原理：
断点续传的关键是断点，所以在制定传输协议的时候要设计好，如上图，我自定义了一个交互协议，每次下载请求都会带上下载的起始点，这样就可以支持从断点下载了，其实HTTP里的断点续传也是这个原理，在HTTP的头里有个可选的字段RANGE，表示下载的范围，下面是我用Java语言实现的下载断点续传示例。
提供下载的服务端代码：
String path = "文件地址";
BufferedInputStream bis = null;
try {
File file = new File(path);
if (file.exists()) {
long p = 0L;
long toLength = 0L;
long contentLength = 0L;
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 fileLength;
String rangBytes = "";
fileLength = file.length();
// get file content
InputStream ins = new FileInputStream(file);
bis = new BufferedInputStream(ins);
// String d = new SimpleDateFormat("EEE,d MMM yyyy hh:mm:ss 'GMT' ",Locale.US).format(new Date());
// System.out.println(d);
// tell the client to allow accept-ranges
response.reset();
response.setHeader("Accept-Ranges", "bytes");
/*DateTime dateTime = DateTime.now();
System.out.println(dateTime.toString("r"));*/
response.setHeader("Last-Modified","Sat, 27 Jul 2017 12:14:58 GMT");
/*response.setHeader("HTTP/1.0 206 Partial Content", "206");*/
// client requests a file block download start byte
String range = request.getHeader("Range");
log.info("range : "+range);
if (range != null && range.trim().length() > 0 && !"null".equals(range)) {
response.setStatus(javax.servlet.http.HttpServletResponse.SC_PARTIAL_CONTENT);
rangBytes = range.replaceAll("bytes=", "");
if (rangBytes.endsWith("-")) { // bytes=270000-
rangeSwitch = 1;
p = Long.parseLong(rangBytes.substring(0, rangBytes.indexOf("-")));
contentLength = fileLength - p; // 客户端请求的是270000之后的字节（包括bytes下标索引为270000的字节）
} else { // bytes=270000-320000
rangeSwitch = 2;
String temp1 = rangBytes.substring(0, rangBytes.indexOf("-"));
String temp2 = rangBytes.substring(rangBytes.indexOf("-") + 1, rangBytes.length());
p = Long.parseLong(temp1);
toLength = Long.parseLong(temp2);
contentLength = toLength - p + 1; // 客户端请求的是 270000-320000 之间的字节
}
} else {
contentLength = fileLength;
}
// 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。
// 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]
response.setHeader("Content-Length", new Long(contentLength).toString());
// 断点开始
// 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]
if (rangeSwitch == 1) {
String contentRange = new StringBuffer("bytes ").append(new Long(p).toString()).append("-")
.append(new Long(fileLength - 1).toString()).append("/")
.append(new Long(fileLength).toString()).toString();
response.setHeader("Content-Range", contentRange);
bis.skip(p);
} else if (rangeSwitch == 2) {
String contentRange = range.replace("=", " ") + "/" + new Long(fileLength).toString();
response.setHeader("Content-Range", contentRange);
bis.skip(p);
} else {
String contentRange = new StringBuffer("bytes ").append("0-")
.append(fileLength - 1).append("/")
.append(fileLength).toString();
response.setHeader("Content-Range", contentRange);
}
String fileName = file.getName();
response.setContentType("application/octet-stream");
response.addHeader("Content-Disposition", "attachment;filename=" + fileName);
OutputStream out = response.getOutputStream();
BufferedOutputStream buf=new BufferedOutputStream(out);
int n = 0;
long readLength = 0;
int bsize = 1024;
byte[] bytes = new byte[bsize];
if (rangeSwitch == 2) {
// 针对 bytes=27000-39000 的请求，从27000开始写数据
while (readLength <= contentLength - bsize) {
n = bis.read(bytes);
readLength += n;
buf.write(bytes, 0, n);
}
if (readLength <= contentLength) {
n = bis.read(bytes, 0, (int) (contentLength - readLength));
buf.write(bytes, 0, n);
}
} else {
while ((n = bis.read(bytes)) != -1) {
buf.write(bytes,0,n);
}
}
buf.flush();
buf.close();
out.close();
bis.close();
} else {
if (log.isDebugEnabled()) {
log.debug("Error: file " + path + " not found.");
}
}
} catch (IOException ie) {
// 忽略 ClientAbortException 之类的异常
} catch (Exception e) {
log.error(e.getMessage());
e.printStackTrace();
}
下载的客户端代码：
/**
* request:get0startIndex0
* response:fileLength0fileBinaryStream
*
* @param filepath
* @throws Exception
*/
public void Get(String filepath) throws Exception {
Socket socket = new Socket();
// 建立连接
socket.connect(new InetSocketAddress("127.0.0.1", 8888));
// 获取网络流
OutputStream out = socket.getOutputStream();
InputStream in = socket.getInputStream();
// 文件传输协定命令
byte[] cmd = "get".getBytes();
out.write(cmd);
out.write(0);// 分隔符
int startIndex = 0;
// 要发送的文件
File file = new File(filepath);
if(file.exists()){
startIndex = (int) file.length();
}
System.out.println("Client startIndex : " + startIndex);
// 文件写出流
RandomAccessFile access = new RandomAccessFile(file,"rw");
// 断点
out.write(String.valueOf(startIndex).getBytes());
out.write(0);
out.flush();
// 文件长度
int temp = 0;
StringWriter sw = new StringWriter();
while((temp = in.read()) != 0){
sw.write(temp);
sw.flush();
}
int length = Integer.parseInt(sw.toString());
System.out.println("Client fileLength : " + length);
// 二进制文件缓冲区
byte[] buffer = new byte[1024*10];
// 剩余要读取的长度
int tatol = length - startIndex;
//
access.skipBytes(startIndex);
while (true) {
// 如果剩余长度为0则结束
if (tatol == 0) {
break;
}
// 本次要读取的长度假设为剩余长度
int len = tatol;
// 如果本次要读取的长度大于缓冲区的容量
if (len > buffer.length) {
// 修改本次要读取的长度为缓冲区的容量
len = buffer.length;
}
// 读取文件，返回真正读取的长度
int rlength = in.read(buffer, 0, len);
// 将剩余要读取的长度减去本次已经读取的
tatol -= rlength;
// 如果本次读取个数不为0则写入输出流，否则结束
if (rlength > 0) {
// 将本次读取的写入输出流中
access.write(buffer, 0, rlength);
} else {
break;
}
System.out.println("finish : " + ((float)(length -tatol) / length) *100 + " %");
}
System.out.println("finished!");
// 关闭流
access.close();
out.close();
in.close();
}
public static void main(String[] args) {
FTPClient client = new FTPClient();
try {
client.Get("E:\\ceshi\\test\\mm.pdf");
} catch (Exception e) {
e.printStackTrace();
}
}代码直接copy过去就可以了 服务端的代码需要改下url
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