解析JSONArray并在ListView中填充它

Question

以下是Activity，它应该从JSON中获取值（用户），并将它们放在ArrayList<User>中，其中User是我创建的类。执行此操作后，应该使用这些值填充ListView。问题是，每当我尝试打开活动时，它都会崩溃。

任何人都可以告诉我，我的问题是什么？这个课程是否适用于我想要的目的？

public class search extends AppCompatActivity {

    @Override
     protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_search);
      }
     public void onClick(View v) {
         switch (v.getId()){
            case R.id.imageButton2: new StoreUserDataAsyncTask().execute();
                break;
             }
     }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
         // Inflate the menu; this adds items to the action bar if it is present.
       getMenuInflater().inflate(R.menu.menu_searchh, menu);
       return true;
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
          return true;
    }

          return super.onOptionsItemSelected(item);
    }

     ArrayList<User> userlist1=null;

     public class StoreUserDataAsyncTask extends AsyncTask<Void,Void,ArrayList<User>> {
     User user;
     GetUserCallback userCallback;

     ProgressDialog progressDialog;
     Spinner s11 = (Spinner) findViewById(R.id.spinner);
     Spinner s2 = (Spinner) findViewById(R.id.spinner2);
     String bloodd = s11.getSelectedItem().toString();
     String loc = s2.getSelectedItem().toString();

     protected ArrayList<User> doInBackground(Void... params) {

     User returnedUser=null;
     ArrayList<User> userlist=null;
     final int CONNECTION_TIMEOUT = 1000 * 15;
     final String SERVER_ADDRESS = "http://xxxxx.comli.com/";
     ArrayList<NameValuePair> dataToSend = new ArrayList<>();
     dataToSend.add(new BasicNameValuePair("bloodtype", bloodd));
     dataToSend.add(new BasicNameValuePair("location", loc));

     HttpParams httpRequestParams = new BasicHttpParams();
     HttpConnectionParams.setConnectionTimeout(httpRequestParams, CONNECTION_TIMEOUT);
     HttpConnectionParams.setSoTimeout(httpRequestParams, CONNECTION_TIMEOUT);

     HttpClient client = new DefaultHttpClient(httpRequestParams);
     HttpPost post = new HttpPost(SERVER_ADDRESS + "/Search.php");

     try {
         post.setEntity(new UrlEncodedFormEntity(dataToSend));
         HttpResponse httpResponse = client.execute(post);

         HttpEntity entity = httpResponse.getEntity();
         String result = EntityUtils.toString(entity);
         JSONObject jObject = new JSONObject(result);

         if (jObject.length() == 0) {
             userlist = null;
             Context context = getApplicationContext();
             CharSequence text = "no available donor!";
             int duration = Toast.LENGTH_SHORT;

          } else {
             String resultt = EntityUtils.toString(entity);
             JSONArray jsonArray = new JSONArray(resultt);
             for (int i = 0; i < jsonArray.length(); i++) {
                     JSONObject jsonObject = jsonArray.getJSONObject(i);
                     String fname = jsonObject.getString("firstname");
                     String lname = jsonObject.getString("lastname");
                     String blood = jsonObject.getString("blood");
                     String location = jsonObject.getString("location");
                     String email = jsonObject.getString("email");
                     String age = jsonObject.getString("age");
                     String password = jsonObject.getString("password");
                     String phone = jsonObject.getString("phone");
                     int agee = Integer.parseInt(age);
                     int phonee = Integer.parseInt(phone);

                     returnedUser = new User(fname, lname, blood, location, email, password, agee, phonee);
                     userlist.add(returnedUser);
                  }
                } 
             } catch (Exception e) {
                e.printStackTrace();
            }
            userlist1=userlist;
            return userlist;          
         }
         protected void onPostExecute(ArrayList<User> aVoid) {
         progressDialog.dismiss();
         displayCountryList();
        }
     }

     private void displayCountryList(){
     MyCustomAdapter dataAdapter = null;
     try {
              //create an ArrayAdaptar from the String Array
              dataAdapter = new MyCustomAdapter(this,
              R.layout.listv, userlist1);
             ListView listView = (ListView) findViewById(R.id.listView);
        // Assign adapter to ListView
              listView.setAdapter(dataAdapter);

        //enables filtering for the contents of the given ListView
              listView.setTextFilterEnabled(true);
         }
         catch (Exception e) {
             e.printStackTrace();
         }
     }

       class MyCustomAdapter extends ArrayAdapter<User> {

         private ArrayList<User> userlist;

         public MyCustomAdapter(Context context, int textViewResourceId,
                           ArrayList<User> userlist) {
            super(context, textViewResourceId, userlist);
            this.userlist = new ArrayList<User>();
            this.userlist.addAll(userlist);
          }

         private class ViewHolder {
           TextView name;
           TextView phone;
           TextView email;
         }

         @Override
         public View getView(int position, View convertView, ViewGroup parent) {

             ViewHolder holder = null;
             Log.v("ConvertView", String.valueOf(position));
             if (convertView == null) {

                 LayoutInflater vi = (LayoutInflater)getSystemService(
                        Context.LAYOUT_INFLATER_SERVICE);
                 convertView = vi.inflate(R.layout.listv, null);

                 holder = new ViewHolder();
                 holder.name = (TextView) convertView.findViewById(R.id.textView31);
                 holder.phone = (TextView) convertView.findViewById(R.id.textView33);
                 holder.email = (TextView) convertView.findViewById(R.id.textView34);

                 convertView.setTag(holder);

            } else {
                holder = (ViewHolder) convertView.getTag();
             }

            User user = userlist.get(position);
            holder.name.setText(user.getFirstname());
            holder.phone.setText(user.getPhone());
            holder.email.setText(user.getEmail());
            return convertView;
        }
      }
    }