Download a file using Async Task

From the initial days of Android Application development, It recommends not to use Threads in the Main Thread or Class. But from ICS, it strictly says not to use Thread in main class and if do so, it will throw ' NetworkOnMainThreadException ' . So its wise to use Async task in all network operations.


Here I will show you a quick demo of how to download a Pdf file and saving that in the Downloads folder of your SD card. Will also open the file after downloading.

As we all know AsyncTask class have 3 methods.

1. onPreExecute()
2. doInBackground(String... urls)
3. onPostExecute(String result)

So in onPreExecute, We will configure where it should download the file. You can change the folder as you need.

@Override protected void onPreExecute() { dialog = ProgressDialog.show(context, "", "Loading...Please wait..."); try { pdfFile = new File(Environment.getExternalStorageDirectory() "/Download/"pdf_url); } catch (NullPointerException e) { e.printStackTrace(); } }

In doInBackground, We will do the network operations. I'm showing the progress bar here is to prevent other events to happen until the downloaded file got downloaded and its opened. If you are downloading other files which don't want to open after downloading, you can skip the progress bar so that the downloading will happen in the background and you other operation will also happen Asynchronously. 

@Override protected String doInBackground(String... urls) { DefaultHttpClient client = new DefaultHttpClient(); String ret = ""; for (String url : urls) { try { HttpGet getMethod = new HttpGet(url); HttpResponse response = client.execute(getMethod); InputStream inStream = response.getEntity().getContent(); FileOutputStream fileWriter = new FileOutputStream(Environment.getExternalStorageDirectory()+ "/Download/" + pdf_url); int dataByte = inStream.read(); while (dataByte > -1) { dataByte = inStream.read(); } fileWriter.close(); } catch (Exception e) { e.printStackTrace(); } } return ret; }

Finally after the file got downloaded, onPostExecute method will be called. Lets open the downloaded file inside this method.

@Override protected void onPostExecute(String result) { dialog.dismiss(); Intent getPdfIntent = new Intent(Intent.ACTION_VIEW).setDataAndType( Uri.fromFile(pdfFile),"application/pdf"); context.startActivity(getPdfIntent); }

Thus, a file got downloaded and its brought to foreground using Intent.ACTION_VIEW. To call this class, 

                 DownloadAndOpenPDF pdfFile = new DownloadAndOpenPDF(this,pdfUrl);
pdfFile.execute(new String[] { pdfUrl });

And, below is the class comprises of all three methods together.

import java.io.File; import java.io.FileOutputStream; import java.io.InputStream; import org.apache.http.HttpResponse; import org.apache.http.client.methods.HttpGet; import org.apache.http.impl.client.DefaultHttpClient; import android.app.ProgressDialog; import android.content.Context; import android.content.Intent; import android.net.Uri; import android.os.AsyncTask; import android.os.Environment; public class DownloadAndOpenPDF extends AsyncTask { static public ProgressDialog dialog; static Context context; static int call; static String method; static HttpResponse execute; File pdfFile; String pdfUrl; public DownloadAndOpenPDF(Context context, String pdfUrl) { DownloadAndOpenPDF.context = context; this.pdfUrl = pdfUrl; } @Override protected String doInBackground(String... urls) { DefaultHttpClient client = new DefaultHttpClient(); String ret = ""; for (String url : urls) { try { HttpGet getMethod = new HttpGet(url); HttpResponse response = client.execute(getMethod); InputStream inStream = response.getEntity().getContent(); FileOutputStream fileWriter = new FileOutputStream( Environment.getExternalStorageDirectory() + "/Download/" + pdf_url); int dataByte = inStream.read(); while (dataByte > -1) { fileWriter.write(dataByte); dataByte = inStream.read(); } fileWriter.close(); } catch (Exception e) { e.printStackTrace(); } } return ret; } @Override protected void onPreExecute() { dialog = ProgressDialog.show(context, "", "Loading...Please wait..."); try { pdfFile = new File(Environment.getExternalStorageDirectory() + "/Download/" + pdf_url); } catch (NullPointerException e) { e.printStackTrace(); } } @Override protected void onPostExecute(String result) { dialog.dismiss(); Intent getPdfIntent = new Intent(Intent.ACTION_VIEW).setDataAndType( Uri.fromFile(pdfFile), "application/pdf"); context.startActivity(getPdfIntent); } }