There are many ways to download files. Following I will post most common ways; it is up to you to decide which method is better for your app.

1. Use AsyncTask and show the download progress in a dialog

This method will allow you to execute some background processes and update the UI at the same time (in this case, we'll update a progress bar).

Imports:

import android.os.PowerManager;import java.io.InputStream;import java.io.OutputStream;import java.io.FileOutputStream;import java.net.HttpURLConnection;

This is an example code:

// declare the dialog as a member field of your activityProgressDialog mProgressDialog;// instantiate it within the onCreate methodmProgressDialog = new ProgressDialog(YourActivity.this);mProgressDialog.setMessage("A message");mProgressDialog.setIndeterminate(true);mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);mProgressDialog.setCancelable(true);// execute this when the downloader must be firedfinal DownloadTask downloadTask = new DownloadTask(YourActivity.this);downloadTask.execute("the url to the file you want to download");mProgressDialog.setOnCancelListener(new DialogInterface.OnCancelListener() {    @Override    public void onCancel(DialogInterface dialog) {        downloadTask.cancel(true); //cancel the task    }});

The AsyncTask will look like this:

// usually, subclasses of AsyncTask are declared inside the activity class.// that way, you can easily modify the UI thread from hereprivate class DownloadTask extends AsyncTask<String, Integer, String> {    private Context context;    private PowerManager.WakeLock mWakeLock;    public DownloadTask(Context context) {        this.context = context;    }    @Override    protected String doInBackground(String... sUrl) {        InputStream input = null;        OutputStream output = null;        HttpURLConnection connection = null;        try {            URL url = new URL(sUrl[0]);            connection = (HttpURLConnection) url.openConnection();            connection.connect();            // expect HTTP 200 OK, so we don't mistakenly save error report            // instead of the file            if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {                return "Server returned HTTP " + connection.getResponseCode()                        + " " + connection.getResponseMessage();            }            // this will be useful to display download percentage            // might be -1: server did not report the length            int fileLength = connection.getContentLength();            // download the file            input = connection.getInputStream();            output = new FileOutputStream("/sdcard/file_name.extension");            byte data[] = new byte[4096];            long total = 0;            int count;            while ((count = input.read(data)) != -1) {                // allow canceling with back button                if (isCancelled()) {                    input.close();                    return null;                }                total += count;                // publishing the progress....                if (fileLength > 0) // only if total length is known                    publishProgress((int) (total * 100 / fileLength));                output.write(data, 0, count);            }        } catch (Exception e) {            return e.toString();        } finally {            try {                if (output != null)                    output.close();                if (input != null)                    input.close();            } catch (IOException ignored) {            }            if (connection != null)                connection.disconnect();        }        return null;    }

The method above (doInBackground) runs always on a background thread. You shouldn't do any UI tasks there. On the other hand, the onProgressUpdate and onPreExecute run on the UI thread, so there you can change the progress bar:

    @Override    protected void onPreExecute() {        super.onPreExecute();        // take CPU lock to prevent CPU from going off if the user         // presses the power button during download        PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);        mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,             getClass().getName());        mWakeLock.acquire();        mProgressDialog.show();    }    @Override    protected void onProgressUpdate(Integer... progress) {        super.onProgressUpdate(progress);        // if we get here, length is known, now set indeterminate to false        mProgressDialog.setIndeterminate(false);        mProgressDialog.setMax(100);        mProgressDialog.setProgress(progress[0]);    }    @Override    protected void onPostExecute(String result) {        mWakeLock.release();        mProgressDialog.dismiss();        if (result != null)            Toast.makeText(context,"Download error: "+result, Toast.LENGTH_LONG).show();        else            Toast.makeText(context,"File downloaded", Toast.LENGTH_SHORT).show();    }

For this to run, you need the WAKE_LOCK permission.

<uses-permission android:name="android.permission.WAKE_LOCK" />

2. Download from Service

The big question here is: how do I update my activity from a service?. In the next example we are going to use two classes you may not be aware of: ResultReceiver and IntentService. ResultReceiver is the one that will allow us to update our thread from a service; IntentService is a subclass of Service which spawns a thread to do background work from there (you should know that a Service runs actually in the same thread of your app; when you extends Service, you must manually spawn new threads to run CPU blocking operations).

