用Java实现断点续传 (HTTP)

断点续传的原理

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

200
Content-Length=106786028
Accept-Ranges=bytes
Date=Mon, 30 Apr 2001 12:56:11 GMT
ETag=W/"02ca57e173c11:95b"
Content-Type=application/octet-stream
Server=Microsoft-IIS/5.0
Last-Modified=Mon, 30 Apr 2001 12:56:11 GMT

GET /down.zip HTTP/1.0
User-Agent: NetFox
RANGE: bytes=2000070-
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2

RANGE: bytes=2000070-

206
Content-Length=106786028
Content-Range=bytes 2000070-106786027/106786028
Date=Mon, 30 Apr 2001 12:55:20 GMT
ETag=W/"02ca57e173c11:95b"
Content-Type=application/octet-stream
Server=Microsoft-IIS/5.0
Last-Modified=Mon, 30 Apr 2001 12:55:20 GMT

Content-Range=bytes 2000070-106786027/106786028

Java 实现断点续传的关键几点

RANGE: bytes=2000070-

URL url = new URL("http://www.sjtu.edu.cn/down.zip");

HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection();

// 设置 User-Agent
httpConnection.setRequestProperty("User-Agent","NetFox");

// 设置断点续传的开始位置
httpConnection.setRequestProperty("RANGE","bytes=2000070");

// 获得输入流
InputStream input = httpConnection.getInputStream();

RandomAccess oSavedFile = new RandomAccessFile("down.zip","rw");
long nPos = 2000070;

// 定位文件指针到 nPos 位置
oSavedFile.seek(nPos);
byte[] b = new byte[1024];
int nRead;

// 从输入流中读入字节流，然后写到文件中
while((nRead=input.read(b,0,1024)) > 0) {
oSavedFile.write(b,0,nRead);
}

