android HTTP实现文件多线程下载

                           文件多线程下载                                                 

 

 

一、文件多线程下载步骤

        1、  获取网络文件的长度，在android客户端生成一个与网络文件长度相等的空文件

        2、  开启N条线程下载文件，计算每条线程负责下载的数据量

        3、  依次创建，启动多条线程分别从网络文件的不同位置下载数据，并从本地文件的相同位置写入数据，要计算出每条线程从网络文件的什么位置开始下载数据，到什么位置结束下载

 

具体步骤

        1、  打开HTTP协议的连接对象，为对象设置属性

    //path为源路径

             URL url = new URL(path);

                   HttpURLConnection conn = (HttpURLConnection) url.openConnection();

                   conn.setConnectTimeout(5000);

                   conn.setRequestMethod("GET");

            2、判断请求是否成功

                   if(conn.getResponseCode() == 200){

             //步骤3 4 5 6

                   }

             (以下步骤均在请求成功的基础上去实实现)

            3、取得网络文件的长度

             int  length = conn.getContentLength();

 

            4、随机文件访问类来生成一个与网络文件长度相等的本地文件

                1>取得网络文件名称

                     String  filename = path.substring(path.lastIndexOf("/")+1);//从最后一个"/"开始获取

               2>创建文件

                   File file = new File(mubiao,filename); // mubiao指文件保存路径  filename为文件名称

                3>随机文件访问

                    RandomAccessFile accessFile = new RandomAccessFile(file,"rwd");

                4>设置文件长度

                    accessFile.setLength(length);

                     accessFile.close();

          5、计算每个线程负责下载的数据量

 

               int block  = length%threadsize == 0 ? length/threadsize :length/threadsize +1;

 

           6、循环实现线程的下载

            for(int  threadid = 0 ;threadid<threadsize;threadid++){

                            new DownloadThread(threadid,block,url,file).start();

            }

 

           7、创建线程类实现下载

            private class DownloadThread extends Thread{

                   private int threadid;

                   private int block;

                   private URL url;

                   private File file;

 

                   public DownloadThread(int threadid, int block, URL url, File file) {

                            this.threadid=threadid;

                            this.block=block;

                            this.url=url;

                            this.file=file;

                   }

                  

                   public void run(){

                             //计算该线程从网络文件的什么位置开始下载

                                int start = threadid * block ;

                               //下载到什么位置结束

                                int end= (threadid+1) * block-1 ;

                               try{

                                        //建随机文件访问类对象

                                        RandomAccessFile accessFile = new RandomAccessFile(file,"rwd");

                                       //指定从本地文件的某一个位置写入数据

                                        accessFile.seek(start);

//请求网络文件某一个区域的数据

                                        HttpURLConnection conn = (HttpURLConnection) url.openConnection();

                                                   conn.setConnectTimeout(5000);

                                                   conn.setRequestMethod("GET");

                                                   conn.setRequestProperty("Range", "bytes="+start+"-"+end);

                                                   if(conn.getResponseCode() == 206){

                                                              InputStream instream = conn.getInputStream();

                                                   //循环读入数据

                                                   byte[] buffer = new byte[1024];

                                                   int len=0; //返回的读取到的长度

                                                   while((len = instream.read(buffer))!=-1){

                                                       //-------将数据写入本地文件

                                                       accessFile.write(buffer,0,len);

                                                   }      

                                                   accessFile.close();

                                                   instream.close();

                                                //   System.out.println(threadid+1+"线程下载完成") ;

                                                   }

                                                   else{

                                                   //      System.out.println(threadid+1+"线程下载失败") ;

                                                   }

                                      

                                       

                               }catch(Exception e){

                                        e.printStackTrace();

                               }

                   }       

         }

 

 重写线程的run方法

   1>计算该线程从网络文件的什么位置开始下载

    int start = threadid * block ;

   2>下载到什么位置结束

    int end= (threadid+1) * block-1 ;

    3>建随机文件访问类对象

    RandomAccessFile accessFile = new RandomAccessFile(file,"rwd");

    4>指定从本地文件的某一个位置写入数据

    accessFile.seek(start);

    5>请求网络文件某一个区域的数据

      1 得到HttpURLConnection对象,并设置属性

          HttpURLConnection conn = (HttpURLConnection) url.openConnection();

                    conn.setConnectTimeout(5000);

                    conn.setRequestMethod("GET");

      2 使用HTTP的Range头字段指定每条线程从文件的什么位置开始下载

        下载到什么位置结束

        conn.setRequestProperty("Range", "bytes="+start+"-"+end);

     

      3 判断请求是否成功断点下载的返回码为206

        if(conn.getResponseMethod()==206){ 

             //步骤4

 

 

        }

       

        4 得到输入流，通过输入流得到数据，将数据写入本地文件

          InputStream instream = conn.getInputStream();

          //循环读入数据

          byte[] buffer = new byte[1024];

          int len=0; //返回的读取到的长度

          while((len = instream.read(buffer))!=-1){  //如果不为-1说明数据没有读完

              //-------将数据写入本地文件

              accessFile.write(buffer,0,len);

          }      

          accessFile.close();

          instream.close();