Download service can look like this:

public class DownloadService extends IntentService {    public static final int UPDATE_PROGRESS = 8344;    public DownloadService() {        super("DownloadService");    }    @Override    protected void onHandleIntent(Intent intent) {        String urlToDownload = intent.getStringExtra("url");        ResultReceiver receiver = (ResultReceiver) intent.getParcelableExtra("receiver");        try {            //create url and connect            URL url = new URL(urlToDownload);            URLConnection connection = url.openConnection();            connection.connect();            // this will be useful so that you can show a typical 0-100% progress bar            int fileLength = connection.getContentLength();            // download the file            InputStream input = new BufferedInputStream(connection.getInputStream());            String path = "/sdcard/BarcodeScanner-debug.apk" ;            OutputStream output = new FileOutputStream(path);            byte data[] = new byte[1024];            long total = 0;            int count;            while ((count = input.read(data)) != -1) {                total += count;                // publishing the progress....                Bundle resultData = new Bundle();                resultData.putInt("progress" ,(int) (total * 100 / fileLength));                receiver.send(UPDATE_PROGRESS, resultData);                output.write(data, 0, count);            }            // close streams             output.flush();            output.close();            input.close();        } catch (IOException e) {            e.printStackTrace();        }        Bundle resultData = new Bundle();        resultData.putInt("progress" ,100);        receiver.send(UPDATE_PROGRESS, resultData);    }}

Add the service to your manifest:

<service android:name=".DownloadService"/>

And the activity will look like this:

// initialize the progress dialog like in the first example// this is how you fire the downloadermProgressDialog.show();Intent intent = new Intent(this, DownloadService.class);intent.putExtra("url", "url of the file to download");intent.putExtra("receiver", new DownloadReceiver(new Handler()));startService(intent);

Here is were ResultReceiver comes to play:

private class DownloadReceiver extends ResultReceiver{    public DownloadReceiver(Handler handler) {        super(handler);    }    @Override    protected void onReceiveResult(int resultCode, Bundle resultData) {        super.onReceiveResult(resultCode, resultData);        if (resultCode == DownloadService.UPDATE_PROGRESS) {            int progress = resultData.getInt("progress"); //get the progress            dialog.setProgress(progress);            if (progress == 100) {                dialog.dismiss();            }        }    }}

2.1 Use Groundy library

Groundy is a library that basically helps you run pieces of code in a background service, and it is based on the ResultReceiver concept shown above. This library is deprecated at the moment. This is how the whole code would look like:

The activity where you are showing the dialog...

public class MainActivity extends Activity {    private ProgressDialog mProgressDialog;    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        findViewById(R.id.btn_download).setOnClickListener(new View.OnClickListener() {            public void onClick(View view) {                String url = ((EditText) findViewById(R.id.edit_url)).getText().toString().trim();                Bundle extras = new Bundler().add(DownloadTask.PARAM_URL, url).build();                Groundy.create(DownloadExample.this, DownloadTask.class)                        .receiver(mReceiver)                        .params(extras)                        .queue();                mProgressDialog = new ProgressDialog(MainActivity.this);                mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);                mProgressDialog.setCancelable(false);                mProgressDialog.show();            }        });    }    private ResultReceiver mReceiver = new ResultReceiver(new Handler()) {        @Override        protected void onReceiveResult(int resultCode, Bundle resultData) {            super.onReceiveResult(resultCode, resultData);            switch (resultCode) {                case Groundy.STATUS_PROGRESS:                    mProgressDialog.setProgress(resultData.getInt(Groundy.KEY_PROGRESS));                    break;                case Groundy.STATUS_FINISHED:                    Toast.makeText(DownloadExample.this, R.string.file_downloaded, Toast.LENGTH_LONG);                    mProgressDialog.dismiss();                    break;                case Groundy.STATUS_ERROR:                    Toast.makeText(DownloadExample.this, resultData.getString(Groundy.KEY_ERROR), Toast.LENGTH_LONG).show();                    mProgressDialog.dismiss();                    break;            }        }    };}

A GroundyTask implementation used by Groundy to download the file and show the progress:

public class DownloadTask extends GroundyTask {        public static final String PARAM_URL = "com.groundy.sample.param.url";    @Override    protected boolean doInBackground() {        try {            String url = getParameters().getString(PARAM_URL);            File dest = new File(getContext().getFilesDir(), new File(url).getName());            DownloadUtils.downloadFile(getContext(), url, dest, DownloadUtils.getDownloadListenerForTask(this));            return true;        } catch (Exception pokemon) {            return false;        }    }}

And just add this to the manifest:

<service android:name="com.codeslap.groundy.GroundyService"/>

It couldn't be easier I think. Just grab the latest jar from Github and you are ready to go. Keep in mind that Groundy's main purpose is to make calls to external REST apis in a background service and post results to the UI with easily. If you are doing something like that in your app, it could be really useful.