断点续传内核的实现

/**
* SiteFileFetch.java
*/
package NetFox;
import java.io.*;
import java.net.*;
public class SiteFileFetch extends Thread {
SiteInfoBean siteInfoBean = null; // 文件信息 Bean
long[] nStartPos; // 开始位置
long[] nEndPos; // 结束位置
FileSplitterFetch[] fileSplitterFetch; // 子线程对象
long nFileLength; // 文件长度
boolean bFirst = true; // 是否第一次取文件
boolean bStop = false; // 停止标志
File tmpFile; // 文件下载的临时信息
DataOutputStream output; // 输出到文件的输出流
public SiteFileFetch(SiteInfoBean bean) throws IOException {
siteInfoBean = bean;
//tmpFile = File.createTempFile ("zhong","1111",new File(bean.getSFilePath()));
tmpFile = new File(bean.getSFilePath()+File.separator + bean.getSFileName()+".info");
if(tmpFile.exists ())
{
bFirst = false;
read_nPos();
}
else
{
nStartPos = new long[bean.getNSplitter()];
nEndPos = new long[bean.getNSplitter()];
}
}
public void run() {
// 获得文件长度
// 分割文件
// 实例 FileSplitterFetch
// 启动 FileSplitterFetch 线程
// 等待子线程返回
try{
if(bFirst){
nFileLength = getFileSize();
if(nFileLength == -1){
System.err.println("File Length is not known!");
}else if(nFileLength == -2){
System.err.println("File is not access!");
}else{
for(int i=0;i<nStartPos.length;i++){
nStartPos[i] = (long)(i*(nFileLength/nStartPos.length));
}
for(int i=0;i<nEndPos.length-1;i++){
nEndPos[i] = nStartPos[i+1];
}
nEndPos[nEndPos.length-1] = nFileLength;
}
}
// 启动子线程
fileSplitterFetch = new FileSplitterFetch[nStartPos.length];
for(int i=0;i<nStartPos.length;i++){
fileSplitterFetch[i] = new FileSplitterFetch(siteInfoBean.getSSiteURL(),
siteInfoBean.getSFilePath() + File.separator + siteInfoBean.getSFileName(),
nStartPos[i],nEndPos[i],i);
Utility.log("Thread " + i + " , nStartPos = " + nStartPos[i] + ", nEndPos = "
+ nEndPos[i]);
fileSplitterFetch[i].start();
}
// fileSplitterFetch[nPos.length-1] = new FileSplitterFetch(siteInfoBean.getSSiteURL(),
siteInfoBean.getSFilePath() + File.separator
+ siteInfoBean.getSFileName(),nPos[nPos.length-1],nFileLength,nPos.length-1);
// Utility.log("Thread " +(nPos.length-1) + ",nStartPos = "+nPos[nPos.length-1]+", nEndPos = " + nFileLength);
// fileSplitterFetch[nPos.length-1].start();
// 等待子线程结束
//int count = 0;
// 是否结束 while 循环
boolean breakWhile = false;
while(!bStop){
write_nPos();
Utility.sleep(500);
breakWhile = true;
for(int i=0;i<nStartPos.length;i++){
if(!fileSplitterFetch[i].bDownOver){
breakWhile = false;
break;
}
}
if(breakWhile)
break;
//count++;
//if(count>4)
//    siteStop();
}
System.err.println("文件下载结束！");
}catch(Exception e){
e.printStackTrace ();
}
}
// 获得文件长度
public long getFileSize(){
int nFileLength = -1;
try{
URL url = new URL(siteInfoBean.getSSiteURL());
HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection ();
httpConnection.setRequestProperty("User-Agent","NetFox");
int responseCode=httpConnection.getResponseCode();
if(responseCode>=400){
processErrorCode(responseCode);
return -2; //-2 represent access is error
}
String sHeader;
for(int i=1;;i++){
//DataInputStream in = new DataInputStream(httpConnection.getInputStream ());
//Utility.log(in.readLine());
sHeader=httpConnection.getHeaderFieldKey(i);
if(sHeader!=null){
if(sHeader.equals("Content-Length")){
nFileLength = Integer.parseInt(httpConnection.getHeaderField(sHeader));
break;
}
}
else
break;
}
}
catch(IOException e){e.printStackTrace ();}
catch(Exception e){e.printStackTrace ();}
Utility.log(nFileLength);
return nFileLength;
}
// 保存下载信息（文件指针位置）
private void write_nPos(){
try{
output = new DataOutputStream(new FileOutputStream(tmpFile));
output.writeInt(nStartPos.length);
for(int i=0;i<nStartPos.length;i++){
// output.writeLong(nPos[i]);
output.writeLong(fileSplitterFetch[i].nStartPos);
output.writeLong(fileSplitterFetch[i].nEndPos);
}
output.close();
}
catch(IOException e){e.printStackTrace ();}
catch(Exception e){e.printStackTrace ();}
}
// 读取保存的下载信息（文件指针位置）
private void read_nPos(){
try{
DataInputStream input = new DataInputStream(new FileInputStream(tmpFile));
int nCount = input.readInt();
nStartPos = new long[nCount];
nEndPos = new long[nCount];
for(int i=0;i<nStartPos.length;i++){
nStartPos[i] = input.readLong();
nEndPos[i] = input.readLong();
}
input.close();
}
catch(IOException e){e.printStackTrace ();}
catch(Exception e){e.printStackTrace ();}
}
private void processErrorCode(int nErrorCode){
System.err.println("Error Code : " + nErrorCode);
}
// 停止文件下载
public void siteStop(){
bStop = true;
for(int i=0;i<nStartPos.length;i++)
fileSplitterFetch[i].splitterStop();
}
}

