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Java 回实现断点续传

发布时间： 2012-12-24 10:43:13 作者： rapoo

Java 来实现断点续传


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。


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Java 实现断点续传的关键几点

1.(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。

2.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。


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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。


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; // 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();
?}
?}
?

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