Android网络下载图片

package net.learn2develop.Networking;

import android.app.Activity;
import android.os.Bundle;

import java.io.IOException;
import java.io.InputStream;

import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import android.util.Log;

import android.widget.ImageView;
import android.widget.Toast;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
public class NetworkingActivity 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, Void, Bitmap> {
        protected Bitmap doInBackground(String... urls) {
            return DownloadImage(urls[0]);
        }
        
        protected void onPostExecute(Bitmap result) {
            ImageView img = (ImageView) findViewById(R.id.img);
            img.setImageBitmap(result);
        }
    }

@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        new DownloadImageTask().execute("http://www.mayoff.com/5-01cablecarDCP01934.jpg");
}
}

因为DownloadImage()方法是同步的——这意味着，在图片下载完之前不会返回控制权——所以直接调用它会导致活动UI被冻结，使用AsyncTask允许在单独的线程中执行后台任务，然后在UI线程中返回结果。