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！