java 多线程断点下载demo

源码链接

 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());
                         }
                     }
                 }
             }
         }
     }
 }