多线程下载和断点续传技术的实现原理。

package com.zzw.constant;

public class Contant 
{
    public static int threadCount = 3;
}


  1 package com.zzw.download;
  2 
  3 import java.io.BufferedReader;
  4 import java.io.File;
  5 import java.io.FileInputStream;
  6 import java.io.FileOutputStream;
  7 import java.io.IOException;
  8 import java.io.InputStream;
  9 import java.io.InputStreamReader;
 10 import java.io.RandomAccessFile;
 11 import java.net.HttpURLConnection;
 12 import java.net.MalformedURLException;
 13 import java.net.URL;
 14 import java.net.URLConnection;
 15 
 16 import javax.tools.FileObject;
 17 
 18 import com.zzw.constant.Contant;
 19 
 20 public class TestDownload 
 21 {
 22     //public static int threadCount = 3;
 23     public static int blockSize = 0;
 24 
 25     public static void main(String[] args) throws Exception 
 26     {
 27         URL url = new URL("http://localhost:8080/download/cc.zip");
 28         
 29         HttpURLConnection conn = (HttpURLConnection) url.openConnection();
 30         conn.setRequestMethod("GET");
 31         conn.setReadTimeout(10000);
 32         
 33         long length = conn.getContentLength();
 34         
 35         conn.connect(); //连接
 36         File file = new File("cc.zip");
 37         
 38         RandomAccessFile randomFile = new RandomAccessFile(file,"rwd");
 39         randomFile.setLength(length);
 40         
 41         //每一个线程他需要下载的大小
 42         System.out.println("总大小:" + length);
 43         blockSize = (int) (length / Contant.threadCount);
 44         System.out.println("每一个线程下载的大小：" + blockSize);
 45         long start = 1;
 46         long end = 0;
 47         for(int i=1; i < (Contant.threadCount+1); i++)
 48         {
 49             
 50             //1 >> 0 - 46354938 123456
 51             //2 >> 46354939 -(46354939 + 46354938)
 52             //3 >> 
 53             start = blockSize * (i -1);
 54             end = blockSize*i -1;
 55             if(i == Contant.threadCount)
 56             {
 57                 end = length;
 58             }
 59             System.out.println("第" + i + "个线程下载的区间:" + start + " - " + end);
 60             new downloadThread(file, i, start, end).start();
 61         }
 62         
 63         conn.disconnect();
 64     }
 65 }
 66 
 67 class downloadThread extends Thread
 68 {
 69     private File path; //本地磁盘的地址
 70     private String url = "http://localhost:8080/download/cc.zip" ; //HTTP请求的地址
 71     private Integer threadID; //当前是第几个线程
 72     private long start; //该线程下载的起始位置
 73     private long end; //该线程的结束位置
 74     
 75     public downloadThread(File path, Integer threadID, long start,
 76             long end) {
 77         this.path = path;
 78         this.threadID = threadID;
 79         this.start = start;
 80         this.end = end;
 81         
 82     }
 83 
 84     @Override
 85     public void run() 
 86     {
 87         try {
 88             System.out.println(threadID + "开启读取");
 89             File contextFile = new File(threadID +".txt");
 90             if(!contextFile.exists())
 91             {
 92                 contextFile.createNewFile();
 93             }
 94             
 95             //读取断线信息里面的数据来重新的指定从哪里开始写
 96             BufferedReader reader = new BufferedReader(new InputStreamReader(new FileInputStream(contextFile)));
 97             String info = reader.readLine();
 98             if(null != info && info.length() > 0)
 99             {
100                 start = start + Integer.parseInt(info);
101             }
102             
103             System.out.println("start........=" + start);
104             URL uri = new URL(url);
105             HttpURLConnection conn = (HttpURLConnection) uri.openConnection();
106             conn.setRequestProperty("Range", "bytes="+start +"-" + end);
107             conn.setReadTimeout(50000);
108             conn.setConnectTimeout(50000);
109             
110             conn.connect();
111             
112             //让http请求只请求一个片段
113             System.out.println("bytes="+start +"-" + end);
114             
115             //判断一下是否为200？
116             int status = conn.getResponseCode();
117             
118             if(status / 100 == 2)
119             {
120                 int total = 0; //总共下载的字节数
121                 //FileOutputStream out = new FileOutputStream(new File(threadID +".txt")); 不靠谱了
122                 RandomAccessFile random = new RandomAccessFile(path, "rw");
123                 
124                 random.seek(start);
125                 InputStream in = conn.getInputStream();
126                 byte[] buffer = new byte[1024];
127                 int len = 0;
128                 
129                 while((len = in.read(buffer)) != -1)
130                 {
131                     RandomAccessFile duandian = new RandomAccessFile(contextFile, "rwd");
132                     random.write(buffer, 0, len);
133                     total += len;
134                     duandian.write(String.valueOf(total).getBytes());
135                     duandian.close();
136                 }
137                 in.close();
138                 random.close();
139             }
140             conn.disconnect();
141             System.out.println(threadID + "读取完成");
142         } catch (MalformedURLException e) {
143             e.printStackTrace();
144         } catch (IOException e) {
145             e.printStackTrace();
146         }finally
147         {
148             synchronized (TestDownload.class)
149             {
150                 Contant.threadCount --;
151                 System.out.println("Contant.threadCount = " + Contant.threadCount);
152                 if(Contant.threadCount == 0)
153                 {
154                     for(int i=1; i < 4; i++)
155                     {
156                         File contextFile = new File(i +".txt");
157                         System.out.println(contextFile.delete() +"   ccc");
158                     }
159                 }
160             }
161         }
162         
163     }
164 }