<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
 xmlns:tools="http://schemas.android.com/tools" 
 android:layout_width="match_parent" 
 android:layout_height="match_parent" 
 android:orientation="vertical" > 

 <Button 
  android:id="@+id/btn_download" 
  android:layout_width="wrap_content" 
  android:layout_height="wrap_content" 
  android:layout_gravity="center_horizontal" 
  android:text="點擊下載" /> 

 <FrameLayout 
  android:layout_width="fill_parent" 
  android:layout_height="fill_parent" > 

  <ImageView 
   android:id="@+id/iv_image" 
   android:layout_width="fill_parent" 
   android:layout_height="fill_parent" 
   android:scaleType="fitCenter" /> 

 </FrameLayout> 
</LinearLayout> 

package com.example.asynctask; 
import java.io.BufferedInputStream; 
import java.io.ByteArrayOutputStream; 
import java.io.IOException; 
import java.io.InputStream; 
import java.net.MalformedURLException; 
import java.net.URLConnection; 
import android.os.AsyncTask; 
import android.os.Bundle; 
import android.app.Activity; 
import android.app.ProgressDialog; 
import android.graphics.Bitmap; 
import android.graphics.BitmapFactory; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
import android.widget.ImageView; 
public class MainActivity extends Activity implements OnClickListener{ 
 private ImageView image; 
 private ProgressDialog progress; 
 private Button btn_download; 
 private static String URL="http://img4.imgtn.bdimg.com/it/u=1256159061,743487979&fm=21&gp=0.jpg"; 
 @Override 
 protected void onCreate(Bundle savedInstanceState) { 
  super.onCreate(savedInstanceState); 
  setContentView(R.layout.activity_main); 
  image=(ImageView) findViewById(R.id.iv_image); 
  btn_download=(Button) findViewById(R.id.btn_download); 
  progress=new ProgressDialog(this); 
  progress.setIcon(R.drawable.ic_launcher); 
  progress.setTitle("提示信息"); 
  progress.setMessage("正在下載，請稍候..."); 
  progress.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL); 
  btn_download.setOnClickListener(this); 
 } 
 @Override 
 public void onClick(View v) { 
  // TODO Auto-generated method stub 
  new MyAsyncTask().execute(URL); 
 } 
 /* 
  * String*********對應(yīng)我們的URL類型 
  * Integer********進(jìn)度條的進(jìn)度值 
  * BitMap*********異步任務(wù)完成后返回的類型 
  * */ 
 class MyAsyncTask extends AsyncTask<String, Integer, Bitmap>{ 

  //執(zhí)行異步任務(wù)（doInBackground）之前執(zhí)行，并且在ui線程中執(zhí)行 
  @Override 
  protected void onPreExecute() { 
   // TODO Auto-generated method stub 
   super.onPreExecute(); 
   if(image!=null){ 
    image.setVisibility(View.GONE); 
   } 
   //開始下載 對話框進(jìn)度條顯示 
   progress.show(); 
   progress.setProgress(0); 
  } 
  @Override 
  protected Bitmap doInBackground(String... params) { 
   // TODO Auto-generated method stub 
   //params是一個可變長的數(shù)組 在這里我們只傳進(jìn)來了一個url 
   String url=params[0]; 
   Bitmap bitmap=null; 
   URLConnection connection; 
   InputStream is;//用于獲取數(shù)據(jù)的輸入流 
   ByteArrayOutputStream bos;//可以捕獲內(nèi)存緩沖區(qū)的數(shù)據(jù)，轉(zhuǎn)換成字節(jié)數(shù)組。 
   int len; 
   float count=0,total;//count為圖片已經(jīng)下載的大小 total為總大小 
   try { 
    //獲取網(wǎng)絡(luò)連接對象 
    connection=(URLConnection) new java.net.URL(url).openConnection(); 
    //獲取當(dāng)前頁面的總長度 
    total=(int)connection.getContentLength(); 
    //獲取輸入流 
    is=connection.getInputStream(); 
    bos=new ByteArrayOutputStream(); 
    byte []data=new byte[1024]; 
    while((len=is.read(data))!=-1){ 
     count+=len; 
     bos.write(data,0,len); 
     //調(diào)用publishProgress公布進(jìn)度,最后onProgressUpdate方法將被執(zhí)行 
     publishProgress((int)(count/total*100)); 
     //為了顯示出進(jìn)度 人為休眠0.5秒 
     Thread.sleep(500); 
    } 
    bitmap=BitmapFactory.decodeByteArray(bos.toByteArray(), 0, bos.toByteArray().length); 
    is.close(); 
    bos.close(); 
   } catch (MalformedURLException e) { 
    // TODO Auto-generated catch block 
    e.printStackTrace(); 
   } catch (IOException e) { 
    // TODO Auto-generated catch block 
    e.printStackTrace(); 
   } catch (InterruptedException e) { 
    // TODO Auto-generated catch block 
    e.printStackTrace(); 
   } 
   return bitmap; 
  } 
  //在ui線程中執(zhí)行 可以操作ui 
  @Override 
  protected void onPostExecute(Bitmap bitmap) { 
   // TODO Auto-generated method stub 
   super.onPostExecute(bitmap); 
   //下載完成 對話框進(jìn)度條隱藏 
   progress.cancel(); 
   image.setImageBitmap(bitmap); 
   image.setVisibility(View.VISIBLE); 
  } 
  /* 
   * 在doInBackground方法中已經(jīng)調(diào)用publishProgress方法 更新任務(wù)的執(zhí)行進(jìn)度后 
   * 調(diào)用這個方法 實現(xiàn)進(jìn)度條的更新 
   * */ 
  @Override 
  protected void onProgressUpdate(Integer... values) { 
   // TODO Auto-generated method stub 
   super.onProgressUpdate(values); 
   progress.setProgress(values[0]); 
  } 
 } 
}