2.2 Use https://github.com/koush/ion

3. Use DownloadManager class (GingerBread and newer only)

GingerBread brought a new feature, DownloadManager, which allows you to download files easily and delegate the hard work of handling threads, streams, etc. to the system.

First, let's see a utility method:

/** * @param context used to check the device version and DownloadManager information * @return true if the download manager is available */public static boolean isDownloadManagerAvailable(Context context) {    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD) {        return true;    }    return false;}

Method's name explains it all. Once you are sure DownloadManager is available, you can do something like this:

String url = "url you want to download";DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));request.setDescription("Some descrition");request.setTitle("Some title");// in order for this if to run, you must use the android 3.2 to compile your appif (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {    request.allowScanningByMediaScanner();    request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);}request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "name-of-the-file.ext");// get download service and enqueue fileDownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);manager.enqueue(request);

Download progress will be showing in the notification bar.

Final thoughts

First and second methods are just the tip of the iceberg. There are lots of things you have to keep in mind if you want your app to be robust. Here is a brief list:

• You must check whether user has an internet connection available
• Make sure you have the right permissions (INTERNET and WRITE_EXTERNAL_STORAGE); also ACCESS_NETWORK_STATE if you want to check internet availability.
• Make sure the directory were you are going to download files exist and has write permissions.
• If download is too big you may want to implement a way to resume the download if previous attempts failed.
• Users will be grateful if you allow them to interrupt the download.

Unless you need detailed control of the download process, then consider using DownloadManager (3) because it already handles most of the items listed above.

But also consider that your needs may change. For example, DownloadManager does no response caching. It will blindly download the same big file multiple times. There's no easy way to fix it after the fact. Where if you start with a basic HttpURLConnection (1, 2), then all you need is to add an HttpResponseCache. So the initial effort of learning the basic, standard tools can be a good investment.

This class was deprecated in API level 26. ProgressDialog is a modal dialog, which prevents the user from interacting with the app. Instead of using this class, you should use a progress indicator like ProgressBar, which can be embedded in your app's UI. Alternatively, you can use a notification to inform the user of the task's progress. For more details Link

Don't forget to add permissions to your manifest file if you're gonna be downloading stuff from the internet!

<manifest xmlns:android="http://schemas.android.com/apk/res/android"    package="com.example.helloandroid"    android:versionCode="1"    android:versionName="1.0">        <uses-sdk android:minSdkVersion="10" />        <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission>        <uses-permission android:name="android.permission.INTERNET"></uses-permission>        <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"></uses-permission>        <uses-permission android:name="android.permission.READ_PHONE_STATE"></uses-permission>        <application             android:icon="@drawable/icon"             android:label="@string/app_name"             android:debuggable="true">        </application></manifest>

Yes the code above will work .But if you are updating your progressbar in onProgressUpdate of Asynctask and you press back button or finish your activity AsyncTask looses its track with your UI .And when you go back to your activity, even if download is running in background you will see no update on progressbar. So on OnResume() try to run a thread like runOnUIThread with a timer task that updates ur progressbar with values updating from the AsyncTask running background.

private void updateProgressBar(){    Runnable runnable = new updateProgress();    background = new Thread(runnable);    background.start();}public class updateProgress implements Runnable {    public void run() {        while(Thread.currentThread()==background)            //while (!Thread.currentThread().isInterrupted()) {            try {                Thread.sleep(1000);                 Message msg = new Message();                progress = getProgressPercentage();                        handler.sendMessage(msg);            } catch (InterruptedException e) {                Thread.currentThread().interrupt();            } catch (Exception e) {        }    }}private Handler handler = new Handler(){    @Override    public void handleMessage(Message msg) {        progress.setProgress(msg.what);    }};

Don't forget to Destroy the thread when ur activity is not visible.

private void destroyRunningThreads() {    if (background != null) {        background.interrupt();        background=null;    }}