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(); } } }