android实现多线程下载

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.ProtocolException;
import java.net.URL;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

import android.os.Environment;
import android.os.Handler;
import android.os.Message;
/**
 * 多线程下载
 * @author raid
 *
 */
public class Download {
    /**
     * 创建一个线程池
     */
    private Executor threadPool = Executors.newFixedThreadPool(3);
    /**
     * 接收一个Handler对象，用于获取下载进度
     */
    private Handler handler;
    /**
     * 默认的下载线程个数为3
     */
    public static final int THREAD_COUNT = 3;
    
    public Download(Handler handler) {
        super();
        this.handler = handler;
    }

    /**
     * 下载线程，这里定义成静态类
     * @author raid
     *
     */
    static class DownLoadRunnable implements Runnable {
        private String url;
        private String fileName;
        private long start;
        private long end;
        private Handler handler;
        

        /**
         * 构造方法
         * @param url 需要下载的网站url
         * @param fileName 下载回本地的文件名
         * @param start 开始位置
         * @param end 终止位置
         * @param handler 用于更新UI显示下载进度
         */
        public DownLoadRunnable(String url, String fileName, long start,
                long end, Handler handler) {
            super();
            this.url = url;
            this.fileName = fileName;
            this.start = start;
            this.end = end;
            this.handler = handler;
        }



        @Override
        public void run() {
            // TODO Auto-generated method stub
            RandomAccessFile access = null;
            InputStream in = null;
            try {
                HttpURLConnection conn = getConnection(url);
                //设置请求读取文件的开始字节数和结束字节数
                conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
                access = new RandomAccessFile(fileName, "rwd");
                access.seek(start);
                in = conn.getInputStream();
                byte[] b = new byte[4*1024];
                int len = 0;
                while((len=in.read(b)) != -1) {
                    access.write(b, 0, len);
                }
                
                Message msg = Message.obtain();
                msg.what = 1;
                handler.sendMessage(msg);
                
            } catch (MalformedURLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } finally {
                if (access != null) {
                    try {
                        access.close();
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                if (in != null) {
                    try {
                        in.close();
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }
        }
        
    }
    
    /**
     * 调用多线程下载的方法
     * @param url 下载的资源的地址
     */
    public void downloadFile(String url) {
        HttpURLConnection conn = getConnection(url);
        int count = conn.getContentLength();
        int block = count / THREAD_COUNT;
        
        String fileName = getFileName(url);
        File parent = Environment.getExternalStorageDirectory();
        File fileDownload = new File(parent, fileName);
        for(int i = 0; i < THREAD_COUNT; i++) {
            long start = i*block;
            long end = (i+1) * block - 1;
            if (i == THREAD_COUNT-1) {
                end = count;
            }
            DownLoadRunnable runnable =
                    new DownLoadRunnable(url,
                            fileDownload.getAbsolutePath(),
                            start, end, handler);
            threadPool.execute(runnable);
        }
    }
    
    private String getFileName(String url) {
        return url.substring(url.lastIndexOf("/")+1);
    }
    
    private static HttpURLConnection getConnection(String url) {
        URL httpUrl;
        try {
            httpUrl = new URL(url);
            HttpURLConnection conn = (HttpURLConnection) httpUrl.openConnection();
            conn.setReadTimeout(5000);
            conn.setRequestMethod("GET");
            
            return conn;
        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ProtocolException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        
        return null;
    }
}