Android 断点续传的原理剖析与实例讲解

 本文所要讲的是android断点续传的内容，以实例的形式进行了详细介绍。

       一、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www.jizhuomi.com/android，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。

       二、java实现断点续传的关键几点

       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：

java代码

url url = new url("http://www.jizhuomi.com/android/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。

    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：

java代码
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。

    三、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。

    四、实例源码

    下面是源程序：

java代码
/* 
**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 = (long)(i*(nfilelength/nstartpos.length));  
}  
for(int i=0;i<nendpos.length-1;i++)  
{  
nendpos = 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 = new filesplitterfetch(siteinfobean.getssiteurl(),  
siteinfobean.getsfilepath() + file.separator + siteinfobean.getsfilename(),  
nstartpos,nendpos,i);  
utility.log("thread " + i + " , nstartpos = " + nstartpos + ", nendpos = " + nendpos);  
filesplitterfetch.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.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);  
output.writelong(filesplitterfetch.nstartpos);  
output.writelong(filesplitterfetch.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 = input.readlong();  
nendpos = 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.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; //threads 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; //sites url  
private string sfilepath; //saved files path  
private string sfilename; //saved files 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();  
}  
}  

以上就是对android 断点续传的资料整理，后续继续补充相关资料，谢谢大家对本站的支持！