, params[0]);
                 PropertyInfo sayHelloPI = new PropertyInfo();
                 sayHelloPI.setName("page");
                 sayHelloPI.setValue("1");
                 sayHelloPI.setType(Integer.class);
                 request.addProperty(sayHelloPI);
                 SoapSerializationEnvelope envelope = new SoapSerializationEnvelope(
                         SoapEnvelope.VER11);
                 envelope.dotNet = true;
                 envelope.setOutputSoapObject(request);
                 HttpTransportSE androidHttpTransport = new HttpTransportSE(URL);
                     androidHttpTransport.call(SOAP_ACTION + params[0], envelope);
                     SoapPrimitive response = (SoapPrimitive) envelope.getResponse();
                     responseString = response.toString();
                 if (!responseString.equalsIgnoreCase("Error occured")) {
                     JSONArray array = new JSONArray(responseString);
                     for (int i = 0; i < array.length(); i++) {
                         JSONObject objct = array.getJSONObject(i);
                         dummy = new ThemeModel();
                         dummy.preview = objct.getString("Preview");
                         dummy.name = objct.getString("Name");
                         model.add(dummy);
                     }
                 }

             } catch (Exception e) {
                 model = null;
                 return model;
             }
             return model;
         }

         @Override
         protected void onPostExecute(ArrayList<ThemeModel> model) {
             if(model!=null) {
                 gridView.setAdapter(new ImageAdapter(getActivity().getApplicationContext(), model));
                 firstVisiblepos = gridView.getFirstVisiblePosition();
             }else{
                 try {
                     Toast.makeText(getActivity().getApplicationContext(), "Site Unreachable", Toast.LENGTH_LONG).show();
                 }catch (Exception e){
                     Log.d("MyApp","exception");
                 }
             }
         }
     }
 }

这是我的图像适配器

 public class ImageAdapter extends BaseAdapter implements View.OnClickListener{

     private Context context;
     private LayoutInflater inflater;
     private ArrayList<ThemeModel> model;
     public ImageAdapter(Context context,ArrayList<ThemeModel> model) {
         this.inflater = LayoutInflater.from(context);
         this.context = context;
         this.model = model;

     }

     @Override
     public int getCount() {
         return model.size();
     }

     @Override
     public Object getItem(int position) {
         return null;
     }

     @Override
     public long getItemId(int position) {
         return 0;
     }

     @Override
     public View getView(int position, View convertView, ViewGroup parent) {
         convertView = inflater.inflate(R.layout.custom_item,null);
         CardView cardView = (CardView)convertView.findViewById(R.id.cardView);
         cardView.setOnClickListener(this);
         ImageView imageView = (ImageView)convertView.findViewById(R.id.imageView);
         final ProgressBar progressBar = (ProgressBar)convertView.findViewById(R.id.progressbar);
         TextView textView = (TextView)convertView.findViewById(R.id.contentName);
         ImageButton imageButton = (ImageButton)convertView.findViewById(R.id.downloadButton);
         textView.setText(model.get(position).getName());
         Picasso.with(context).load(model.get(position).getPreview()).into(imageView, new Callback() {
             @Override
             public void onSuccess() {
                 progressBar.setVisibility(View.GONE);
             }

             @Override
             public void onError() {

             }
         });
         return convertView;
     }

     @Override
     public void onClick(View v) {

     }
 }

我不断地在下面的线上施加压力

gridView.setAdapter(new ImageAdapter(getActivity().getApplicationContext(), model));

错误是

java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.support.v4.app.FragmentActivity.getApplicationContext()' on a null object reference

我尝试过使用 getActivity()、getContext() 等，但它们都不起作用

Answer 1

原因是因为当你在onPostExecute()中调用getActivity()时，Fragment已经从中分离出来了Activity(例如，当 AsyncTask 正在执行时，用户单击后退按钮退出 Activity)，因此 getActivity() 将为 null。

解决方法是在getActivity()之前，检查isAdded()是否为真，如果不为真，说明 fragment 已经分离，调用 >getActivity() 将返回 null。

请记住，在 AsyncTask 中，在调用 getActivity() 之前，您最好再次检查 isAdded()，因为用户可能会在任何时候退出 Activity AsyncTask 正在执行。