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(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有所不同而已。
打個比方，浏覽器請求服務器上的一個文時，所發出的請求如下：
假設服務器域名為wwww.sjtu.edu.cn，文件名為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
服務器收到請求後，按要求尋找請求的文件，提取文件的信息，然後返回給浏覽器，返回信息如下：
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，也就是要從文件已經下載的地方開始繼續下載。所以在客戶端浏覽器傳給
Web服務器的時候要多加一條信息--從哪裡開始。
下面是用自己編的一個"浏覽器"來傳遞請求信息給Web服務器，要求從2000070字節開始。
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服務器down.zip這個文件從2000070字節開始傳，前面的字節不用傳了。
服務器收到這個請求以後，返回的信息如下：
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
和前面服務器返回的信息比較一下，就會發現增加了一行：
Content-Range=bytes 2000070-106786027/106786028
返回的代碼也改為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編程了。
(二)Java實現斷點續傳的關鍵幾點
(1)用什麼方法實現提交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
();
//設置User-Agent
httpConnection.setRequestProperty("User-Agent","NetFox");
//設置斷點續傳的開始位置
httpConnection.setRequestProperty("RANGE","bytes=2000070");
//獲得輸入流
InputStream input = httpConnection.getInputStream();
從輸入流中取出的字節流就是down.zip文件從2000070開始的字節流。
大家看，其實斷點續傳用Java實現起來還是很簡單的吧。
接下來要做的事就是怎麼保存獲得的流到文件中去了。
保存文件采用的方法。
我采用的是IO包中的RandAccessFile類。
操作相當簡單，假設從2000070處開始保存文件，代碼如下：
RandomAccess oSavedFile = new RandomAccessFile("down.zip","rw");
long nPos = 2000070;
//定位文件指針到nPos位置
oSavedFile.seek(nPos);
byte[] b = new byte[1024];
int nRead;
//從輸入流中讀入字節流，然後寫到文件中
while((nRead=input.read(b,0,1024)) > 0)
{
oSavedFile.write(b,0,nRead);
}
怎麼樣，也很簡單吧。
接下來要做的就是整合成一個完整的程序了。包括一系列的線程控制等等。
(三)斷點續傳內核的實現
主要用了6個類，包括一個測試類。
SiteFileFetch.java負責整個文件的抓取，控制內部線程(FileSplitterFetch類)。
FileSplitterFetch.java負責部分文件的抓取。
FileAccess.java負責文件的存儲。
SiteInfoBean.java要抓取的文件的信息，如文件保存的目錄，名字，抓取文件的URL等。
Utility.java工具類，放一些簡單的方法。
TestMethod.java測試類。
下面是源程序：
/*
**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; //開始位置
long[] nEndPos; //結束位置
FileSplitterFetch[] fileSplitterFetch; //子線程對象
long nFileLength; //文件長度
boolean bFirst = true; //是否第一次取文件
boolean bStop = false; //停止標志
File tmpFile; //文件下載的臨時信息
DataOutputStream output; //輸出到文件的輸出流
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()
{
//獲得文件長度
//分割文件
//實例FileSplitterFetch
//啟動FileSplitterFetch線程
//等待子線程返回
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 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;i4)
// siteStop();
}
System.err.println("文件下載結束！");
}
catch(Exception e){e.printStackTrace ();}
}
//獲得文件長度
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;
}
//保存下載信息（文件指針位置）
private void write_nPos()
{
try{
output = new DataOutputStream(new FileOutputStream(tmpFile));
output.writeInt(nStartPos.length);
for(int i=0;iDataInputStream(new FileInputStream(tmpFile));
int nCount = input.readInt();
nStartPos = new long[nCount];
nEndPos = new long[nCount];
for(int i=0;i 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 ();}
}
}
//打印回應的頭信息
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: emp","weblogic60b2_win.exe",5);
//SiteInfoBean bean = new SiteInfoBean("http://localhost:8080/down.zip"," L: emp","weblogic60b2_win.exe",5);
SiteFileFetch fileFetch = new SiteFileFetch(bean);
fileFetch.start();
}
catch(Exception e){e.printStackTrace ();}
}
public static void main(String[] args)
{
new TestMethod();
}
}
{
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: emp","weblogic60b2_win.exe",5);
//SiteInfoBean bean = new SiteInfoBean("http://localhost:8080/down.zip"," L: emp","weblogic60b2_win.exe",5);
SiteFileFetch fileFetch = new SiteFileFetch(bean);
fileFetch.start();
}
catch(Exception e){e.printStackTrace ();}
}
public static void main(String[] args)
{
new TestMethod();
}
}
/*
**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: emp","weblogic60b2_win.exe",5);
//SiteInfoBean bean = new SiteInfoBean("http://localhost:8080/down.zip"," L: emp","weblogic60b2_win.exe",5);
SiteFileFetch fileFetch = new SiteFileFetch(bean);
fileFetch.start();
}
catch(Exception e){e.printStackTrace ();}
}
public static void main(String[] args)
{
new TestMethod();
}
}
