AlertDialog setmessage not working inside Asynctask

Below is my code my trying to display my progress in Asyntask via onProgressUpdate method but it aint showing in the alert diaalog. Only the initial message is shown.

 class DownloadFileFromURL extends AsyncTask<String, String, String> {

        private AlertDialog.Builder alert;
        private int progress = 0;
        /**
         * Before starting background thread Show Progress Bar Dialog
         */
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            alert = new AlertDialog.Builder(context);
            alert.setTitle("Downloading..");
            alert.setMessage("1");
            alert.setCancelable(false);
            alert.show();
        }

        /**
         * Downloading file in background thread
         */
        @Override
        protected String doInBackground(String... f_url) {
            int count;
            try {
                URL url = new URL(f_url[0]);
                URLConnection conection = url.openConnection();
                conection.connect();

                // this will be useful so that you can show a tipical 0-100%
                // progress bar
                int lenghtOfFile = conection.getContentLength();

                // download the file
                InputStream input = new BufferedInputStream(url.openStream(),
                        8192);

                // Output stream
                OutputStream output = new FileOutputStream(file);

                byte data[] = new byte[1024];

                long total = 0;

                while ((count = input.read(data)) != -1) {
                    total += count;
                    // publishing the progress....
                    // After this onProgressUpdate will be called
                    publishProgress("" + (int) ((total * 100) / lenghtOfFile));

                    // writing data to file
                    output.write(data, 0, count);
                }

                // flushing output
                output.flush();

                // closing streams
                output.close();
                input.close();

            } catch (Exception e) {
                Log.e("Error: ", e.getMessage());
            }

            return null;
        }

        /**
         * Updating progress bar
         */
        @Override
        protected void onProgressUpdate(String... progress) {
            Log.d("Myapp","progress :"+progress[0]);
            alert.setMessage(""+progress[0]);
        }

        /**
         * After completing background task Dismiss the progress dialog
         **/
        @Override
        protected void onPostExecute(String file_url) {
            // dismiss the dialog after the file was downloaded
}
}

As i have also written a log to show progress update in onProgressUpdate, the log is printed but the alert.setMessage in onProgressUpdate seems not set the message to my alert dialog.

As per your code, alert is an AlertDialog.Builder and not an AlertDialog itself. This raised a concern to me because the reason it might not be changing is because you already showed the builder, but not give to an AlertDialog. So I tried out a simple code:

public class MainActivity extends AppCompatActivity {

    private AlertDialog.Builder alert;
    private AlertDialog ad;

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

        alert = new AlertDialog.Builder(this);
        alert.setTitle("Downloading..");
        alert.setMessage("1");
        alert.setCancelable(false);
        ad = alert.show();


        Log.d("SAMPLE", "SET MESSAGE 2");
        alert.setMessage("2");

        Log.d("SAMPLE", "SET MESSAGE 3");
        ad.setMessage("3");
    }

}

At first, I just used the alert.setMessage (this is the AlertDialog.Builder), and the message did not change at all. But after putting it in an AlertDialog and then setting the message of the AlertDialog instance, the message changed. Care to try this approach out. Pass the AlertDialog.Builder to an AlertDialog first then, setMessage using the AlertDialog instance.

Docs for AlertDialog and AlertDialog.Builder.