doInBackground not updating variable

Issue

I'm working on a basic android app that makes a POST with HttpURLConnection. I want to return the response Message from my Web API.

My MainActivity.java

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final TextView mTextView = findViewById(R.id.textView);
        AsyncExample asyncExample = new AsyncExample();
        asyncExample.execute();
        mTextView.setText(asyncExample.getResponseMsg());
    }
}

My AsyncExample.java

class AsyncExample extends AsyncTask<Void, Void, Void> {
    private HttpURLConnection con;
    private String responseMsg;

    protected void onPreExecute() {
        responseMsg = "empty message";
    }

    @Override
    protected Void doInBackground(Void... params) {
        String urlParameters = "param1=data1";

        byte[] postData = urlParameters.getBytes(Charset.forName("UTF-8"));
        int postDataLength = postData.length;
        String request = "http://192.168.1.30:6262";
        URL url = null;
        try {
            url = new URL(request);
            con = (HttpURLConnection) url.openConnection();
            con.setDoOutput(true);
            con.setInstanceFollowRedirects(false);
            con.setRequestMethod("POST");
            con.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
            con.setRequestProperty("charset", "utf-8");
            con.setRequestProperty("Content-Length", Integer.toString(postDataLength));
            responseMsg = con.getResponseMessage();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    public String getResponseMsg() {
        return responseMsg;
    }
}

After running the app, i get empty message in my TextView. Why it is not getting updated my doInBackground? Even if con.getResponseMessage() is null, it should be updated?

Solution

The problem is that your AsyncTask is executed asynchronously, while you try to retrieve the value immediately. You need to implement this a little bit differently. Either leverage the API of AsyncTask, sine it posts callbacks for your on the UI thread. You can update your TextView directly in your AsyncTask

class MyAwesomeAsyncTask extends AsyncTask<Void, Void, String> {

    @Override
    protected void onPreExecute() {
        myTextView.setText("progress started");
    }

    @Override
    protected String doInBackground(final Void... voids) {
        String s = amazingCallToBackend();
        return s;
    }

    @Override
    protected void onPostExecute(final String s) {
        myTextView.setText(s);
    }
}

Or if you just want the value, you can pass a Callback to your async task that will deliver the value to you, something like that

interface Callback {
    void onValueReceived(String value);
    void onFailure();
}

class MyAwesomeAsyncTask extends AsyncTask<Void, Void, String> {

    private Callback callback;

    MyAwesomeAsyncTask(final Callback callback) {
        this.callback = callback;
    }

    @Override
    protected String doInBackground(final Void... voids) {
        String s = amazingCallToBackend();
        return s;
    }

    @Override
    protected void onPostExecute(final String s) {
        callback.onValueReceived(s);
    }
}

Here's how you create it

Callback callback = new Callback() {
        @Override
        public void onValueReceived(final String value) {

        }

        @Override
        public void onFailure() {

        }
    };

new MyAwesomeAsyncTask(callback).execute();

However, be careful because if for some reason your activity/fragment is gone/finished before your AsyncTask is done, this can cause memory leaks.

A quick Google search will tell you all you want about AsyncTask leaking memory :)

Answered By - elmorabea