客户端；
import java.io.BufferedInputStream;
import java.io.File;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class ArcSyncHttpClient {
public static void main(String[] args) throws Exception {


String strURL = "http://localhost:8080/ArcSyncHttpDownload/httpServlet";
URL downUrl = new URL(strURL);
HttpURLConnection cn = (HttpURLConnection) downUrl.openConnection();
if (cn.getResponseCode() == 200) {
// InputStream is = cn.getInputStream();
int part_count = 10;

int contentLength = cn.getContentLength();
System.out.println("cn.getContentLength():"+cn.getContentLength());
int firstLength = contentLength / part_count;
System.out.println("firstLength():" + firstLength);
cn.disconnect();
// part_count = 1;
for (int i = 1; i < part_count+1; i++) {
if (i==part_count) {
// int lastLength = contentLength -firstLength*(part_count-1);
File saveFile = new File("E:/test/123",String.format("123_0%d.mp3", i));
TestDownload.download(saveFile.getName(), saveFile.getParent(),
strURL, (i-1)*firstLength, contentLength);
}else {
File saveFile = new File("E:/test/123",String.format("123_0%d.mp3", i));
TestDownload.download(saveFile.getName(), saveFile.getParent(),
strURL, (i-1)*firstLength, firstLength*i);
}

}
}
}

public static void download( String fileName, String filePath,String remoteUrl, long start ,long end) {
byte[] buf = new byte[10240];
HttpURLConnection httpURLConnection;
URL url;
BufferedInputStream bis;
int size;
RandomAccessFile rndFile;
// 下载文件
try {
url = new URL(remoteUrl);
httpURLConnection = (HttpURLConnection)url.openConnection();
// 设置User-Agent
httpURLConnection.setRequestProperty("User-Agent", "Net");
// 设置续传开始
httpURLConnection.setRequestProperty("Range", "bytes=" + start + "-"+ end);
// 获取输入流
bis = new BufferedInputStream(httpURLConnection.getInputStream());
rndFile = new RandomAccessFile(filePath + "\\" + fileName, "rw");
rndFile.seek(0);
int i = 0;
while ((size = bis.read(buf)) != -1) {
//if (i > 500) break;
rndFile.write(buf, 0, size);

i++;
}
System.out.println("i=" + i);
httpURLConnection.disconnect();
bis.close();
rndFile.close();
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}
}

public static long getRemoteFileSzie(String url) {
long size = 0;
try {
HttpURLConnection httpUrl = (HttpURLConnection)(new URL(url)).openConnection();
size = httpUrl.getContentLength();
httpUrl.disconnect();
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}
return size;
}

public static void mergeFiles(String outFile, List<String> files) {
FileChannel outChannel = null;
System.out.println("Merge " + files + " into " + outFile);
try {
outChannel = new FileOutputStream(outFile).getChannel();
for (String f : files) {
FileChannel fc = new FileInputStream(f).getChannel();
ByteBuffer bb = ByteBuffer.allocate(BUFSIZE);
while (fc.read(bb) != -1) {
bb.flip();
outChannel.write(bb);
bb.clear();
}
fc.close();
}
System.out.println("Merged!! ");
} catch (IOException ioe) {
ioe.printStackTrace();
} finally {
try {
if (outChannel != null) {
outChannel.close();
}
} catch (IOException ignore) {
}
}
}

}

服务端

package com.defonds.cds.common;


import java.io.BufferedOutputStream;
import java.io.File;
import java.io.IOException;
import java.io.OutputStream;
import java.io.RandomAccessFile;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

//HTTP 断点续传 demo（客户端测试工具：快车、迅雷）
public class ArcSyncHttpDownloadServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
// final static Log log = LogFactory.getLog(ArcSyncHttpDownloadServlet.class);

