App needs to get JSON and display it. How to do it using async thread?

Question

As mentioned I get the above error which I know is because my application is doing networking in UI thread. I went through a lot of stackoverflow questions which advise to use AsyncTask for this purpose. From what I understand asynctask is asynchronous and will run independently in the background. But I need to fetch the data from http and display on the main thread. So basically my UI thread should be blocked till I have the JSON fetched so that I can display it.

My questions are 1) Since I need to run http networking in another thread how do I do it? 2) Do I use an async thread? 3) How do I block my UI thread for the async thread to fetch the result? 4) How do I pass the result from async thread back to UI thread?

This is the current JSON parser class that I use.

public class JSONParser {

static InputStream is = null;
static JSONObject jObj = null;
static String json = "";

// constructor
public JSONParser() {

}

public JSONArray getJSONfromURL(String url) {
    // initialize
    InputStream is = null;
    String result = "";
    JSONArray jArray = null;
    // http post
    try {
        HttpClient httpclient = new DefaultHttpClient();
        HttpGet httpGet = new HttpGet(url);
        HttpResponse response = httpclient.execute(httpGet);
        HttpEntity entity = response.getEntity();
        is = entity.getContent();

    } catch (Exception e) {
        Log.e("log_tag", "Error in http connection " + e.toString());
        return null;
    }

    // convert response to string
    try{
        BufferedReader reader = new BufferedReader(new InputStreamReader(
                is, "iso-8859-1"), 8);
        StringBuilder sb = new StringBuilder();
        String line = null;
        while ((line = reader.readLine()) != null) {
            sb.append(line + "\n");
        }
        is.close();
        result = sb.toString();
        Log.e("log_tag", "JSON data" + result);
    } catch (Exception e) {
        Log.e("log_tag", "Error converting result " + e.toString());
        return null;
    }

    // try parse the string to a JSON object
    try {           
        jArray = new JSONArray(result);
    } catch (JSONException e) {
        Log.e("log_tag", "Error parsing data " + e.toString());
        return null;
    }

    return jArray;
}

}

And this is my MainActivity where I call JSONparser to fetch some data that I need to display

    JSONArray json = jParser.getJSONfromURL(temp);

    if (json == null) {
        return -1;
    }

    for (int i = 0; i < json.length(); i++) {
        try {
            JSONObject c = json.getJSONObject(i);
            // Getting Array of Contacts
            // Storing each json item in variable
            asr_iq = c.getString("lMAsr");
            sunrise_iq = c.getString("lMSunrise");
            fajr_iq = c.getString("lMFajr");
            isha_iq = c.getString("lMIsha");
            dhuhr_iq = c.getString("lMDhuhr");
            maghrib_iq = c.getString("lMMaghrib");


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

Answer 1

Load your asynctask on the UI thread.

If you cannot do any network related operation on the UI Thread Honeycomb and later. You will get a NetworkOnMainThread Exception.

    new MyTask(url).execute();// can pass parameter to class constructor 
    // can also pass url to doInBackground.   

Async task

 class MyTask extends AsyncTask<Void,Void,Void>
 {
    String url;   

    public MyTask(String url)
    {
         this.url =url
    }      
@Override
protected Void doInBackground(Void... params) {
    // all your network related operation
            // invoked on the background thread
            // all code from getJSONfromURL(param)     
            // do not update ui here   
    return null;
}

@Override
protected void onPostExecute(Void result) { // invoked on the ui thread
    // TODO Auto-generated method stub
    super.onPostExecute(result);
            // dismiss progress dialog
            // update ui here
}

@Override
protected void onPreExecute() {
    // TODO Auto-generated method stub
    super.onPreExecute();
            // display progress dialog  
}

@Override
protected void onProgressUpdate(Void... values) {
    // TODO Auto-generated method stub
    super.onProgressUpdate(values);
}

}

Detailed information @ http://developer.android.com/reference/android/os/AsyncTask.html

Edit:

Use a Handler. Return result in doInBaCkground().

Example in onPostExecute()

 Message msg=new Message();
 msg.obj=result.getProperty(0).toString();
 mHandler.sendMessage(msg);

In your activity onCreate()

 Handler mHandler = new Handler() { 
   @Override public void handleMessage(Message msg) { 
      //ArrayList s=(ArrayList)msg.obj;
      SoapObject s =(SoapObject) msg.obj;
      tv.setText("Result = "+s.toString());

   }
};

You can also use runonuithread to update ui from doInBackGround()

     runOnUiThread(new Runnable() //run on ui thread
                 {
                  public void run() 
                  { 

                      _tv.setText("update from doinbackground");

                 }
                 });

Answer 2

Using Core Java,

1. Have your getJson execution logic in a Runnable/Callable(Java concurrency class), submit it via executors so that its an Asynch call.

2. Then with in your Runnable/Callable once json is retrived call the class which will have logic to display the json, this clas can be designed as a listener and you may publish an even after getting json response