java - Using AsyncTask with passing a value

Question

I have been working on this and I have hit a point where I dont know what to do. What I am trying to do is use one class to download and parse out a file into a string and then send that string to another class to parse out the JSON stuff. All the parts work fine by themselves and I have tested everything separately. I just dont know how to send the value to the Json parses to start the parsing.

So this is my filedownloader class.

  public class JsonFileDownloader extends AsyncTask<String, Void, String> {
  //used to access the website
  String username = "admin";
  String password = "admin";
  public String ret = "";



  @Override
  protected String doInBackground(String... params) {
      Log.d("Params ", params[0].toString());
      readFromFile(params[0]);
      return ret;
  }

  private String readFromFile(String myWebpage) {

      HttpURLConnection urlConnection = null;

      try {
          //Get the url connection
          URL url = new URL(myWebpage);
          Authenticator.setDefault(new Authenticator() {
              @Override
             protected PasswordAuthentication getPasswordAuthentication()   {
                  return new PasswordAuthentication(username,   password.toCharArray());
            }
          });
          urlConnection = (HttpURLConnection) url.openConnection();

          InputStream inputStream = urlConnection.getInputStream();


          if (inputStream != null) {
              ret = streamToString(inputStream);
              inputStream.close();
              Log.d("Final String", ret);
          }

      } catch (FileNotFoundException e) {
          e.printStackTrace();
      } catch (IOException e) {
          e.printStackTrace();
      } finally {
          if (urlConnection != null) {
              urlConnection.disconnect();
          }
          return ret;

      }
  }
  public static String streamToString(InputStream is) throws IOException   {

      StringBuilder sb = new StringBuilder();
      BufferedReader rd = new BufferedReader(new InputStreamReader(is));
      String line;
        while ((line = rd.readLine()) != null) {
          sb.append(line);
      }
       return sb.toString();
   }

  public String getJsonData()
  {
      return ret;
   }

}

This works fine I have tested it over and over with no errors. The next is the Json parser which is like this.

public class JSONParser {

JSONObject jsonResponse;
String jsonData;

//Consturctor
public JSONParser()
{
    //this.jsonData = jsonData;
    // this.OutputData = outPutData;
}

public void parsesData(String promo,
                       ArrayList<String> pictureHTTP,
                       ArrayList<String> pathHTTP,
                       ArrayList<String> labelText) throws IOException {

    //Build the Json String
    JsonFileDownloader jfd = new JsonFileDownloader();
   // jsonData = String.valueOf(jfd.execute(promo));
    jfd.execute(promo);
    //jfd.getResuts(jsonData);
    //jsonData = jfd.ret;

    Log.d("JsonData String = " , jsonData);


    //Try to parse the data
    try
    {

        Log.d("Jsondata " , jsonData);
        //Creaate a new JSONObject ith the name/value mapping from the JSON string
        jsonResponse = new JSONObject(jsonData);
        //Returns the value mapped by the name if it exists and is a JSONArry
        JSONArray jsonMainNode = jsonResponse.optJSONArray("");

        //Proccess the JSON node
        int lenghtJsonArrar = jsonMainNode.length();
        for (int i = 0; i<lenghtJsonArrar; i++)
        {
            //Get object for each json node
            JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);
            //Get the node values
            //int song_id = Integer.parseInt(jsonChildNode.optString("song_id").toString());
            String picture = jsonChildNode.optString("picture").toString();
            String pathName = jsonChildNode.optString("path").toString();
            String lableName = jsonChildNode.optString("label".toString());
            //Debug Testing code
            pictureHTTP.add(picture);
            pathHTTP.add(pathName);
            labelText.add(lableName);


        }

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

}

Now I know where the problem is occurring. When i try to assign a value to the jsonData it never is assigned so it is null and the system fails. I have tried a few things after the jfd.exicute() but I just dont know how to get the value from the final string output into the jsonData. Thank you for any help with this.

Answer 1

Alright, here is a pretty flexible pattern for the overall usage of using AsyncTask to download web content and getting the results from it back to the UI thread.

Step 1 Define an interface that will act as a message bus between the AsyncTask and where you want the data.

public interface AsyncResponse<T> {
    void onResponse(T response);
}

Step 2 Create a generic AsyncTask extension that will take any URL and return the results from it. You basically had this already, but I made some tweaks. Most importantly, allowing the setting of the AsyncResponse callback interface.

public class WebDownloadTask extends AsyncTask<String, Void, String> {

    private AsyncResponse<String> callback;

    // Optional parameters
    private String username;
    private String password;

    // Make a constuctor to store the parameters
    public WebDownloadTask(String username, String password) {
        this.username = username;
        this.password = password;
    }

    // Don't forget to call this
    public void setCallback(AsyncResponse<String> callback) {
        this.callback = callback;
    }

    @Override
    protected String doInBackground(String... params) {
        String url = params[0];
        return readFromFile(url);
    }

    @Override
    protected void onPostExecute(String s) {
        super.onPostExecute(s);
        if (callback != null) {
            callback.onResponse(s);
        } else {
            Log.w(WebDownloadTask.class.getSimpleName(), "The response was ignored");
        }
    }

    /******* private helper methods *******/

    private String streamToString(InputStream is) throws IOException {

        StringBuilder sb = new StringBuilder();
        BufferedReader rd = new BufferedReader(new InputStreamReader(is));
        String line;
        while ((line = rd.readLine()) != null) {
            sb.append(line);
        }
        return sb.toString();
    }

    private String readFromFile(String myWebpage) {

        String response = null;
        HttpURLConnection urlConnection = null;

        try {
            //Get the url connection
            URL url = new URL(myWebpage);

            // Unnecessary for general AsyncTask usage
            /* 
            Authenticator.setDefault(new Authenticator() {
                @Override
                protected PasswordAuthentication getPasswordAuthentication() {
                    return new PasswordAuthentication(username, password.toCharArray());
                }
            });
            */

            urlConnection = (HttpURLConnection) url.openConnection();

            InputStream inputStream = urlConnection.getInputStream();

            if (inputStream != null) {
                response = streamToString(inputStream);
                inputStream.close();
                Log.d("Final String", response);
            }

        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
        }

        return response;
    }
}

Step 3 Go forth and use that AsyncTask wherever you wish. Here is an example. Note that if you do not use setCallback, you will be unable to get the data that came from the AsyncTask.

public class MainActivity extends Activity {

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

        WebDownloadTask task = new WebDownloadTask("username", "password");
        task.setCallback(new AsyncResponse<String>() {
            @Override
            public void onResponse(String response) {
                // Handle response here. E.g. parse into a JSON object
                // Then put objects into some list, then place into an adapter... 
                Toast.makeText(getApplicationContext(), response, Toast.LENGTH_SHORT).show();
            }
        });

        // Use any URL, this one returns a list of 10 users in JSON
        task.execute("http://jsonplaceholder.typicode.com/users");
    }
}