JAVA下实现多线程断点下载

Posted 2020-10-03 starry

多线程断点下载：顾名思义是用多线程实现的，断点是当第三方因素（断电、断网等）中断下载时，下次下载可以继续上次下载的地方下载。

 

1、通过getContentLength可以获取要下载文件的大小，这样可以在本机上创建一个相同大小的文件用来下载。

int fileLength = connection.getContentLength();

2、由于是多线程，所以要给每一个线程均分分配要下载的位置。

for(int i = 0; i < threadCount; i ++) {
    int startThread = i * blockSize;
    int endThread = (i + 1) * blockSize - 1;
    if( i == blockSize - 1) endThread = fileLength -1;
    new DownloadThread(i, startThread, endThread).start();
                    
}

3、启动每个线程下载时，请求头需要Range参数，值是bytes:xxx-xxx某事。比如"Range:0-10100",代表要下载的位置是从0到10100。

connection.setRequestProperty("Range", "bytes:"+startThred+"-" + endThread);

4、然后每次用RandomAccessFile写入数据到本机文件里。

while((length = inputStream.read(buffer)) != -1) {
    randomAccessFile.write(buffer, 0, length);
}

 

5、当然每次下载时需要记录本线程下载了多少，以便断点时，下载的时候可以从下次下载的地方下载。

total += length;
int currentThreadPostion = startThred + total;
RandomAccessFile randomAccessFile2 = new RandomAccessFile(file, "rwd");
randomAccessFile2.write(String.valueOf(currentThreadPostion).getBytes());
randomAccessFile2.close();

 

继承Thread类的DownloadThread类代码：

 1 public static class DownloadThread extends Thread {
 2         private int threadId;
 3         private int endThread;
 4         private int startThred;
 5         public DownloadThread(int threadId, int startThred, int endThread) {
 6             this.threadId = threadId;
 7             this.startThred = startThred;
 8             this.endThread = endThread;
 9         }
10         public void run() {
11             //分段请求网络连接，分段保存在本地
12             synchronized (DownloadThread.class) {
13                 currentRunThreadCount += 1;
14             }
15             try {
16                 System.err.println("理论线程:"+threadId+",开始位置:"+startThred+",结束位置:"+endThread);
17                 URL url = new URL(path);
18                 HttpURLConnection connection = (HttpURLConnection) url.openConnection();
19                 connection.setRequestMethod("GET");
20                 connection.setConnectTimeout(10 * 1000);
21                 File file = new File(threadId+".txt");
22                 if(file.exists()) {    //是否断点
23                     BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(new FileInputStream(file)));
24                     String lastPostion_str = bufferedReader.readLine();
25                     startThred = Integer.parseInt(lastPostion_str);
26                     bufferedReader.close();
27                 }
28                 //设置分段下载的头信息  Range:做分段
29                 connection.setRequestProperty("Range", "bytes:"+startThred+"-" + endThread);
30                 int code = connection.getResponseCode();
31                 System.out.println(code);
32                 if(code == 206) {    //200:请求全部资源成功  206:代表部分资源请求成功
33                     InputStream inputStream = connection.getInputStream();
34                     System.out.println(getFileName(path));
35                     RandomAccessFile randomAccessFile = new RandomAccessFile(new File(getFileName(path)), "rw");
36                     randomAccessFile.seek(startThred);
37                     byte[] buffer = new byte[1024*10];
38                     int length = -1;
39                     int total = 0;//记录下载的总量
40                     System.err.println("实际线程:"+threadId+",开始位置:"+startThred+",结束位置:"+endThread);
41                     while((length = inputStream.read(buffer)) != -1) {
42                         randomAccessFile.write(buffer, 0, length);
43                         total += length;
44                         int currentThreadPostion = startThred + total;
45                         RandomAccessFile randomAccessFile2 = new RandomAccessFile(file, "rwd");
46                         randomAccessFile2.write(String.valueOf(currentThreadPostion).getBytes());
47                         randomAccessFile2.close();
48                     }
49                     randomAccessFile.close();
50                     inputStream.close();
51                     System.err.println("线程:"+threadId+"下载完毕");
52                     synchronized (DownloadThread.class) {
53                         currentRunThreadCount -= 1;
54                         if(currentRunThreadCount == 0){
55                             for(int i = 0; i < threadCount; i ++) {
56                                 File file2 = new File(i+".txt");
57                                 file2.delete();
58                             }
59                         }
60                     }
61                 }
62                 
63             } catch (Exception e) {
64                 e.printStackTrace();
65             }
66             
67              
68             super.run();
69         }
70     }

