断点续传的原理：一个文件下载了一部分后，由于服务器或客户端原因，当前下载进度中断，用户可继续重新建立网络连接继续下载未下完的部分

断点三个最主要的属性:   Code:连接返回响应状态,状态码206支持断点续传

                       

                        Range属性:下载区域，它接收是一个区间范围，比如:Range:bytes=0-10000

                        例如：下载8.zip文件要求从10086字节开始传，前面的字节不用传，那么RANGE: bytes=10086-

                        Content-Range：bytes start- (fileSize-1)/fileSize;注释:fileSize文件总大小，start续传区间，从哪个位置开始续传的位置

首次请求支持断点续传服务器:

请求：

GET /down.zip HTTP/1.1

Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-

excel, application/msword, application/vnd.ms-powerpoint, */*

Accept-Language: zh-cn

Accept-Encoding: gzip, deflate

User-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)

Connection: Keep-Alive

响应：

200

Content-Length=106786028

Accept-Ranges=bytes

Date=Mon, 30 Apr 2013 12:56:11 GMT

ETag=W/"02ca57e173c11:95b"

Content-Type=application/octet-stream

Server=Microsoft-IIS/5.0

Last-Modified=Mon, 30 Apr 2013 12:56:11 GMT

第二次请求支持断点续传服务器：

请求：

GET /down.zip HTTP/1.0

User-Agent: NetFox

RANGE: bytes=2000070-

Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2

响应：

206

Content-Length=106786028

Content-Range=bytes 2000070-106786027/106786028

Date=Mon, 30 Apr 2013 12:55:20 GMT

ETag=W/"02ca57e173c11:95b"

Content-Type=application/octet-stream

Server=Microsoft-IIS/5.0

Last-Modified=Mon, 30 Apr 2013 12:55:20 GMT

示例图：

 

代码示例:

