1、Last-Modified=Mon, 30 Apr 2001 12: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 NetFoxRANGE: bytes=2000070- text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2仔细看一下就会发现多了一行RANGE: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,
2、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:206Content-Range=bytes 2000070-106786027/10678602855:20 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: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(HttpURLConnecti
3、on httpConnection = (HttpURLConnection)url.openConnection();/设置User-AgenthttpConnection.setRequestProperty(User-Agent,NetFox);/设置断点续传的开始位置RANGEbytes=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
4、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.ziprwlong nPos = 2000070;/定位文件指针到nPos位置oSavedFile.seek(nPos);byte b = new byte1024;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)
5、;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。下面是源程序: /*SiteFileFetch.j
6、ava*/package NetFox;import java.io.*;import .*;public class SiteFileFetch extends Thread SiteInfoBean siteInfoBean = null; /文件信息Beanlong 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Fi
7、le 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 IOExceptionsiteInfoBean = bean;/tmpFile = File.createTempFile (zhong1111,new File(bean.getSFilePath();tmpFile = new File(bean.getSFilePath()+File.separator + bean.getSFileName()+.infoif(tmpFile.ex
8、ists ()bFirst = false;read_nPos();elsenStartPos = new longbean.getNSplitter();nEndPos = new longbean.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tryif(bFirst)nFileLength = getFileSize();if(nFileLength = -1)System.err.println(File Length is not known!
9、else if(nFileLength = -2)File is not access!for(int i=0;i<nStartPos.length;i+)nStartPosi = (long)(i*(nFileLength/nStartPos.length);nEndPos.length-1;nEndPosi = nStartPosi+1;nEndPosnEndPos.length-1 = nFileLength;/启动子线程fileSplitterFetch = new FileSplitterFetchnStartPos.length;fileSplitterFetchi = ne
10、w FileSplitterFetch(siteInfoBean.getSSiteURL(),siteInfoBean.getSFilePath() + File.separator + siteInfoBean.getSFileName(),nStartPosi,nEndPosi,i);Utility.log(Thread + i + , nStartPos = + nStartPosi + , nEndPos = + nEndPosi);fileSplitterFetchi.start();/ fileSplitterFetchnPos.length-1 = new FileSplitte
11、rFetch(siteInfoBean.getSSiteURL(),siteInfoBean.getSFilePath() + File.separator + siteInfoBean.getSFileName(),nPosnPos.length-1,nFileLength,nPos.length-1);/ Utility.log( + (nPos.length-1) + + nPosnPos.length-1 + ,nEndPos = + nFileLength);/ fileSplitterFetchnPos.length-1.start();/等待子线程结束/int count = 0
12、;/是否结束while循环boolean breakWhile = false;while(!bStop)write_nPos();Utility.sleep(500);breakWhile = true;if(!fileSplitterFetchi.bDownOver)breakWhile = false;break;if(breakWhile)/count+;/if(count&4)/ siteStop();文件下载结束!catch(Exception e)e.printStackTrace ();public long getFileSize()int nFileLength = -1;
13、URL url = new URL(siteInfoBean.getSSiteURL();HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection ();int responseCode=httpConnection.getResponseCode();if(responseCode&=400)processErrorCode(responseCode);return -2; /-2 represent access is errorString sHeader;for(int i=1;/DataInput
14、Stream 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);catch(IOException e)e.printStackTrace ();Uti
15、lity.log(nFileLength);return nFileLength;/保存下载信息(文件指针位置)private void write_nPos()output = new DataOutputStream(new FileOutputStream(tmpFile);output.writeInt(nStartPos.length);/ output.writeLong(nPosi);output.writeLong(fileSplitterFetchi.nStartPos);output.writeLong(fileSplitterFetchi.nEndPos);output.
16、close();/读取保存的下载信息(文件指针位置)private void read_nPos()DataInputStream input = new DataInputStream(new FileInputStream(tmpFile);int nCount = input.readInt();nStartPos = new longnCount;nEndPos = new longnCount;nStartPosi = input.readLong();nEndPosi = input.readLong();input.close();private void processErro
17、rCode(int nErrorCode)Error Code : + nErrorCode);/停止文件下载public void siteStop()bStop = true;fileSplitterFetchi.splitterStop();*FileSplitterFetch.javapublic class FileSplitterFetch extends Thread String sURL; /File URLlong nStartPos; /File Snippet Start Positionlong nEndPos; /File Snippet End Positioni
18、nt nThreadID; /Threads IDboolean bDownOver = false; /Downing is over /Stop identicalFileAccessI fileAccessI = null; /File Access interfacepublic FileSplitterFetch(String sURL,String sName,long nStart,long nEnd,int id) throws IOExceptionthis.sURL = sURL;this.nStartPos = nStart;this.nEndPos = nEnd;nTh
19、readID = id;fileAccessI = new FileAccessI(sName,nStartPos);while(nStartPos & nEndPos & !URL url = new URL(sURL);String sProperty = bytes=+nStartPos+-,sProperty);Utility.log(sProperty);/logResponseHead(httpConnection); 0 & nStartPos &nStartPos += fileAccessI.write(b,0,nRead);/if(nThreadID = 1)nStartP
20、os = + nStartPos + + nEndPos); + nThreadID + is over!bDownOver = true;/nPos = fileAccessI.write (b,0,nRead);/打印回应的头信息public void logResponseHead(HttpURLConnection con)String header=con.getHeaderFieldKey(i);if(header!/responseHeaders.put(header,httpConnection.getHeaderField(header);Utility.log(header
21、+ :+con.getHeaderField(header);public void splitterStop()*FileAccess.javapublic class FileAccessI implements SerializableRandomAccessFile oSavedFile;long nPos;public FileAccessI() throws IOExceptionthis(,0);public FileAccessI(String sName,long nPos) throws IOExceptionoSavedFile = new RandomAccessFil
22、e(sName,this.nPos = 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.javapublic class SiteInfoBean private String sSiteURL; /Sites URLprivate String sFilePath; /Saved Files Pathprivate String sFi
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