斷點續(xù)傳的原理
其實斷點續(xù)傳的原理很簡單,就是在Http的請求上和一般的下載有所不同而已。
打個比方,瀏覽器請求服務(wù)器上的一個文時,所發(fā)出的請求如下:
假設(shè)服務(wù)器域名為wwww.sjtu.edu.cn,文件名為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
服務(wù)器收到請求后,按要求尋找請求的文件,提取文件的信息,然后返回給瀏覽器,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
所謂斷點續(xù)傳,也就是要從文件已經(jīng)下載的地方開始繼續(xù)下載。所以在客戶端瀏覽器傳給 Web服務(wù)器的時候要多加一條信息--從哪里開始。
下面是用自己編的一個"瀏覽器"來傳遞請求信息給Web服務(wù)器,要求從2000070字節(jié)開始。
GET /down.zip HTTP/1.0
User-Agent: NetFox
RANGE: bytes=2000070-
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
仔細看一下就會發(fā)現(xiàn)多了一行RANGE: bytes=2000070-
這一行的意思就是告訴服務(wù)器down.zip這個文件從2000070字節(jié)開始傳,前面的字節(jié)不用傳了。
服務(wù)器收到這個請求以后,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
和前面服務(wù)器返回的信息比較一下,就會發(fā)現(xiàn)增加了一行:
Content-Range=bytes 2000070-106786027/106786028
返回的代碼也改為206了,而不再是200了。
知道了以上原理,就可以進行斷點續(xù)傳的編程了。
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Java實現(xiàn)斷點續(xù)傳的關(guān)鍵幾點
-
(1)用什么方法實現(xiàn)提交RANGE: bytes=2000070-。
當然用最原始的Socket是肯定能完成的,不過那樣太費事了,其實Java的net包中提供了這種功能。代碼如下:
URL url = new URL("http://www.sjtu.edu.cn/down.zip");
HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection();
//設(shè)置User-Agent
httpConnection.setRequestProperty("User-Agent","NetFox");
//設(shè)置斷點續(xù)傳的開始位置
httpConnection.setRequestProperty("RANGE","bytes=2000070");
//獲得輸入流
InputStream input = httpConnection.getInputStream();
從輸入流中取出的字節(jié)流就是down.zip文件從2000070開始的字節(jié)流。大家看,其實斷點續(xù)傳用Java實現(xiàn)起來還是很簡單的吧。接下來要做的事就是怎么保存獲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類。
操作相當簡單,假設(shè)從2000070處開始保存文件,代碼如下:
RandomAccess oSavedFile = new RandomAccessFile("down.zip","rw");
long nPos = 2000070;
//定位文件指針到nPos位置
oSavedFile.seek(nPos);
byte[] b = new byte[1024];
int nRead;
//從輸入流中讀入字節(jié)流,然后寫到文件中
while((nRead=input.read(b,0,1024)) > 0)
{
oSavedFile.write(b,0,nRead);
}
怎么樣,也很簡單吧。接下來要做的就是整合成一個完整的程序了。包括一系列的線程控制等等。
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斷點續(xù)傳內(nèi)核的實現(xiàn)
主要用了6個類,包括一個測試類。
SiteFileFetch.java負責整個文件的抓取,控制內(nèi)部線程(FileSplitterFetch類)。
FileSplitterFetch.java負責部分文件的抓取。
FileAccess.java負責文件的存儲。
SiteInfoBean.java要抓取的文件的信息,如文件保存的目錄,名字,抓取文件的URL等。
Utility.java工具類,放一些簡單的方法。
TestMethod.java測試類。
下面是源程序:
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/* **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; //開始位置 long[] nEndPos; //結(jié)束位置 FileSplitterFetch[] fileSplitterFetch; //子線程對象 long nFileLength; //文件長度 boolean bFirst = true; //是否第一次取文件 boolean bStop = false; //停止標志 File tmpFile; //文件下載的臨時信息 DataOutputStream output; //輸出到文件的輸出流 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() { //獲得文件長度 //分割文件 //實例FileSplitterFetch //啟動FileSplitterFetch線程 //等待子線程返回 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; } } //啟動子線程 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]); <!-- code sample is too wide -->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); <!-- code sample is too wide -->// Utility.log("Thread " + (nPos.length-1) + " , nStartPos = " + nPos[nPos.length-1] + ", nEndPos = " + nFileLength); // fileSplitterFetch[nPos.length-1].start(); //等待子線程結(jié)束 //int count = 0; //是否結(jié)束while循環(huán) 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("文件下載結(jié)束!"); } catch(Exception e){e.printStackTrace ();} } //獲得文件長度 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; } //保存下載信息(文件指針位置) 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 ();} } //讀取保存的下載信息(文件指針位置) 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); } //停止文件下載 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 <!-- code sample is too wide -->{ 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 ();} } } //打印回應(yīng)的頭信息 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); <!-- code sample is too wide -->//SiteInfoBean bean = new SiteInfoBean("http://localhost:8080/down.zip","L:\\temp","weblogic60b2_win.exe",5); <!-- code sample is too wide -->SiteFileFetch fileFetch = new SiteFileFetch(bean); fileFetch.start(); } catch(Exception e){e.printStackTrace ();} } public static void main(String[] args) { new TestMethod(); } }
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