用 Java 实现断点续传 (HTTP)（转）

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。

---

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。

---

 

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：

  1 /* 
  2  /*
  3  * SiteFileFetch.java 
  4  */ 
  5  package NetFox; 
  6  import java.io.*; 
  7  import java.net.*; 
  8  public class SiteFileFetch extends Thread { 
  9  SiteInfoBean siteInfoBean = null; // 文件信息 Bean 
 10  long[] nStartPos; // 开始位置
 11  long[] nEndPos; // 结束位置
 12  FileSplitterFetch[] fileSplitterFetch; // 子线程对象
 13  long nFileLength; // 文件长度
 14  boolean bFirst = true; // 是否第一次取文件
 15  boolean bStop = false; // 停止标志
 16  File tmpFile; // 文件下载的临时信息
 17  DataOutputStream output; // 输出到文件的输出流
 18  public SiteFileFetch(SiteInfoBean bean) throws IOException 
 19  { 
 20  siteInfoBean = bean; 
 21  //tmpFile = File.createTempFile ("zhong","1111",new File(bean.getSFilePath())); 
 22  tmpFile = new File(bean.getSFilePath()+File.separator + bean.getSFileName()+".info");
 23  if(tmpFile.exists ()) 
 24  { 
 25  bFirst = false; 
 26  read_nPos(); 
 27  } 
 28  else 
 29  { 
 30  nStartPos = new long[bean.getNSplitter()]; 
 31  nEndPos = new long[bean.getNSplitter()]; 
 32  } 
 33  } 
 34  public void run() 
 35  { 
 36  // 获得文件长度
 37  // 分割文件
 38  // 实例 FileSplitterFetch 
 39  // 启动 FileSplitterFetch 线程
 40  // 等待子线程返回
 41  try{ 
 42  if(bFirst) 
 43  { 
 44  nFileLength = getFileSize(); 
 45  if(nFileLength == -1) 
 46  { 
 47  System.err.println("File Length is not known!"); 
 48  } 
 49  else if(nFileLength == -2) 
 50  { 
 51  System.err.println("File is not access!"); 
 52  } 
 53  else 
 54  { 
 55  for(int i=0;i) 
 56  { 
 57  nStartPos[i] = (long)(i*(nFileLength/nStartPos.length)); 
 58  } 
 59  for(int i=0;i) 
 60  { 
 61  nEndPos[i] = nStartPos[i+1]; 
 62  } 
 63  nEndPos[nEndPos.length-1] = nFileLength; 
 64  } 
 65  } 
 66  // 启动子线程
 67  fileSplitterFetch = new FileSplitterFetch[nStartPos.length]; 
 68  for(int i=0;i) 
 69  { 
 70  fileSplitterFetch[i] = new FileSplitterFetch(siteInfoBean.getSSiteURL(), 
 71  siteInfoBean.getSFilePath() + File.separator + siteInfoBean.getSFileName(), 
 72  nStartPos[i],nEndPos[i],i); 
 73  Utility.log("Thread " + i + " , nStartPos = " + nStartPos[i] + ", nEndPos = " 
 74  + nEndPos[i]); 
 75  fileSplitterFetch[i].start(); 
 76  } 
 77  // fileSplitterFetch[nPos.length-1] = new FileSplitterFetch(siteInfoBean.getSSiteURL(),
 78  siteInfoBean.getSFilePath() + File.separator 
 79  + siteInfoBean.getSFileName(),nPos[nPos.length-1],nFileLength,nPos.length-1); 
 80  // Utility.log("Thread " +(nPos.length-1) + ",nStartPos = "+nPos[nPos.length-1]+",
 81  nEndPos = " + nFileLength); 
 82  // fileSplitterFetch[nPos.length-1].start(); 
 83  // 等待子线程结束
 84  //int count = 0; 
 85  // 是否结束 while 循环
 86  boolean breakWhile = false; 
