a、對於網路上的一個資源，首先發送一個請求，從返回的Content-Length中回去需要下載檔案的大小，然後根據檔案大小建立一個檔案。
b、根據執行緒數和檔案大小，為每個執行緒分配下載的位元組區間，然後每個執行緒向伺服器傳送請求，獲取這段位元組區間的檔案內容。
c、利用RandomAccessFile的seek方法，多執行緒同時往一個檔案中寫入位元組。

【HTTP之Range】

　1、什麼是Range？

　　當用戶在聽一首歌的時候，如果聽到一半（網路下載了一半），網路斷掉了，使用者需要繼續聽的時候，檔案伺服器不支援斷點的話，則使用者需要重新下載這個檔案。而Range支援的話，客戶端應該記錄了之前已經讀取的檔案範圍，網路恢復之後，則向伺服器傳送讀取剩餘Range的請求，服務端只需要傳送客戶端請求的那部分內容，而不用整個檔案傳送回客戶端，以此節省網路頻寬。

　2、HTTP1.1規範的Range是怎樣一個約定呢？

　　如果Server支援Range，首先就要告訴客戶端，咱支援Range，之後客戶端才可能發起帶Range的請求。這裡套用唐僧的一句話，你不說我怎麼知道呢。response.setHeader(‘Accept-Ranges’, ‘bytes’);

　　Server通過請求頭中的Range: bytes=0-xxx來判斷是否是做Range請求，如果這個值存在而且有效，則只發回請求的那部分檔案內容，響應的狀態碼變成206，表示Partial Content，並設定Content-Range。如果無效，則返回416狀態碼，表明Request Range Not Satisfiable（

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  
{  

    /** 
     * 需要下載資源的地址 
     */  
    private String urlStr;  
    /** 
     * 下載的檔案 
     */  
    private File localFile;  
    /** 
     * 需要下載檔案的存放的本地資料夾路徑 
     */  
    private String dirStr;  
    /** 
     * 儲存到本地的檔名 
     */  
    private String filename;  

    /** 
     * 開啟的執行緒數量 
     */  
    private int threadCount;  
    /** 
     * 下載檔案的大小 
     */  
    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();  

        /** 
         * 計算每個執行緒需要下載的資料長度 
         */  
        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();  
        }  

    }  

    /** 
     * 根據資源的URL獲取資源的大小，以及在本地建立檔案 
     */  
    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();// 根據響應獲取檔案大小  
            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("需要下載的檔案大小為 :" + this.fileSize + " , 儲存位置為： "  
                    + dirStr + "/" + filename);  

        } else  
        {  
            throw new RuntimeException("url that you conneted has error ...");  
        }  
    }  

    private class DownloadThread extends Thread  
    {  
        /**  
         * 下載檔案的URI  
         */  
        private URL url;  
        /** 
         * 存的本地路徑 
         */  
        private File localFile;  
        /** 
         * 是否結束 
         */  
        private boolean isFinish;  
        /** 
         * 開始的位置 
         */  
        private Long startPos;  
        /** 
         * 結束位置 
         */  
        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() + "開始下載...");  
            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);// 設定獲取實體資料的範圍  

                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();  

                /**  
                 * 代表伺服器已經成功處理了部分GET請求  
                 */  
                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()  
                            + "完成下載  ： " + 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();  
        }  

    }  



}