/**
*FileSplitterFetch.java
*/
package NetFox;
import java.io.*;
import java.net.*;
public class FileSplitterFetch extends Thread {
String sURL; //File URL
long nStartPos; //File Snippet Start Position
long nEndPos; //File Snippet End Position
int nThreadID; //Thread's ID
boolean bDownOver = false; //Downing is over
boolean bStop = false; //Stop identical
FileAccessI fileAccessI = null; //File Access interface
public FileSplitterFetch(String sURL,String sName,long nStart,long nEnd,int id)
throws IOException {
this.sURL = sURL;
this.nStartPos = nStart;
this.nEndPos = nEnd;
nThreadID = id;
fileAccessI = new FileAccessI(sName,nStartPos);
}
public void run(){
while(nStartPos < nEndPos && !bStop){
try{
URL url = new URL(sURL);
HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection ();
httpConnection.setRequestProperty("User-Agent","NetFox");
String sProperty = "bytes="+nStartPos+"-";
httpConnection.setRequestProperty("RANGE",sProperty);
Utility.log(sProperty);
InputStream input = httpConnection.getInputStream();
//logResponseHead(httpConnection);
byte[] b = new byte[1024];
int nRead;
while((nRead=input.read(b,0,1024)) > 0 && nStartPos < nEndPos
&& !bStop) {
nStartPos += fileAccessI.write(b,0,nRead);
//if(nThreadID == 1)
// Utility.log("nStartPos = " + nStartPos + ", nEndPos = " + nEndPos);
}
Utility.log("Thread " + nThreadID + " is over!");
bDownOver = true;
//nPos = fileAccessI.write (b,0,nRead);
}
catch(Exception e){e.printStackTrace ();}
}
}
// 打印回应的头信息
public void logResponseHead(HttpURLConnection con)
{
for(int i=1;;i++){
String header=con.getHeaderFieldKey(i);
if(header!=null)
//responseHeaders.put(header,httpConnection.getHeaderField(header));
Utility.log(header+" : "+con.getHeaderField(header));
else
break;
}
}
public void splitterStop(){
bStop = true;
}
}

/**
*FileAccess.java
*/
package NetFox;
import java.io.*;
public class FileAccessI implements Serializable{
RandomAccessFile oSavedFile;
long nPos;
public FileAccessI() throws IOException{
this("",0);
}
public FileAccessI(String sName,long nPos) throws IOException{
oSavedFile = new RandomAccessFile(sName,"rw");
this.nPos = nPos;
oSavedFile.seek(nPos);
}
public synchronized int write(byte[] b,int nStart,int nLen){
int n = -1;
try{
oSavedFile.write(b,nStart,nLen);
n = nLen;
}catch(IOException e){
e.printStackTrace ();
}
return n;
}
}

/**
*SiteInfoBean.java
*/
package NetFox;
public class SiteInfoBean {
private String sSiteURL; //Site's URL
private String sFilePath; //Saved File's Path
private String sFileName; //Saved File's Name
private int nSplitter; //Count of Splited Downloading File
public SiteInfoBean() {
//default value of nSplitter is 5
this("","","",5);
}
public SiteInfoBean(String sURL,String sPath,String sName,int nSpiltter) {
sSiteURL= sURL;
sFilePath = sPath;
sFileName = sName;
this.nSplitter = nSpiltter;
}
public String getSSiteURL() {
return sSiteURL;
}
public void setSSiteURL(String value) {
sSiteURL = value;
}
public String getSFilePath() {
return sFilePath;
}
public void setSFilePath(String value) {
sFilePath = value;
}
public String getSFileName() {
return sFileName;
}
public void setSFileName(String value) {
sFileName = value;
}
public int getNSplitter() {
return nSplitter;
}
public void setNSplitter(int nCount) {
nSplitter = nCount;
}
}

/**
*Utility.java
*/
package NetFox;
public class Utility {
public Utility() {}

public static void sleep(int nSecond) {
try{
Thread.sleep(nSecond);
}
catch(Exception e) {
e.printStackTrace ();
}
}
public static void log(String sMsg) {
System.err.println(sMsg);
}
public static void log(int sMsg) {
System.err.println(sMsg);
}
}

/**
*TestMethod.java
*/
package NetFox;
public class TestMethod {
public TestMethod() {    ///xx/weblogic60b2_win.exe
try{
SiteInfoBean bean = new SiteInfoBean("http://localhost/xx/weblogic60b2_win.exe",
"L:\\temp","weblogic60b2_win.exe",5);
//SiteInfoBean bean = new SiteInfoBean("http://localhost:8080/down.zip","L:\\temp",
"weblogic60b2_win.exe",5);
SiteFileFetch fileFetch = new SiteFileFetch(bean);
fileFetch.start();
} catch(Exception e) {
e.printStackTrace ();
}
}

public static void main(String[] args) {
new TestMethod();
}
}

参考资料