 87  while(!bStop) 
 88  { 
 89  write_nPos(); 
 90  Utility.sleep(500); 
 91  breakWhile = true; 
 92  for(int i=0;i) 
 93  { 
 94  if(!fileSplitterFetch[i].bDownOver) 
 95  { 
 96  breakWhile = false; 
 97  break; 
 98  } 
 99  } 
100  if(breakWhile) 
101  break; 
102  //count++; 
103  //if(count>4) 
104  // siteStop(); 
105  } 
106  System.err.println("文件下载结束！"); 
107  } 
108  catch(Exception e){e.printStackTrace ();} 
109  } 
110  // 获得文件长度
111  public long getFileSize() 
112  { 
113  int nFileLength = -1; 
114  try{ 
115  URL url = new URL(siteInfoBean.getSSiteURL()); 
116  HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection ();
117  httpConnection.setRequestProperty("User-Agent","NetFox"); 
118  int responseCode=httpConnection.getResponseCode(); 
119  if(responseCode>=400) 
120  { 
121  processErrorCode(responseCode); 
122  return -2; //-2 represent access is error 
123  } 
124  String sHeader; 
125  for(int i=1;;i++) 
126  { 
127  //DataInputStream in = new DataInputStream(httpConnection.getInputStream ()); 
128  //Utility.log(in.readLine()); 
129  sHeader=httpConnection.getHeaderFieldKey(i); 
130  if(sHeader!=null) 
131  { 
132  if(sHeader.equals("Content-Length")) 
133  { 
134  nFileLength = Integer.parseInt(httpConnection.getHeaderField(sHeader)); 
135  break; 
136  } 
137  } 
138  else 
139  break; 
140  } 
141  } 
142  catch(IOException e){e.printStackTrace ();} 
143  catch(Exception e){e.printStackTrace ();} 
144  Utility.log(nFileLength); 
145  return nFileLength; 
146  } 
147  // 保存下载信息（文件指针位置）
148  private void write_nPos() 
149  { 
150  try{ 
151  output = new DataOutputStream(new FileOutputStream(tmpFile)); 
152  output.writeInt(nStartPos.length); 
153  for(int i=0;i) 
154  { 
155  // output.writeLong(nPos[i]); 
156  output.writeLong(fileSplitterFetch[i].nStartPos); 
157  output.writeLong(fileSplitterFetch[i].nEndPos); 
158  } 
159  output.close(); 
160  } 
161  catch(IOException e){e.printStackTrace ();} 
162  catch(Exception e){e.printStackTrace ();} 
163  } 
164  // 读取保存的下载信息（文件指针位置）
165  private void read_nPos() 
166  { 
167  try{ 
168  DataInputStream input = new DataInputStream(new FileInputStream(tmpFile)); 
169  int nCount = input.readInt(); 
170  nStartPos = new long[nCount]; 
171  nEndPos = new long[nCount]; 
172  for(int i=0;i) 
173  { 
174  nStartPos[i] = input.readLong(); 
175  nEndPos[i] = input.readLong(); 
176  } 
177  input.close(); 
178  } 
179  catch(IOException e){e.printStackTrace ();} 
180  catch(Exception e){e.printStackTrace ();} 
181  } 
182  private void processErrorCode(int nErrorCode) 
183  { 
184  System.err.println("Error Code : " + nErrorCode); 
185  } 
186  // 停止文件下载
187  public void siteStop() 
188  { 
189  bStop = true; 
190  for(int i=0;i) 
191  fileSplitterFetch[i].splitterStop(); 
192  } 
193  }

