用Java实现断点续传(HTTP)，java断点续传,其实断点续传的原理很简单

用Java实现断点续传(HTTP)，java断点续传,其实断点续传的原理很简单

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; MSIE5.01; 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=106786028Accept-Ranges=bytes Date=Mon, 30 Apr 2001 12:56:11 GMTETag=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： 206Content-Length=106786028 Content-Range=bytes 2000070-106786027/106786028Date=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, 30Apr 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=bytes2000070-106786027/106786028 返回的代码也改为206了

package NetFox;import java.io.*;import java.net.*;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;}}elsebreak;}}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();}}//该片段来自于http://byrx.net