项目中用到的，要求一次下载30张图片。开始时使用谷歌bitmap fun中提供的ImageFetcher来下载，但是发现一个蛋疼无比现象，图片总是莫名其妙的少几张。

   排除了图片地址存在无效链接外，怀疑是并发下载线程个数太多，线程池满了以后，使用抛弃策略将以前的下载线程抛弃了。

   求人不如求己，自己写一个吧。

   在这里使用线程池，支持并发下载。线程池可以自己选择，使用 newSingleThreadExecutor，newFixedThreadPool，newCachedThreadPool中的任意一种。使用 时，自己实现监听器，当监听下载个数与url集合的个数相同时，会回调监听器的onSuccess()方法。

   源码如下，希望大家指正

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.UnsupportedEncodingException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.RejectedExecutionException;
import android.util.Log;
 
/**
 * 批量图片下载类 无需与界面交互的下载类
 *
 * @Description:
 *
 * @author- liubing
 * @create- 2013-5 -6
 * @modify by:
 * @modify- time
 */
public class DownloadService {
     private static String TAG = "DownloadService" ;
     public static final int IO_BUFFER_SIZE = 8 * 1024;
     private static final String CACHE_FILENAME_PREFIX = "cache_";
     private static ExecutorService SINGLE_TASK_EXECUTOR = null;
     private static ExecutorService LIMITED_TASK_EXECUTOR = null;
     private static final ExecutorService FULL_TASK_EXECUTOR = null;
     private static final ExecutorService DEFAULT_TASK_EXECUTOR ;
     private static Object lock = new Object();
     static {
           // SINGLE_TASK_EXECUTOR = (ExecutorService)
           // Executors.newSingleThreadExecutor();
           LIMITED_TASK_EXECUTOR = (ExecutorService) Executors
                   . newFixedThreadPool(1);
           // FULL_TASK_EXECUTOR = (ExecutorService)
           // Executors.newCachedThreadPool();
           DEFAULT_TASK_EXECUTOR = LIMITED_TASK_EXECUTOR ;
     };
     // 下载状态监听，提供回调
     DownloadStateListener listener;
     // 下载目录
     private String downloadPath;
 
     // 下载链接集合
     private List<String> listURL;
     // 下载个数
     private int size = 0;
 
     // 下载完成回调接口
     public interface DownloadStateListener {
           public void onFinish();
 
           public void onFailed();
     }
 
     public DownloadService(String downloadPath, List<String> listURL,
              DownloadStateListener listener) {
           this.downloadPath = downloadPath;
           this.listURL = listURL;
           this.listener = listener;
     }
 
     /**
      * 暂未提供设置
      */
     public void setDefaultExecutor() {
 
     }
 
     /**
      * 开始下载
      */
     public void startDownload() {
           // 首先检测path是否存在
          File downloadDirectory = new File(downloadPath );
           if (!downloadDirectory.exists()) {
              downloadDirectory.mkdirs();
          }
 
           for (final String url : listURL) {
               //捕获线程池拒绝执行异常
               try {
                    // 线程放入线程池
                    DEFAULT_TASK_EXECUTOR.execute(new Runnable() {
 
                         @Override
                         public void run() {
                             downloadBitmap(url);
                        }
                   });
              } catch (RejectedExecutionException e) {
                   e.printStackTrace();
                   Log. e(TAG, "thread pool rejected error");
                    listener.onFailed();
              } catch (Exception e) {
                   e.printStackTrace();
                    listener.onFailed();
              }
 
          }
 
     }
 
     /**
      * 下载图片
      *
      * @param urlString
      * @return
      */
     private File downloadBitmap(String urlString) {
          String fileName = urlString;
           // 图片命名方式
           final File cacheFile = new File(createFilePath(new File(
                    downloadPath), fileName));
 
          HttpURLConnection urlConnection = null;
          BufferedOutputStream out = null;
 
           try {
               final URL url = new URL(urlString);
              urlConnection = (HttpURLConnection) url.openConnection();
               final InputStream in = new BufferedInputStream(
                        urlConnection.getInputStream(), IO_BUFFER_SIZE);
              out = new BufferedOutputStream(new FileOutputStream(cacheFile),
                         IO_BUFFER_SIZE);
 
               int b;
               while ((b = in.read()) != -1) {
                   out.write(b);
              }
               // 每下载成功一个，统计一下图片个数
              statDownloadNum();
               return cacheFile;
 
          } catch (final IOException e) {
               // 有一个下载失败，则表示批量下载没有成功
              Log. e(TAG, "download " + urlString + " error");
               listener.onFailed();
 
          } finally {
               if (urlConnection != null) {
                   urlConnection.disconnect();
              }
               if (out != null ) {
                    try {
                        out.close();
                   } catch (final IOException e) {
                        Log. e(TAG, "Error in downloadBitmap - " + e);
                   }
              }
          }
 
           return null ;
     }
 
     /**
      * Creates a constant cache file path given a target cache directory and an
      * image key.
      *
      * @param cacheDir
      * @param key
      * @return
      */
     public static String createFilePath(File cacheDir, String key) {
           try {
               // Use URLEncoder to ensure we have a valid filename, a tad hacky
               // but it will do for
               // this example
               return cacheDir.getAbsolutePath() + File.separator + CACHE_FILENAME_PREFIX
                        + URLEncoder.encode(key.replace("*", ""), "UTF-8" );
          } catch (final UnsupportedEncodingException e) {
              Log. e(TAG, "createFilePath - " + e);
          }
 
           return null ;
     }
 
 
     /**
      * 统计下载个数
      */
     private void statDownloadNum() {
           synchronized (lock ) {
               size++;
               if (size == listURL .size()) {
                   Log. d(TAG, "download finished total " + size);
                    // 释放资源
                    DEFAULT_TASK_EXECUTOR.shutdownNow();
                    // 如果下载成功的个数与列表中 url个数一致，说明下载成功
                    listener.onFinish(); // 下载成功回调
              }
          }
     }
}

使用方法如下：

new DownloadService( "/mnt/sdcard/test", listUrl, new DownloadStateListener() {
               
               @Override
               public void onFinish() {
                    //图片下载成功后，实现您的代码
                    
              }
               
               @Override
               public void onFailed() {
                    //图片下载成功后，实现您的代码
                    
              }
          }).startDownload();