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
this.doPost(req, resp);
}

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) {
File downloadFile = new File("E:/test/123.mp3");//要下载的文件
long fileLength = downloadFile.length();//记录文件大小
System.out.println("fileLength："+fileLength);
long pastLength = 0;//记录已下载文件大小
int rangeSwitch = 0;//0：从头开始的全文下载；1：从某字节开始的下载（bytes=27000-）；2：从某字节开始到某字节结束的下载（bytes=27000-39000）
long toLength = 0;//记录客户端需要下载的字节段的最后一个字节偏移量（比如bytes=27000-39000，则这个值是为39000）
long contentLength = 0;//客户端请求的字节总量
String rangeBytes = "";//记录客户端传来的形如“bytes=27000-”或者“bytes=27000-39000”的内容
RandomAccessFile raf = null;//负责读取数据
OutputStream os = null;//写出数据
OutputStream out = null;//缓冲
byte b[] = new byte[1024];//暂存容器

if (request.getHeader("Range") != null) {// 客户端请求的下载的文件块的开始字节
response.setStatus(javax.servlet.http.HttpServletResponse.SC_PARTIAL_CONTENT);
System.out.println("request.getHeader(\"Range\")=" + request.getHeader("Range"));
rangeBytes = request.getHeader("Range").replaceAll("bytes=", "");
int index = rangeBytes.indexOf(‘-‘);
int length = rangeBytes.length()-1;
if (index == 1) {//bytes=969998336-
rangeSwitch = 1;
// rangeBytes = rangeBytes.substring(0, rangeBytes.indexOf(‘-‘));
String temp0 = rangeBytes.substring(0,rangeBytes.indexOf(‘-‘));
String temp2 = rangeBytes.substring(rangeBytes.indexOf(‘-‘) + 1, rangeBytes.length());
pastLength = Long.parseLong(temp0.trim());
if ("".equals(temp2)) {
contentLength = fileLength - pastLength + 1;//客户端请求的是 969998336 之后的字节
}else {
toLength = Long.parseLong(temp2);//bytes=1275856879-1275877358，到第 1275877358 个字节结束
contentLength = toLength - pastLength + 1;//客户端请求的是 1275856879-1275877358 之间的字节
}
} else {//bytes=1275856879-1275877358
rangeSwitch = 2;
String temp0 = rangeBytes.substring(0,rangeBytes.indexOf(‘-‘));
String temp2 = rangeBytes.substring(rangeBytes.indexOf(‘-‘) + 1, rangeBytes.length());
pastLength = Long.parseLong(temp0.trim());//bytes=1275856879-1275877358，从第 1275856879 个字节开始下载
toLength = Long.parseLong(temp2);//bytes=1275856879-1275877358，到第 1275877358 个字节结束
contentLength = toLength - pastLength + 1;//客户端请求的是 1275856879-1275877358 之间的字节
}
} else {//从开始进行下载
contentLength = fileLength;//客户端要求全文下载
}

/**
* 如果设设置了Content-Length，则客户端会自动进行多线程下载。如果不希望支持多线程，则不要设置这个参数。
* 响应的格式是:
* Content-Length: [文件的总大小] - [客户端请求的下载的文件块的开始字节]
* ServletActionContext.getResponse().setHeader("Content-Length",
* new Long(file.length() - p).toString());
*/
response.reset();//告诉客户端允许断点续传多线程连接下载,响应的格式是:Accept-Ranges: bytes
response.setHeader("Accept-Ranges", "bytes");//如果是第一次下,还没有断点续传,状态是默认的 200,无需显式设置;响应的格式是:HTTP/1.1 200 OK
if (pastLength != 0) {
//不是从最开始下载,
//响应的格式是:
//Content-Range: bytes [文件块的开始字节]-[文件的总大小 - 1]/[文件的总大小]
System.out.println("----------------------------不是从开始进行下载！服务器即将开始断点续传...");
switch (rangeSwitch) {
case 1 : {//针对 bytes=27000- 的请求
String contentRange = new StringBuffer("bytes ").append(new Long(pastLength).toString()).append("-").append(new Long(fileLength - 1).toString()).append("/").append(new Long(fileLength).toString()).toString();
response.setHeader("Content-Range", contentRange);
break;
}
case 2 : {//针对 bytes=27000-39000 的请求
String contentRange = rangeBytes + "/" + new Long(fileLength).toString();
response.setHeader("Content-Range", contentRange);
break;
}
default : {
break;
}
}
} else {
//是从开始下载
System.out.println("----------------------------是从开始进行下载！");
}