  1 /* 
  2  **FileSplitterFetch.java 
  3  */ 
  4  package NetFox; 
  5  import java.io.*; 
  6  import java.net.*; 
  7  public class FileSplitterFetch extends Thread { 
  8  String sURL; //File URL 
  9  long nStartPos; //File Snippet Start Position 
 10  long nEndPos; //File Snippet End Position 
 11  int nThreadID; //Thread's ID 
 12  boolean bDownOver = false; //Downing is over 
 13  boolean bStop = false; //Stop identical 
 14  FileAccessI fileAccessI = null; //File Access interface 
 15  public FileSplitterFetch(String sURL,String sName,long nStart,long nEnd,int id)
 16  throws IOException 
 17  { 
 18  this.sURL = sURL; 
 19  this.nStartPos = nStart; 
 20  this.nEndPos = nEnd; 
 21  nThreadID = id; 
 22  fileAccessI = new FileAccessI(sName,nStartPos); 
 23  } 
 24  public void run() 
 25  { 
 26  while(nStartPos < nEndPos && !bStop) 
 27  { 
 28  try{ 
 29  URL url = new URL(sURL); 
 30  HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection (); 
 31  httpConnection.setRequestProperty("User-Agent","NetFox"); 
 32  String sProperty = "bytes="+nStartPos+"-"; 
 33  httpConnection.setRequestProperty("RANGE",sProperty); 
 34  Utility.log(sProperty); 
 35  InputStream input = httpConnection.getInputStream(); 
 36  //logResponseHead(httpConnection); 
 37  byte[] b = new byte[1024]; 
 38  int nRead; 
 39  while((nRead=input.read(b,0,1024)) > 0 && nStartPos < nEndPos 
 40  && !bStop) 
 41  { 
 42  nStartPos += fileAccessI.write(b,0,nRead); 
 43  //if(nThreadID == 1) 
 44  // Utility.log("nStartPos = " + nStartPos + ", nEndPos = " + nEndPos); 
 45  } 
 46  Utility.log("Thread " + nThreadID + " is over!"); 
 47  bDownOver = true; 
 48  //nPos = fileAccessI.write (b,0,nRead); 
 49  } 
 50  catch(Exception e){e.printStackTrace ();} 
 51  } 
 52  } 
 53  // 打印回应的头信息
 54  public void logResponseHead(HttpURLConnection con) 
 55  { 
 56  for(int i=1;;i++) 
 57  { 
 58  String header=con.getHeaderFieldKey(i); 
 59  if(header!=null) 
 60  //responseHeaders.put(header,httpConnection.getHeaderField(header)); 
 61  Utility.log(header+" : "+con.getHeaderField(header)); 
 62  else 
 63  break; 
 64  } 
 65  } 
 66  public void splitterStop() 
 67  { 
 68  bStop = true; 
 69  } 
 70  } 
 71  
 72  /* 
 73  **FileAccess.java 
 74  */ 
 75  package NetFox; 
 76  import java.io.*; 
 77  public class FileAccessI implements Serializable{ 
 78  RandomAccessFile oSavedFile; 
 79  long nPos; 
 80  public FileAccessI() throws IOException 
 81  { 
 82  this("",0); 
 83  } 
 84  public FileAccessI(String sName,long nPos) throws IOException 
 85  { 
 86  oSavedFile = new RandomAccessFile(sName,"rw"); 
 87  this.nPos = nPos; 
 88  oSavedFile.seek(nPos); 
 89  } 
 90  public synchronized int write(byte[] b,int nStart,int nLen) 
 91  { 
 92  int n = -1; 
 93  try{ 
 94  oSavedFile.write(b,nStart,nLen); 
 95  n = nLen; 
 96  } 
 97  catch(IOException e) 
 98  { 
 99  e.printStackTrace (); 
100  } 
101  return n; 
102  } 
103  } 
104  
105  /* 
106  **SiteInfoBean.java 
107  */ 
108  package NetFox; 
109  public class SiteInfoBean { 
110  private String sSiteURL; //Site's URL 
111  private String sFilePath; //Saved File's Path 
112  private String sFileName; //Saved File's Name 
113  private int nSplitter; //Count of Splited Downloading File 
114  public SiteInfoBean() 
115  { 
116  //default value of nSplitter is 5 
117  this("","","",5); 
118  } 
119  public SiteInfoBean(String sURL,String sPath,String sName,int nSpiltter)
120  { 
121  sSiteURL= sURL; 
122  sFilePath = sPath; 
123  sFileName = sName; 
124  this.nSplitter = nSpiltter; 
125  } 
126  public String getSSiteURL() 
127  { 
128  return sSiteURL; 
129  } 
130  public void setSSiteURL(String value) 
131  { 
132  sSiteURL = value; 
133  } 
134  public String getSFilePath() 
135  { 
136  return sFilePath; 
137  } 
138  public void setSFilePath(String value) 
139  { 
140  sFilePath = value; 
141  } 
142  public String getSFileName() 
143  { 
144  return sFileName; 
145  } 
146  public void setSFileName(String value) 
147  { 
148  sFileName = value; 
149  } 
150  public int getNSplitter() 
151  { 
152  return nSplitter; 
153  } 
154  public void setNSplitter(int nCount) 
155  { 
156  nSplitter = nCount; 
157  } 
158  } 
159  
160  /* 
161  **Utility.java 
162  */ 
163  package NetFox; 
164  public class Utility { 
165  public Utility() 
166  { 
167  } 
168  public static void sleep(int nSecond) 
169  { 
170  try{ 
171  Thread.sleep(nSecond); 
172  } 
173  catch(Exception e) 
174  { 
175  e.printStackTrace (); 
176  } 
177  } 
178  public static void log(String sMsg) 
179  { 
180  System.err.println(sMsg); 
181  } 
182  public static void log(int sMsg) 
183  { 
184  System.err.println(sMsg); 
185  } 
186  } 
187  
188  /* 
189  **TestMethod.java 
190  */ 
191  package NetFox; 
192  public class TestMethod { 
193  public TestMethod() 
194  { ///xx/weblogic60b2_win.exe 
195  try{ 
196  SiteInfoBean bean = new SiteInfoBean("http://localhost/xx/weblogic60b2_win.exe",
197      "L:\\temp","weblogic60b2_win.exe",5); 
198  //SiteInfoBean bean = new SiteInfoBean("http://localhost:8080/down.zip","L:\\temp",
199      "weblogic60b2_win.exe",5); 
200  SiteFileFetch fileFetch = new SiteFileFetch(bean); 
201  fileFetch.start(); 
202  } 
203  catch(Exception e){e.printStackTrace ();} 
204  } 
205  public static void main(String[] args) 
206  { 
207  new TestMethod(); 
208  } 
209  }

 

原文链接：http://www.ibm.com/developerworks/cn/java/joy-down/