支持断点续传后台服务：

import java.io.File;import java.io.FileInputStream;import java.io.IOException;import java.io.PrintWriter;import java.io.RandomAccessFile;import javax.servlet.ServletException;import javax.servlet.ServletOutputStream;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;@SuppressWarnings("serial")public class RetryBokenDownloadsServlet extends HttpServlet {		public RetryBokenDownloadsServlet() {		super();	}	public void init() throws ServletException {		super.init();	}		public void doGet(HttpServletRequest request, HttpServletResponse response)			throws ServletException, IOException {				    //定义一个任意访问文件位置的RandomAccessFiled对象		    RandomAccessFile raFile = null;		    //获取响应输出流对象		    ServletOutputStream sos = null;			 try {			//下载网络杂志的相对路径			String downloadFile = request.getParameter("downloadFile");			//下载文件名			String fileName = request.getParameter("fileName");			//获取绝对路径下网络杂志文件大小			int fileSize = getFileByte(FileUtil.SITE_PATH + downloadFile);			//设置下载文件大小			response.addHeader("content-file-length", "" + (fileSize));			//定义一个任意访问文件位置的RandomAccessFiled对象			raFile = new RandomAccessFile(FileUtil.SITE_PATH + downloadFile, "r");	        //设置获取客户端请求RANGE,获取下载文件的字节开始位置和结束位置			String range = request.getHeader("RANGE");			int status =200; //返回的状态码，默认200，首次下载			//如果range下载区域为空，则首次下载，			if(range ==null){				range="bytes=0-";			}else{				//非首次下载通过下载区域下载使用206状态码支持断点续传				status =206;			}			int start = 0, end = 0;			if (null != range && range.startsWith("bytes=")) {				String[] values = range.split("=")[1].split("-");				start = Integer.parseInt(values[0]);				//如果服务器端没有设置end结尾，默认取下载全部				if(values.length==1){					end = fileSize;				}else{					end = Integer.parseInt(values[1]);				}							}			//此次数据响应大小			int responseSize = 0;			if (end != 0 && end > start) {				responseSize = end - start + 1;				//返回当前连接下载的数据大小,也就是此次数据传输大小				response.addHeader("content-length", "" + (responseSize));			} else {				responseSize = Integer.MAX_VALUE;			}                        //设置缓存为要下载的文件大小一半，减少太频繁并发连接数超过了其承载量而出现 ClientAbortException:  java.io.IOException异常，在windows7本地调试不会，在window2003和linux会出现，待排查                        //超过20M，缓存设置默认20M左右，如视频，防止内存溢出                        int maxSize = 1024*1024*20;                        byte[] buffer = null;                        if (fileSize > maxSize){                                buffer = new byte[1024*1024*20];                        }else{
                                 buffer =  new byte[fileSize/2];                        }			//设置响应状态码			response.setStatus(status);			if(status == 206){				//设置断点续传的Content-Range传输字节和总字节				response.addHeader("Content-Range","bytes "+start+"-"+(fileSize-1)+"/"+fileSize);			}			//设置响应客户端内容类型			response.setContentType("application/x-download");			//设置响应客户端头			response.addHeader("Content-Disposition", "attachment;filename="+ fileName);			//response.addHeader("Content-Range","bytes ""1000-5000/5001" );			//获取响应输出流对象			sos = response.getOutputStream();			//当前需要下载文件的大小			int needSize = responseSize;			//将下载网络杂志记录指针定位到start位置			raFile.seek(start);			while (needSize > 0) {				int len = raFile.read(buffer);				if (needSize < buffer.length) {					sos.write(buffer, 0, needSize);				} else {					sos.write(buffer, 0, len);					//如果读取文件大小小于缓冲字节大小，表示已写入完，直接跳出					if (len < buffer.length) {						break;					}				}				//不断更新当前可下载文件大小				needSize -= buffer.length;			}	     		} catch (Exception e) {			e.printStackTrace();		}finally{			//关闭流			if(raFile != null){			   raFile.close();			}			if(sos != null){			   sos.close();			}		}		}	public void doPost(HttpServletRequest reuqest,			HttpServletResponse response) throws ServletException, IOException {		doGet(reuqest,response);		response.setContentType("text/html");		PrintWriter out = response.getWriter();		out.println("");		out.println("");		out.println("");		out.flush();		out.close();	}	/**	 * 获取文件字节总大小	 * @param filePath 文件绝对路径（硬盘路径）	 * @return	 * @throws IOException 	 */	private static int getFileByte(String filePath) throws IOException {		File file = new File(filePath);		FileInputStream fs = null;		int fileSize = 0;		try {			fs = new FileInputStream(file);            fileSize = fs.available();		} catch (Exception e) {			e.printStackTrace();		}finally{			if(fs!=null){				fs.close();			}		}		return fileSize;	}			public void destroy() {		super.destroy(); 	}	}

 

测试类：

/**    * 断点续传下载     * @param start 开始位置     * @param end 结束位置     * @throws MalformedURLException     * @throws FileNotFoundException    */    public static void retryBokenDownloads(int start, int end){                    String testUrl= "xxxxxxx.zip";        try {            File file = new File("E:/temp/8.zip");           if(file.exists()){              System.out.println("获取已下载的文件大小="+file.length());           }else{             System.out.println("获取已下载的文件大小0");           }            RandomAccessFile raFile = new RandomAccessFile("E:/temp/8.zip", "rw");              URL url = new URL(endpoint);              HttpURLConnection conn = (HttpURLConnection) url.openConnection();               conn.setRequestProperty("Content-Type","text/html; charset=UTF-8");               conn.setRequestProperty("RANGE","bytes="+start+"-"+end);               conn.connect();            System.out.println(conn.getResponseCode());    //获取服务端的返回的响应码,200-返回连接成功              System.out.println(conn.getContentLength());   //获取服务端的内容长度              System.out.println(conn.getContentType());    //获取服务端的内容类型              System.out.println(conn.getHeaderField("content-file-length"));  //获取服务端的文件大小              System.out.println(conn.getHeaderField("Content-Range"));        //获取服务端的文件大小             //System.out.println(conn.getHeaderField("content-length"));      //连接服务端下载的大小              InputStream ins = (InputStream)conn.getContent();              raFile.seek(start);                            byte[] buffer = new byte[4096];              int len = -1;              while((len = ins.read(buffer))!=-1){                  raFile.write(buffer,0,len);              }              raFile.close();              conn.disconnect();          } catch (Exception e) {              e.printStackTrace();          }      }}