import java.io.BufferedReader;

 import java.io.File;

 import java.io.FileInputStream;

 import java.io.InputStream;

 import java.io.InputStreamReader;

 import java.io.RandomAccessFile;

 import java.net.HttpURLConnection;

 import java.net.MalformedURLException;

 import java.net.URL;

 import java.net.URLConnection;

 public class MutileThreadDownload {

     /**

      * 线程的数量

      */

     private static int threadCount = 3;

     /**

      * 每个下载区块的大小

      */

     private static long blocksize;

     /**

      * 正在运行的线程的数量

      */

     private static int runningThreadCount;

     /**

      *

      * @param args

      * @throws Exception

      */

     public static void main(String[] args) throws Exception {

         //服务器路径

         String path = "http://download.java.net/jdk/jdk-api-localizations/jdk-api-zh-cn/publish/1.6.0/chm/JDK_API_1_6_zh_CN.CHM";

         URL url = new URL(path);

         HttpURLConnection conn = (HttpURLConnection) url.openConnection();

         conn.setRequestMethod("GET");

         conn.setReadTimeout(5000);

         int code = conn.getResponseCode();

         if(code == 200) {

             long size = conn.getContentLength();//得到服务器端返回的文件的大小

             System.out.println("服务器文件的大小：" + size);

             blocksize = size / threadCount;

             //1.首先在本地创建一个大小跟服务器一模一样的空白文件

             File file = new File("temp.CHM");

             RandomAccessFile raf = new RandomAccessFile(file, "rw");

             raf.setLength(size);

             //2.开启若干个子线程分别去下载对应的资源

             runningThreadCount = threadCount;

             for(int i = 1;i <= threadCount; i++) {

                 long startIndex = (i - 1) * blocksize;

                 long endIndex = i * blocksize - 1;

                 if(i == threadCount) {

                     //最后一个线程

                     endIndex = size - 1;

                 }

                 System.out.println("开启线程：" + i + "下载的位置：" + startIndex + "~" + endIndex);

                 new DownloadThread(i, startIndex, endIndex, path).start();

             }

         }

         conn.disconnect();

     }

     private static class DownloadThread extends Thread {

         private int threadId;

         private long startIndex;

         private long endIndex;

         private String path;

         public DownloadThread(int threadId, long startIndex, long endIndex,

                 String path) {

             super();

             this.threadId = threadId;

             this.startIndex = startIndex;

             this.endIndex = endIndex;

             this.path = path;

         }

         public void run() {

             try {

                 //当前线程下载的总大小

                 int total = 0;

                 File positionFile = new File(threadId + ".txt");

                 URL url = new URL(path);

                 HttpURLConnection conn = (HttpURLConnection) url.openConnection();

                 conn.setRequestMethod("GET");

                 //接着从上一次的位置继续下载数据

                 if(positionFile.exists() && positionFile.length() > 0) {  //判断是否有记录

                     FileInputStream fis = new FileInputStream(positionFile);

                     BufferedReader br = new BufferedReader(new InputStreamReader(fis));

                     //获取当前线程上次下载的总大小是多少

                     String lasttotalstr = br.readLine();

                     int lastTotal = Integer.valueOf(lasttotalstr);

                     System.out.println("上次线程" + threadId + "下载的总大小：" + lastTotal);

                     startIndex += lastTotal;  //加上上次下载的总大小

                     fis.close();

                 }

                 conn.setRequestProperty("Range", "bytes=" + startIndex + "-" +endIndex);

                 conn.setConnectTimeout(5000);

                 int code = conn.getResponseCode();

                 System.out.println("code=" + code);

                 InputStream is = conn.getInputStream();

                 File file = new File("temp.CHM");

                 RandomAccessFile raf = new RandomAccessFile(file, "rw");

                 raf.seek(startIndex);

                 System.out.println("第" + threadId + "个线程：写文件的开始位置：" + String.valueOf(startIndex));

                 int len = 0;

                 byte[] buffer = new byte[1024 * 1024];

                 while((len = is.read(buffer)) != -1) {

                     RandomAccessFile rf = new RandomAccessFile(positionFile, "rwd");//"rwd"   打开以便读取和写入，对于 "rw"，还要求对文件内容的每个更新都同步写入到底层存储设备。

                     raf.write(buffer, 0, len);

                     total += len;

                     rf.write(String.valueOf(total).getBytes());

                     rf.close();

                 }

                 is.close();

                 raf.close();

                 //conn.disconnect();

             } catch (Exception e) {

                 e.printStackTrace();

             }finally {

                 //只有所有线程都下载完毕后，才可以删除记录文件

                 synchronized (MutileThreadDownload.class) {   //线程安全

                     System.out.println("线程" + threadId + "下载完毕了");

                     runningThreadCount--;

                     if(runningThreadCount < 1) {

                         System.out.println("所有线程都工作完毕了。删除临时记录的文件");

                         for (int i = 1; i <= threadCount; i++) {

                             File f = new File(i + ".txt");

                             System.out.println(f.delete());

                         }

                     }

                 }

             }

         }

     }

 }