public class AsyncTaskActivity2 extends Activity {  private Button btnDown;//圖片框  private ImageView ivImage;//圖片URL  private static String image_path = "http://www.wfu.edu.cn/page/main1406/images/nav2.jpg";  //進度對話框  private ProgressDialog dialog;  @Override  protected void onCreate(Bundle savedInstanceState) {   super.onCreate(savedInstanceState);   setContentView(R.layout.asynctask_activity);   btnDown = (Button) findViewById(R.id.btnDown);   ivImage = (ImageView) findViewById(R.id.ivSinaImage);   //新建一個對話框，不顯示   dialog = new ProgressDialog(this);   dialog.setTitle("提示");   dialog.setMessage("正在下載，請稍后...");   dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);//帶有水平滾動條   dialog.setCancelable(false);//不能   //單擊按鈕后執行AsyncTask，并傳入圖片URL   btnDown.setOnClickListener(new View.OnClickListener() {    @Override    public void onClick(View v) {     // 執行異步任務     new MyTask().execute(image_path);//啟動AsyncTask,參數類型為異步類的第一個參數的類型    }   });  }  //創建一個內部類，用來下載圖片  public class MyTask extends AsyncTask<String, Integer, Bitmap> {   @Override   protected void onPreExecute() {//啟動異步類后首先運行，可以設置    super.onPreExecute();    dialog.show();//顯示對話框   }    //第二步，一般進行復雜處理   @Override   protected Bitmap doInBackground(String... params) {//最重要，復雜處理，后臺，接收execute傳來的參數    Bitmap bitmap = null; //二進制圖片    //字節數組輸出流    ByteArrayOutputStream outputStream = new ByteArrayOutputStream();    InputStream inputStream = null;//字節輸入流    try {     HttpClient httpClient = new DefaultHttpClient();//創建一個Http客戶端     HttpGet httpGet = new HttpGet(params[0]);//發送請求     HttpResponse httpResponse = httpClient.execute(httpGet);//返回     if (httpResponse.getStatusLine().getStatusCode() == 200) {//成功      inputStream = httpResponse.getEntity().getContent();//得到返回內容作為輸入流      long file_length = httpResponse.getEntity().getContentLength();//文件長度      int len = 0;      byte[] data = new byte[1024];//每次讀取的字節      int total_length = 0;      // 以字節的方式讀取圖片數據      while ((len = inputStream.read(data)) != -1) {       total_length += len;       // 計算進度       int values = (int) ((total_length / (float) file_length) * 100);       // 發布進度信息       publishProgress(values);//發布進度，AsyncTask第二個參數類型，觸發onProgressUpdate更新進度條       outputStream.write(data, 0, len);//寫入輸出流      }      byte[] result=outputStream.toByteArray();//將字節數組輸出流轉換為字節數組      //生成二進制圖片      bitmap=BitmapFactory.decodeByteArray(result, 0, result.length);     }    } catch (Exception e) {     e.printStackTrace();    } finally {     try {      if (inputStream != null) {       inputStream.close();      }     } catch (Exception e2) {     }    }    return bitmap;   }   //當有 publishProgress(value)發來的數據時觸發,更新UI   @Override   protected void onProgressUpdate(Integer... values) {    super.onProgressUpdate(values);    // 設置進度對話框的進度值    dialog.setProgress(values[0]);//更新進度條，運行在UI   }   //第四步   @Override   protected void onPostExecute(Bitmap result) {//下載結束后，隱藏對話框    super.onPostExecute(result);    dialog.dismiss();    ivImage.setImageBitmap(result); //更新UI，顯示圖片，運行在UI   }  } }