Android How to - Download a series of images asynchronously

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The following code shows how to Download a series of images asynchronously.

Example

Add the following statement to the AndroidManifest.xml file:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.java2s.Networking"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="14" />
    <uses-permission android:name="android.permission.INTERNET"/>

    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:label="@string/app_name"
            android:name=".NetworkingActivity" >
            <intent-filter >
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

Layout xml file

        <?xml version="1.0" encoding="utf-8"?>
        <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:orientation="vertical" >

        <ImageView
            android:id="@+id/img"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center" />

        </LinearLayout>

Main Activity Java code

package com.java2s.myapplication3.app;
/*w w  w .ja  va 2s. co  m*/
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.widget.ImageView;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.widget.Toast;

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;

public class MainActivity extends Activity {
    ImageView img;

    private InputStream OpenHttpConnection(String urlString) throws IOException
    {
        InputStream in = null;
        int response = -1;

        URL url = new URL(urlString);
        URLConnection conn = url.openConnection();

        if (!(conn instanceof HttpURLConnection))
            throw new IOException("Not an HTTP connection");
        try{
            HttpURLConnection httpConn = (HttpURLConnection) conn;
            httpConn.setAllowUserInteraction(false);
            httpConn.setInstanceFollowRedirects(true);
            httpConn.setRequestMethod("GET");
            httpConn.connect();
            response = httpConn.getResponseCode();
            if (response == HttpURLConnection.HTTP_OK) {
                in = httpConn.getInputStream();
            }
        }
        catch (Exception ex)
        {
            Log.d("Networking", ex.getLocalizedMessage());
            throw new IOException("Error connecting");
        }
        return in;
    }

    private Bitmap DownloadImage(String URL)
    {
        Bitmap bitmap = null;
        InputStream in = null;
        try {
            in = OpenHttpConnection(URL);
            bitmap = BitmapFactory.decodeStream(in);
            in.close();
        } catch (IOException e1) {
            Log.d("NetworkingActivity", e1.getLocalizedMessage());
        }
        return bitmap;
    }

    private class DownloadImageTask extends AsyncTask<String, Bitmap, Long> {
        protected Long doInBackground(String... urls) {
            long imagesCount = 0;
            for (int i = 0; i < urls.length; i++) {
                Bitmap imageDownloaded = DownloadImage(urls[i]);
                if (imageDownloaded != null)  {
                    imagesCount++;
                    try {
                        Thread.sleep(3000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    publishProgress(imageDownloaded);
                }
            }
            return imagesCount;
        }
        protected void onProgressUpdate(Bitmap... bitmap) {
            img.setImageBitmap(bitmap[0]);
        }
        protected void onPostExecute(Long imagesDownloaded) {
            Toast.makeText(getBaseContext(),
                    "Total " + imagesDownloaded + " images downloaded",
                    Toast.LENGTH_LONG).show();
        }
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        img = (ImageView) findViewById(R.id.img);
        new DownloadImageTask().execute(
                "http://www.java2s.com/style/download.png",
                "http://www.java2s.com/style/download.png");
    }

}

Note

When working with the Android emulator, you may frequently need to access data hosted on the local web server using localhost.

In such cases, you should use the special IP address of 10.0.2.2 (not 127.0.0.1) to refer to the host computer's loopback interface. From the Android emulator's perspective, localhost (127.0.0.1) refers to its own loopback interface.

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