try {
response.addHeader("Content-Disposition", "attachment; filename=\"" + downloadFile.getName() + "\"");
response.setContentType( CommonUtil.setContentType(downloadFile.getName()));// set the MIME type.
response.addHeader("Content-Length", String.valueOf(contentLength));
os = response.getOutputStream();
out = new BufferedOutputStream(os);
raf = new RandomAccessFile(downloadFile, "r");
try {
switch (rangeSwitch) {
case 0 : {//普通下载，或者从头开始的下载
//同1
}
case 1 : {//针对 bytes=27000- 的请求
raf.seek(pastLength);//形如 bytes=969998336- 的客户端请求，跳过 969998336 个字节
int n = 0;
while ((n = raf.read(b, 0, 1024)) != -1) {
out.write(b, 0, n);
}
break;
}
case 2 : {//针对 bytes=27000-39000 的请求
raf.seek(pastLength - 1);//形如 bytes=1275856879-1275877358 的客户端请求，找到第 1275856879 个字节
int n = 0;
long readLength = 0;//记录已读字节数
while (readLength <= contentLength - 1024) {//大部分字节在这里读取
n = raf.read(b, 0, 1024);
readLength += 1024;
out.write(b, 0, n);
}
if (readLength <= contentLength) {//余下的不足 1024 个字节在这里读取
n = raf.read(b, 0, (int)(contentLength - readLength));
out.write(b, 0, n);
}
//
// raf.seek(pastLength);//形如 bytes=1275856879-1275877358 的客户端请求，找到第 1275856879 个字节
// while (raf.getFilePointer() < toLength) {
// out.write(raf.read());
// }
break;
}
default : {
break;
}
}
out.flush();
} catch(IOException ie) {
/**
* 在写数据的时候，
* 对于 ClientAbortException 之类的异常，
* 是因为客户端取消了下载，而服务器端继续向浏览器写入数据时，
* 抛出这个异常，这个是正常的。
* 尤其是对于迅雷这种吸血的客户端软件，
* 明明已经有一个线程在读取 bytes=1275856879-1275877358，
* 如果短时间内没有读取完毕，迅雷会再启第二个、第三个。。。线程来读取相同的字节段，
* 直到有一个线程读取完毕，迅雷会 KILL 掉其他正在下载同一字节段的线程，
* 强行中止字节读出，造成服务器抛 ClientAbortException。
* 所以，我们忽略这种异常
*/
//ignore
}
} catch (Exception e) {
System.err.println( e);
} finally {
if (out != null) {
try {
out.close();
} catch (IOException e) {
System.err.println( e);
}
}
if (raf != null) {
try {
raf.close();
} catch (IOException e) {
System.err.println( e);
}
}
}
}
}

package com.defonds.cds.common;
public class CommonUtil {

public static String setContentType(String returnFileName) {
String contentType = "application/octet-stream";
if (returnFileName.lastIndexOf(".") < 0)
return contentType;
returnFileName = returnFileName.toLowerCase();
returnFileName = returnFileName.substring(returnFileName
.lastIndexOf(".") + 1);

