android多线程下载图片

jopen 12年前

很多时候我们需要在Android设备上下载远程服务器上的图片进行显示,今天Android123整理出两种比较好的方法来实现远程图片的下载。 
  方法一、直接通过Android提供的Http类访问远程服务器,这里AndroidHttpClient是SDK 2.2中新出的方法,API Level为8,大家需要注意下,静态访问可以直接调用,如果SDK版本较低可以考虑Apache的Http库,当然HttpURLConnection 或URLConnection也可以。 

   static Bitmap downloadBitmapByCwj(String url) { 
    final AndroidHttpClient client = AndroidHttpClient.newInstance("Android123"); 
    final HttpGet getRequest = new HttpGet(url); 

    try { 
        HttpResponse response = client.execute(getRequest); 
        final int statusCode = response.getStatusLine().getStatusCode(); 
        if (statusCode != HttpStatus.SC_OK) {  
            Log.e("cwjDebug", "Error " + statusCode + " while retrieving bitmap from " + url);  
            return null; 
        } 
         
        final HttpEntity entity = response.getEntity(); 
        if (entity != null) { 
            InputStream inputStream = null; 
            try { 
                inputStream = entity.getContent();  
                final Bitmap bitmap = BitmapFactory.decodeStream(inputStream); 
                return bitmap; 
            } finally { 
                if (inputStream != null) { 
                    inputStream.close();   
                } 
                entity.consumeContent(); 
            } 
        } 
    } catch (Exception e) { 
          getRequest.abort(); 
        Log.e("android123Debug", "Error while retrieving bitmap from " + url, e.toString()); 
    } finally { 
        if (client != null) { 
            client.close(); 
        } 
    } 
    return null; 


  这里Android开发网提醒大家,BitmapFactory类的decodeStream方法在网络超时或较慢的时候无法获取完整的数据,这里我们通过继承FilterInputStream类的skip方法来强制实现flush流中的数据,主要原理就是检查是否到文件末端,告诉http类是否继续。 

static class FlushedInputStream extends FilterInputStream { 
    public FlushedInputStream(InputStream inputStream) { 
        super(inputStream); 
    } 

    @Override 
    public long skip(long n) throws IOException { 
        long totalBytesSkipped = 0L; 
        while (totalBytesSkipped < n) { 
            long bytesSkipped = in.skip(n - totalBytesSkipped); 
            if (bytesSkipped == 0L) { 
                  int byte = read(); 
                  if (byte < 0) { 
                      break;  // we reached EOF 
                  } else { 
                      bytesSkipped = 1; // we read one byte 
                  } 
           } 
            totalBytesSkipped += bytesSkipped; 
        } 
        return totalBytesSkipped; 
    } 


  方法二、AsyncTask异步任务 

  从Android 1.5固件开始Google提供了一个AsyncTask类来帮助开发者处理异步下载的实现,相对于Thread而言他可以运行在UI线程中,其内部的实现是从Java 5开始的并发包concurrent中派生而来的,总体实现比较可靠就是资源占用略大了些。不过使用起来比简单。这里下载图片类 ImageDownloader类的download方法可以很好的处理实现UI显示等操作,参数一url为远程server上文件的url,第二个参数为imageview对象,可以直接让imageview显示出下载的远程图片。 

  public class ImageDownloader { 

    public void download(String url, ImageView imageView) { 
            BitmapDownloaderTask task = new BitmapDownloaderTask(imageView); 
            task.execute(url); 
        } 
    } 



有关具体的AsyncTask类实现,考虑到图片可能较大,为了给JVM充分的空间存储,这里Android123推荐大家使用弱引用来保存ImageView对象。

class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> { 
    private String url; 
    private final WeakReference<ImageView> imageViewReference;  //使用WeakReference解决内存问题 

    public BitmapDownloaderTask(ImageView imageView) { 
        imageViewReference = new WeakReference<ImageView>(imageView); 
    } 

    @Override 
    protected Bitmap doInBackground(String... params) {   //实际的下载线程,内部其实是concurrent线程,所以不会阻塞 
   
         return downloadBitmap(params[0]);   

  } 

    @Override 
     protected void onPostExecute(Bitmap bitmap) {   //下载完后执行的 
        if (isCancelled()) { 
            bitmap = null; 
        } 

        if (imageViewReference != null) { 
            ImageView imageView = imageViewReference.get(); 
            if (imageView != null) { 
                imageView.setImageBitmap(bitmap);  //下载完设置imageview为刚才下载的bitmap对象 
            } 
        } 
    } 
}