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用Java实现断点续传(HTTP)

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

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();
            }
            }
            

posted on 2008-05-16 11:12 都市淘沙者 阅读(511) 评论(0)  编辑  收藏 所属分类: 流媒体服务器/VLC/FFmpeg


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