Http的多线程下载的实现

a、对于网络上的一个资源，首先发送一个请求，从返回的Content-Length中回去需要下载文件的大小，然后根据文件大小创建一个文件。
b、根据线程数和文件大小，为每个线程分配下载的字节区间，然后每个线程向服务器发送请求，获取这段字节区间的文件内容。
c、利用RandomAccessFile的seek方法，多线程同时往一个文件中写入字节。

【HTTP之Range】

　1、什么是Range？

　　当用户在听一首歌的时候，如果听到一半（网络下载了一半），网络断掉了，用户需要继续听的时候，文件服务器不支持断点的话，则用户需要重新下载这个文件。而Range支持的话，客户端应该记录了之前已经读取的文件范围，网络恢复之后，则向服务器发送读取剩余Range的请求，服务端只需要发送客户端请求的那部分内容，而不用整个文件发送回客户端，以此节省网络带宽。

　2、HTTP1.1规范的Range是怎样一个约定呢？

　　如果Server支持Range，首先就要告诉客户端，咱支持Range，之后客户端才可能发起带Range的请求。这里套用唐僧的一句话，你不说我怎么知道呢。response.setHeader(‘Accept-Ranges’, ‘bytes’);

　　Server通过请求头中的Range: bytes=0-xxx来判断是否是做Range请求，如果这个值存在而且有效，则只发回请求的那部分文件内容，响应的状态码变成206，表示Partial Content，并设置Content-Range。如果无效，则返回416状态码，表明Request Range Not Satisfiable（http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.17 ）。如果不包含Range的请求头，则继续通过常规的方式响应。

package com.zhy.mutilthread_download;  import java.io.File;  import java.io.IOException;  import java.io.InputStream;  import java.io.RandomAccessFile;  import java.net.HttpURLConnection;  import java.net.URL;  public class MultipartThreadDownloador  {      /**      * 需要下载资源的地址      */      private String urlStr;      /**      * 下载的文件      */      private File localFile;      /**      * 需要下载文件的存放的本地文件夹路径      */      private String dirStr;      /**      * 存储到本地的文件名      */      private String filename;      /**      * 开启的线程数量      */      private int threadCount;      /**      * 下载文件的大小      */      private long fileSize;      public MultipartThreadDownloador(String urlStr, String dirStr,              String filename, int threadCount)      {          this.urlStr = urlStr;          this.dirStr = dirStr;          this.filename = filename;          this.threadCount = threadCount;      }      public void download() throws IOException      {          createFileByUrl();          /**          * 计算每个线程需要下载的数据长度          */          long block = fileSize % threadCount == 0 ? fileSize / threadCount                  : fileSize / threadCount + 1;          for (int i = 0; i < threadCount; i++)          {              long start = i * block;              long end = start + block >= fileSize ? fileSize : start + block - 1;              new DownloadThread(new URL(urlStr), localFile, start, end).start();          }      }      /**      * 根据资源的URL获取资源的大小，以及在本地创建文件      */      public void createFileByUrl() throws IOException      {          URL url = new URL(urlStr);          HttpURLConnection conn = (HttpURLConnection) url.openConnection();          conn.setConnectTimeout(15 * 1000);          conn.setRequestMethod("GET");          conn.setRequestProperty(                  "Accept",                  "image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*");          conn.setRequestProperty("Accept-Language", "zh-CN");          conn.setRequestProperty("Referer", urlStr);          conn.setRequestProperty("Charset", "UTF-8");          conn.setRequestProperty(                  "User-Agent",                  "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");          conn.setRequestProperty("Connection", "Keep-Alive");          conn.connect();          if (conn.getResponseCode() == 200)          {              this.fileSize = conn.getContentLength();// 根据响应获取文件大小              if (fileSize <= 0)                  throw new RuntimeException(                          "the file that you download has a wrong size ... ");              File dir = new File(dirStr);              if (!dir.exists())                  dir.mkdirs();              this.localFile = new File(dir, filename);              RandomAccessFile raf = new RandomAccessFile(this.localFile, "rw");              raf.setLength(fileSize);              raf.close();              System.out.println("需要下载的文件大小为 :" + this.fileSize + " , 存储位置为： "                      + dirStr + "/" + filename);          } else          {              throw new RuntimeException("url that you conneted has error ...");          }      }      private class DownloadThread extends Thread      {          /**           * 下载文件的URI           */          private URL url;          /**          * 存的本地路径          */          private File localFile;          /**          * 是否结束          */          private boolean isFinish;          /**          * 开始的位置          */          private Long startPos;          /**          * 结束位置          */          private Long endPos;          public DownloadThread(URL url, File savefile, Long startPos, Long endPos)          {              this.url = url;              this.localFile = savefile;              this.startPos = startPos;              this.endPos = endPos;          }          @Override          public void run()          {              System.out.println(Thread.currentThread().getName() + "开始下载...");              try              {                  HttpURLConnection conn = (HttpURLConnection) url                          .openConnection();                  conn.setConnectTimeout(15 * 1000);                  conn.setRequestMethod("GET");                  conn.setRequestProperty(                          "Accept",                          "image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*");                  conn.setRequestProperty("Accept-Language", "zh-CN");                  conn.setRequestProperty("Referer", url.toString());                  conn.setRequestProperty("Charset", "UTF-8");                  conn.setRequestProperty("Range", "bytes=" + startPos + "-"                          + endPos);// 设置获取实体数据的范围                  conn.setRequestProperty(                          "User-Agent",                          "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");                  conn.setRequestProperty("Connection", "Keep-Alive");                  conn.connect();                  /**                   * 代表服务器已经成功处理了部分GET请求                   */                  if (conn.getResponseCode() == 206)                  {                      InputStream is = conn.getInputStream();                      int len = 0;                      byte[] buf = new byte[1024];                      RandomAccessFile raf = new RandomAccessFile(localFile,                              "rwd");                      raf.seek(startPos);                      while ((len = is.read(buf)) != -1)                      {                          raf.write(buf, 0, len);                      }                      raf.close();                      is.close();                      System.out.println(Thread.currentThread().getName()                              + "完成下载  ： " + startPos + " -- " + endPos);                      this.isFinish = true;                  } else                  {                      throw new RuntimeException(                              "url that you conneted has error ...");                  }              } catch (IOException e)              {                  e.printStackTrace();              }          }      }      public static void main(String[] args)      {          try          {              new MultipartThreadDownloador("http://img.firefoxchina.cn/2016/09/5/201609141103140.jpg",                      "D:\\KwDownload", "dd.jpg", 2).download();          } catch (IOException e)          {              e.printStackTrace();          }      }  }