url:http://www.cnblogs.com/KiloNet/articles/1873600.html
(一)断点续传的原理
其实断点续传的原理很简单，就是在Http的请求上和一般的下载有所不同而已。
打个比方，浏览器请求服务器上的一个文时，所发出的请求如下：
假设服务器域名为wwww.sjtu.edu.cn，文件名为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
服务器收到请求后，按要求寻找请求的文件，提取文件的信息，然后返回给浏览器，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
所谓断点续传，也就是要从文件已经下载的地方开始继续下载。所以在客户端浏览器传给
Web服务器的时候要多加一条信息--从哪里开始。
下面是用自己编的一个"浏览器"来传递请求信息给Web服务器，要求从2000070字节开始。
GET /down.zip HTTP/1.0
User-Agent: NetFox
RANGE: bytes=2000070-
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
仔细看一下就会发现多了一行RANGE: bytes=2000070-
这一行的意思就是告诉服务器down.zip这个文件从2000070字节开始传，前面的字节不用传了。
服务器收到这个请求以后，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
和前面服务器返回的信息比较一下，就会发现增加了一行：
Content-Range=bytes 2000070-106786027/106786028
返回的代码也改为206了，而不再是200了。
知道了以上原理，就可以进行断点续传的编程了。
(二)Java实现断点续传的关键几点
(1)用什么方法实现提交RANGE: bytes=2000070-。
当然用最原始的Socket是肯定能完成的，不过那样太费事了，其实Java的net包中提供了这种功能。代码如下：
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();
从输入流中取出的字节流就是down.zip文件从2000070开始的字节流。
大家看，其实断点续传用Java实现起来还是很简单的吧。
接下来要做的事就是怎么保存获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类。
操作相当简单，假设从2000070处开始保存文件，代码如下：
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);
}
怎么样，也很简单吧。
接下来要做的就是整合成一个完整的程序了。包括一系列的线程控制等等。
(三)断点续传内核的实现
主要用了6个类，包括一个测试类。
SiteFileFetch.java负责整个文件的抓取，控制内部线程(FileSplitterFetch类)。
FileSplitterFetch.java负责部分文件的抓取。
FileAccess.java负责文件的存储。
SiteInfoBean.java要抓取的文件的信息，如文件保存的目录，名字，抓取文件的URL等。
Utility.java工具类，放一些简单的方法。
TestMethod.java测试类。
代码
/*
**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();
}
}