(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。 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();}}/***FileSplitterFetch.java*/package NetFox;import java.io.*;import java.net.*;public class FileSplitterFetch extends Thread {String sURL; //File URLlong nStartPos; //File Snippet Start Positionlong nEndPos; //File Snippet End Positionint nThreadID; //Thread's IDboolean bDownOver = false; //Downing is overboolean bStop = false; //Stop identicalFileAccessI fileAccessI = null; //File Access interfacepublic 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));elsebreak;}}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 URLprivate String sFilePath; //Saved File's Pathprivate String sFileName; //Saved File's Nameprivate int nSplitter; //Count of Splited Downloading Filepublic SiteInfoBean(){//default value of nSplitter is 5this("","","",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.exetry{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();}}