用 Java 实现断点续传 (HTTP)-转自 IBM
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。
/* /* * 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(); } }??