requesting JSON document with HttpURLConnection and parsing it

Question

I have an App with a class that is responsible for requesting a JSON from a local server...

Here is the Main Activity:

public class Activity_main extends AppCompatActivity {

    private ArrayList<StructAcount> netAcount = new ArrayList<StructAcount>();
    private TextView textView;


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

        textView = (TextView) findViewById(R.id.txtJSON);

        WebService.readUrl("http://localhost/match_picture/service.php?action=read");

        String result = WebService.getResult();


        if (result != null) {
            try {
                JSONArray tasks = new JSONArray(result);
                for (int i=0; i<tasks.length(); i++) {
                    StructAcount acount= new StructAcount();
                    JSONObject object = tasks.getJSONObject(i);
                    acount.id = object.getLong("user_id");
                    acount.name = object.getString("user_name");
                    acount.email = object.getString("user_email");

                    netAcount.add(acount);
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        } else {
            Toast.makeText(Activity_main.this, "hsh" , Toast.LENGTH_SHORT).show();
        }

        for (StructAcount acount: netAcount) {
            textView.setText(textView.getText() + "username: " + acount.name + "  |  " + "useremail: " + acount.email + "\n");
            Toast.makeText(Activity_main.this, "username: " + acount.name + "\n" + "useremail: " + acount.email , Toast.LENGTH_SHORT).show();
        }
    }

}

...and here's the class for HTTP requests:

public class WebService {

    private static String result;

    public static void readUrl(String server_url) {
       class GetJson extends AsyncTask<String, Void, String> {
           @Override
           protected String doInBackground(String... params) {
               BufferedReader bufferedReader = null;
               String uri = params[0];
               try {
                   URL url = new URL(uri);
                   HttpURLConnection con = (HttpURLConnection) url.openConnection();
                   StringBuilder sb = new StringBuilder();
                   InputStream in = new BufferedInputStream(con.getInputStream());
                   bufferedReader = new BufferedReader(new InputStreamReader(in));

                   String json;
                   while ((json = bufferedReader.readLine()) != null) {
                       sb.append(json);

                   }

                   return sb.toString().trim();


               } catch (MalformedURLException e) {
                   e.printStackTrace();
               } catch (IOException e) {
                   e.printStackTrace();
               }

               return null;

           }

           @Override
           protected void onPostExecute(String s) {
               super.onPostExecute(s);
               WebService.set_result(s);
           }
       }

        GetJson gj = new GetJson();
        gj.execute(server_url);
    }

    public static void set_result(String s) {
        result = s;
    }

    public static String getResult() {
        return result;
    }

}

For the first time that I run the App, nothing was set to TextView.

When the App is still on emulator's screen: if I make a small change, for example, changing the text for a Toast message, I run the App again and TextView's text set by JSON values ...

Can anyone point some references or a solution for this situation?