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。