用Java实现断点续传(HTTP)

http://www.ibm.com/developerworks/cn/java/joy-down/index.html

 

本文介绍了一种利用 Java 来实现断点续传的方法。

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。

回页首

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：

/*
 **SiteFileFetch.java
 */
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;

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
 */
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.io.Serializable;
import java.net.HttpURLConnection;
import java.net.URL;

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
 */
import java.io.*;

public class FileAccessI implements Serializable {
	/**  */
	private static final long serialVersionUID = 265981599353224785L;
	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
 */
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
 */
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
 */

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();
	}
}

 

评论

4 楼 guji528 2009-08-03  

真的很强，有空再测试一下。

3 楼 chilongxph 2008-11-17  

thank you!

2 楼 InviteSun 2008-11-12  

在此谢过。

1 楼 BarryWei 2008-09-10  

Javaeye中高手就是多

这篇文章 受益匪浅 非常感谢