用 Java 实现断点续传 (HTTP)

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。

 

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Java 实现断点续传的关键几点

1. (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。

2. 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。

 

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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 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();
 }
 }