Android 断点续传的原理剖析与实例讲解

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();  }  }