Question

I am using a AsyncTask to download a file from a URL in Android.
This is the class to download the file in the background:

//-----------------------------ASYNC DOWNLOADER--------------------------------
    /**
     * Background Async Task to download file
     * */
    class DownloadFileFromURL extends AsyncTask<String, String, String> {

        /**
         * Before starting background thread
         * Show Progress Bar Dialog
         * */
        @SuppressWarnings("deprecation")
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            showDialog(progress_bar_type);
        }

        //TODO Zustzinfos für alle Methoden hinzufügen
        /**
         * This method is called for executing the background task in the AsyncTask.
         * For this tutorial we are only sleeping the thread for the number of 
         * seconds passed as parameter of the function.
         * 
         * @param numSeconds: life of the task
         * @return the result of the background task
         */
        @Override
        protected String doInBackground(String... f_url) {
            int count;
            try {
                URL url = new URL(f_url[0]);
                URLConnection conection = url.openConnection();
                conection.connect();
                // getting file length
                int lenghtOfFile = conection.getContentLength();     
                // input stream to read file - with 8k buffer
                InputStream input = new BufferedInputStream(url.openStream(), 8192);     
                // Output stream to write file
                OutputStream output = new FileOutputStream("/sdcard/"+Name+".xml");

                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("ASYNC",e.getMessage());
            }
            return null;
        }    
        /**
         * Updating progress bar
         * */
        protected void onProgressUpdate(String... progress) {
            // setting progress percentage
            pDialog.setProgress(Integer.parseInt(progress[0]));
       }     
        /**
        * After completing background task
        * Dismiss the progress dialog
        * **/
        @SuppressWarnings("deprecation")
        @Override
        protected void onPostExecute(String error) {
            // dismiss the dialog after the file was downloaded
            dismissDialog(progress_bar_type);
            String xmlPath = Environment.getExternalStorageDirectory().toString() + "/"+Name+".xml";
            Log.d("XMLDOWNLOADPATH", xmlPath);
            Log.d("DOWNLOADXML","End Download XML file");
        }

    }

I want to be able to recognize in my main activity (the async class is a inner class of the main acticity) if there was a exception and show the error message in a Dialog or Toast.

I tried to return the value from doInBackground() to onPostExecute() and write this String to a global String variable like this:

protected String doInBackground(String... f_url) {
String error = null;
try {
}
catch(Exception e){
error = e.toString();
}
return error;

@Override
protected void onPostExecute(String error) {
globalStringvariable = error;
}

But this does not work properly, the dialog does not always show the exception message. Is this the best way to realize my problem?

Answer 1

e.getMessage();

That’s what you’re looking for.
Also note that if your getting null, it can be because you are catching Exception as a generic, you should always try to catch a specific Exception in order to get the message back.