(一)斷點續傳的原理
其實斷點續傳的原理很簡單，就是在Http的請求上和一般的下載下傳有所不同而已。打個比方，浏覽器請求伺服器上的一個文時，所發出的請求如下：
假設伺服器域名為wwww.sjtu.edu.cn，檔案名為down.zip。
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
伺服器收到請求後，按要求尋找請求的檔案，提取檔案的資訊，然後傳回給浏覽器，傳回資訊如下：
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
所謂斷點續傳，也就是要從檔案已經下載下傳的地方開始繼續下載下傳。是以在用戶端浏覽器傳給
Web伺服器的時候要多加一條資訊--從哪裡開始。
下面是用自己編的一個"浏覽器"來傳遞請求資訊給Web伺服器，要求從2000070位元組開始。
GET /down.zip HTTP/1.0
User-Agent: NetFox
RANGE: bytes=2000070-
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
仔細看一下就會發現多了一行RANGE: bytes=2000070-；這一行的意思就是告訴伺服器down.zip這個檔案從2000070位元組開始傳，前面的位元組不用傳了。
伺服器收到這個請求以後，傳回的資訊如下：
206
Content-Range=bytes 2000070-106786027/106786028
Date=Mon, 30 Apr 2001 12:55:20 GMT
Last-Modified=Mon, 30 Apr 2001 12:55:20 GMT
和前面伺服器傳回的資訊比較一下，就會發現增加了一行：
傳回的代碼也改為206了，而不再是200了。
知道了以上原理，就可以進行斷點續傳的程式設計了。
(1)用什麼方法實作送出RANGE: bytes=2000070-。
當然用最原始的Socket是肯定能完成的，不過那樣太費事了，其實Java的net包中提供了這種功能。代碼如下：
URL url = new URL(" http://www.sjtu.edu.cn/down.zip";;);
HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection
();
//設定User-Agent
httpConnection.setRequestProperty("User-Agent","NetFox");
//設定斷點續傳的開始位置
httpConnection.setRequestProperty("RANGE","bytes=2000070");
//獲得輸入流
InputStream input = httpConnection.getInputStream();
從輸入流中取出的位元組流就是down.zip檔案從2000070開始的位元組流。大家看，其實斷點續傳用Java實作起來還是很簡單的吧。接下來要做的事就是怎麼儲存獲得的流到檔案中去了。
儲存檔案采用的方法
我采用的是IO包中的RandAccessFile類。
操作相當簡單，假設從2000070處開始儲存檔案，代碼如下：
RandomAccess oSavedFile = new RandomAccessFile("down.zip","rw");
long nPos = 2000070;
//定位檔案指針到nPos位置
oSavedFile.seek(nPos);
byte[] b = new byte[1024];
int nRead;
//從輸入流中讀入位元組流，然後寫到檔案中
while((nRead=input.read(b,0,1024)) > 0)
{
oSavedFile.write(b,0,nRead);
}
(三)斷點續傳核心的實作
主要用了6個類，包括一個測試類：
SiteFileFetch.java負責整個檔案的抓取，控制内部線程(FileSplitterFetch類)。
FileSplitterFetch.java負責部分檔案的抓取。
FileAccess.java負責檔案的存儲。
SiteInfoBean.java要抓取的檔案的資訊，如檔案儲存的目錄，名字，抓取檔案的URL等。
Utility.java工具類，放一些簡單的方法。
TestMethod.java測試類。
*/
package NetFox;
import java.io.*;
import java.net.*;
public class SiteFileFetch extends Thread {
SiteInfoBean siteInfoBean = null; //檔案資訊Bean
long[] nStartPos; //開始位置
long[] nEndPos; //結束位置
FileSplitterFetch[] fileSplitterFetch; //子線程對象
long nFileLength; //檔案長度
boolean bFirst = true; //是否第一次取檔案
boolean bStop = false; //停止标志
File tmpFile; //檔案下載下傳的臨時資訊
DataOutputStream output; //輸出到檔案的輸出流
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()
//獲得檔案長度
//分割檔案
//執行個體FileSplitterFetch
//啟動FileSplitterFetch線程
//等待子線程傳回
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;
}
//啟動子線程
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();
//等待子線程結束
//int count = 0;
//是否結束while循環
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("檔案下載下傳結束！");
catch(Exception e){e.printStackTrace ();}
//獲得檔案長度
public long getFileSize()
int nFileLength = -1;
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
catch(IOException e){e.printStackTrace ();}
catch(Exception e){e.printStackTrace ();}
Utility.log(nFileLength);
return nFileLength;
//儲存下載下傳資訊（檔案指針位置）
private void write_nPos()
output = new DataOutputStream(new FileOutputStream(tmpFile));
output.writeInt(nStartPos.length);
// output.writeLong(nPos[i]);
output.writeLong(fileSplitterFetch[i].nStartPos);
output.writeLong(fileSplitterFetch[i].nEndPos);
output.close();
//讀取儲存的下載下傳資訊（檔案指針位置）
private void read_nPos()
DataInputStream input = new DataInputStream(new FileInputStream(tmpFile));
int nCount = input.readInt();
nStartPos = new long[nCount];
nEndPos = new long[nCount];
nStartPos[i] = input.readLong();
nEndPos[i] = input.readLong();
input.close();
private void processErrorCode(int nErrorCode)
System.err.println("Error Code : " + nErrorCode);
//停止檔案下載下傳
public void siteStop()
bStop = true;
fileSplitterFetch[i].splitterStop();
/*
**FileSplitterFetch.java
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);
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 ();}
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));
public void splitterStop()
**FileAccess.java
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;
oSavedFile.write(b,nStart,nLen);
n = nLen;
catch(IOException e)
e.printStackTrace ();
return n;
**SiteInfoBean.java
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
public class Utility {
public Utility()
public static void sleep(int nSecond)
Thread.sleep(nSecond);
catch(Exception e)
e.printStackTrace ();
}
public static void log(String sMsg)
System.err.println(sMsg);
public static void log(int sMsg)
**TestMethod.java
public class TestMethod {
public TestMethod()
{ ///xx/weblogic60b2_win.exe
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();
public static void main(String[] args)
new TestMethod();
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