用 Java 實現斷點續傳 (HTTP)

斷點續傳的原理

其實斷點續傳的原理很簡單，就是在 Http 的請求上和通常的下載有所不一樣而已。 
打個比方，瀏覽器請求服務器上的一個文時，所發出的請求以下： 
假設服務器域名爲 wwww.sjtu.edu.cn，文件名爲 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

服務器收到請求後，按要求尋找請求的文件，提取文件的信息，而後返回給瀏覽器，返回信息以下：

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

所謂斷點續傳，也就是要從文件已經下載的地方開始繼續下載。因此在客戶端瀏覽器傳給 Web 服務器的時候要多加一條信息 -- 從哪裏開始。 
下面是用本身編的一個"瀏覽器"來傳遞請求信息給 Web 服務器，要求從 2000070 字節開始。 
GET /down.zip HTTP/1.0 
User-Agent: NetFox 
RANGE: bytes=2000070- 
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2

仔細看一下就會發現多了一行 RANGE: bytes=2000070- 
這一行的意思就是告訴服務器 down.zip 這個文件從 2000070 字節開始傳，前面的字節不用傳了。 
服務器收到這個請求之後，返回的信息以下： 
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

和前面服務器返回的信息比較一下，就會發現增長了一行： 
Content-Range=bytes 2000070-106786027/106786028 
返回的代碼也改成 206 了，而再也不是 200 了。

知道了以上原理，就能夠進行斷點續傳的編程了。

 

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Java 實現斷點續傳的關鍵幾點

1. (1) 用什麼方法實現提交 RANGE: bytes=2000070-。 
   固然用最原始的 Socket 是確定能完成的，不過那樣太費事了，其實 Java 的 net 包中提供了這種功能。代碼以下： 
   
   URL url = new URL("http://www.sjtu.edu.cn/down.zip"); 
   HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection(); 
   
   // 設置 User-Agent 
   httpConnection.setRequestProperty("User-Agent","NetFox"); 
   // 設置斷點續傳的開始位置 
   httpConnection.setRequestProperty("RANGE","bytes=2000070"); 
   // 得到輸入流 
   InputStream input = httpConnection.getInputStream(); 
   

   從輸入流中取出的字節流就是 down.zip 文件從 2000070 開始的字節流。 你們看，其實斷點續傳用 Java 實現起來仍是很簡單的吧。 接下來要作的事就是怎麼保存得到的流到文件中去了。

2. 保存文件採用的方法。 
   我採用的是 IO 包中的 RandAccessFile 類。 
   操做至關簡單，假設從 2000070 處開始保存文件，代碼以下： 
   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流，而後寫到文件中 
   while((nRead=input.read(b,0,1024)) > 0) 
   { 
   oSavedFile.write(b,0,nRead); 
   }

怎麼樣，也很簡單吧。 接下來要作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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斷點續傳內核的實現

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：

/* 
 /*
 * 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; // 中止標誌
 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() 
 { 
 // 得到文件長度
 // 分割文件
 // 實例 FileSplitterFetch 
 // 啓動 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; 
 } 
 } 
 // 啓動子線程
 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]); 
 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(); 
 // 等待子線程結束
 //int count = 0; 
 // 是否結束 while 循環
 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("文件下載結束！"); 
 } 
 catch(Exception e){e.printStackTrace ();} 
 } 
 // 得到文件長度
 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); 
 } 
 // 中止文件下載
 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 
 { 
 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 ();} 
 } 
 } 
 // 打印迴應的頭信息
 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); 
 //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(); 
 } 
 }