if (returnFileName.equals("html") || returnFileName.equals("htm")
|| returnFileName.equals("shtml")) {
contentType = "text/html";
} else if (returnFileName.equals("css")) {
contentType = "text/css";
} else if (returnFileName.equals("xml")) {
contentType = "text/xml";
} else if (returnFileName.equals("gif")) {
contentType = "image/gif";
} else if (returnFileName.equals("jpeg")
|| returnFileName.equals("jpg")) {
contentType = "image/jpeg";
} else if (returnFileName.equals("js")) {
contentType = "application/x-javascript";
} else if (returnFileName.equals("atom")) {
contentType = "application/atom+xml";
} else if (returnFileName.equals("rss")) {
contentType = "application/rss+xml";
} else if (returnFileName.equals("mml")) {
contentType = "text/mathml";
} else if (returnFileName.equals("txt")) {
contentType = "text/plain";
} else if (returnFileName.equals("jad")) {
contentType = "text/vnd.sun.j2me.app-descriptor";
} else if (returnFileName.equals("wml")) {
contentType = "text/vnd.wap.wml";
} else if (returnFileName.equals("htc")) {
contentType = "text/x-component";
} else if (returnFileName.equals("png")) {
contentType = "image/png";
} else if (returnFileName.equals("tif")
|| returnFileName.equals("tiff")) {
contentType = "image/tiff";
} else if (returnFileName.equals("wbmp")) {
contentType = "image/vnd.wap.wbmp";
} else if (returnFileName.equals("ico")) {
contentType = "image/x-icon";
} else if (returnFileName.equals("jng")) {
contentType = "image/x-jng";
} else if (returnFileName.equals("bmp")) {
contentType = "image/x-ms-bmp";
} else if (returnFileName.equals("svg")) {
contentType = "image/svg+xml";
} else if (returnFileName.equals("jar") || returnFileName.equals("var")
|| returnFileName.equals("ear")) {
contentType = "application/java-archive";
} else if (returnFileName.equals("doc")) {
contentType = "application/msword";
} else if (returnFileName.equals("pdf")) {
contentType = "application/pdf";
} else if (returnFileName.equals("rtf")) {
contentType = "application/rtf";
} else if (returnFileName.equals("xls")) {
contentType = "application/vnd.ms-excel";
} else if (returnFileName.equals("ppt")) {
contentType = "application/vnd.ms-powerpoint";
} else if (returnFileName.equals("7z")) {
contentType = "application/x-7z-compressed";
} else if (returnFileName.equals("rar")) {
contentType = "application/x-rar-compressed";
} else if (returnFileName.equals("swf")) {
contentType = "application/x-shockwave-flash";
} else if (returnFileName.equals("rpm")) {
contentType = "application/x-redhat-package-manager";
} else if (returnFileName.equals("der") || returnFileName.equals("pem")
|| returnFileName.equals("crt")) {
contentType = "application/x-x509-ca-cert";
} else if (returnFileName.equals("xhtml")) {
contentType = "application/xhtml+xml";
} else if (returnFileName.equals("zip")) {
contentType = "application/zip";
} else if (returnFileName.equals("mid")
|| returnFileName.equals("midi")
|| returnFileName.equals("kar")) {
contentType = "audio/midi";
} else if (returnFileName.equals("mp3")) {
contentType = "audio/mpeg";
} else if (returnFileName.equals("ogg")) {
contentType = "audio/ogg";
} else if (returnFileName.equals("m4a")) {
contentType = "audio/x-m4a";
} else if (returnFileName.equals("ra")) {
contentType = "audio/x-realaudio";
} else if (returnFileName.equals("3gpp")
|| returnFileName.equals("3gp")) {
contentType = "video/3gpp";
} else if (returnFileName.equals("mp4")) {
contentType = "video/mp4";
} else if (returnFileName.equals("mpeg")
|| returnFileName.equals("mpg")) {
contentType = "video/mpeg";
} else if (returnFileName.equals("mov")) {
contentType = "video/quicktime";
} else if (returnFileName.equals("flv")) {
contentType = "video/x-flv";
} else if (returnFileName.equals("m4v")) {
contentType = "video/x-m4v";
} else if (returnFileName.equals("mng")) {
contentType = "video/x-mng";
} else if (returnFileName.equals("asx") || returnFileName.equals("asf")) {
contentType = "video/x-ms-asf";
} else if (returnFileName.equals("wmv")) {
contentType = "video/x-ms-wmv";
} else if (returnFileName.equals("avi")) {
contentType = "video/x-msvideo";
}

return contentType;
}
}

http断点下载客户端和服务端