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用Java实现HTTP断点续传

(一)断点续传的道理

着实断点续传的道理很简单,便是在Http的哀求上和一样平常的下载有所不合而已。打个比方,浏览器哀求办事器上的一个文时,所发出的哀求如下:

假设办事器域名为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

办事器收到哀求后,按要求探求哀求的文件,提取文件的信息,然后返回给浏览器,返复书息如下:

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,也便是要从文件已经下载的地方开始继承下载。以是在客户端浏览器传给

Web办事器的时刻要多加一条信息--从哪里开始。

下面是用自己编的一个"浏览器"来通报哀求信息给Web办事器,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

仔细看一下就会发明多了一行RANGE: bytes=2000070-;这一行的意思便是奉告办事器down.zip这个文件从2000070字节开始传,前面的字节不用传了。

办事器收到这个哀求今后,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

和前面办事器返回的信息对照一下,就会发明增添了一行:

Content-Range=bytes 2000070-106786027/106786028

返回的代码也改为206了,而不再是200了。

知道了以上道理,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)用什么措施实现提交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,着实Java的net包中供给了这种功能。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");

//得到输入流

InputStream input = httpConnection.getInputStream();

从输入流中掏出的字撙节便是down.zip文件从2000070开始的字撙节。大年夜家看,着实断点续传用Java实现起来照样很简单的吧。接下来要做的事便是怎么保存得到的流到文件中去了。

保存文件采纳的措施

我采纳的是IO包中的RandAccessFile类。

操作相称简单,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;

//从输入流中读入字撙节,然后写到文件中

while((nRead=input.read(b,0,1024)) > 0)

{

oSavedFile.write(b,0,nRead);

}

接下来要做的便是整合成一个完备的法度榜样了。包括一系列的线程节制等等。

(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认真全部文件的抓取,节制内部线程(FileSplitterFetch类)。

FileSplitterFetch.java认真部分文件的抓取。

FileAccess.java认真文件的存储。

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对象类,放一些简单的措施。

TestMethod.java测试类。

下面是源法度榜样:

/*

**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; //停止位置

FileSplitterFetch[] fileSplitterFetch; //子线程工具

long nFileLength; //文件长度

boolean bFirst = true; //是否第一次取文件

boolean bStop = false; //竣事标志

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()

{

//得到文件长度

//瓜分文件

//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;i4)

// siteStop();

}

System.err.println("文件下载停止!");

}

catch(Exception e){e.printStackTrace ();}

}

//得到文件长度

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 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: 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();

}

}

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