View Code

完整代码：

  1 package cn.starry.muchThreadDown;
  2 
  3 import java.io.BufferedReader;
  4 import java.io.File;
  5 import java.io.FileInputStream;
  6 import java.io.InputStream;
  7 import java.io.InputStreamReader;
  8 import java.io.RandomAccessFile;
  9 import java.net.HttpURLConnection;
 10 import java.net.MalformedURLException;
 11 import java.net.URL;
 12 import java.net.URLConnection;
 13 
 14 import javax.swing.text.StyledEditorKit.BoldAction;
 15 
 16 public class MuchThreadDown {
 17 
 18     /**
 19      * @param args
 20      */
 21     private static int threadCount = 3;
 22     private static int blockSize;
 23     private static String path = "http://192.168.0.112:8080/itheima74/feiq.exe";
 24     private static int currentRunThreadCount = 0;
 25     public static void main(String[] args) {
 26         
 27         try {
 28             URL url = new URL(path);
 29             HttpURLConnection connection = (HttpURLConnection) url.openConnection();
 30             connection.setRequestMethod("GET");
 31             connection.setConnectTimeout(10 * 1000);
 32             int code = connection.getResponseCode();
 33             if(code == 200) {
 34                 int fileLength = connection.getContentLength();
 35                 RandomAccessFile randomAccessFile = new RandomAccessFile(new File(getFileName(path)), "rw");
 36                 randomAccessFile.setLength(fileLength);
 37                 blockSize = fileLength / threadCount;
 38                 for(int i = 0; i < threadCount; i ++) {
 39                     int startThread = i * blockSize;
 40                     int endThread = (i + 1) * blockSize - 1;
 41                     if( i == blockSize - 1) endThread = fileLength -1;
 42                     new DownloadThread(i, startThread, endThread).start();
 43                     
 44                 }
 45             }
 46         } catch (Exception e) {
 47             e.printStackTrace();
 48         }
 49         
 50     }
 51     
 52     
 53     public static class DownloadThread extends Thread {
 54         private int threadId;
 55         private int endThread;
 56         private int startThred;
 57         public DownloadThread(int threadId, int startThred, int endThread) {
 58             this.threadId = threadId;
 59             this.startThred = startThred;
 60             this.endThread = endThread;
 61         }
 62         public void run() {    
 63             synchronized (DownloadThread.class) {
 64                 currentRunThreadCount += 1;
 65             }
 66             //分段请求网络连接，分段保存在本地
 67             try {
 68                 System.err.println("理论线程:"+threadId+",开始位置:"+startThred+",结束位置:"+endThread);
 69                 URL url = new URL(path);
 70                 HttpURLConnection connection = (HttpURLConnection) url.openConnection();
 71                 connection.setRequestMethod("GET");
 72                 connection.setConnectTimeout(10 * 1000);
 73                 File file = new File(threadId+".txt");
 74                 if(file.exists()) {    //是否断点
 75                     BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(new FileInputStream(file)));
 76                     String lastPostion_str = bufferedReader.readLine();
 77                     startThred = Integer.parseInt(lastPostion_str);
 78                     bufferedReader.close();
 79                 }
 80                 //设置分段下载的头信息  Range:做分段
 81                 connection.setRequestProperty("Range", "bytes:"+startThred+"-" + endThread);
 82                 int code = connection.getResponseCode();
 83                 System.out.println(code);
 84                 if(code == 206) {    //200:请求全部资源成功  206:代表部分资源请求成功
 85                     InputStream inputStream = connection.getInputStream();
 86                     System.out.println(getFileName(path));
 87                     RandomAccessFile randomAccessFile = new RandomAccessFile(new File(getFileName(path)), "rw");
 88                     randomAccessFile.seek(startThred);
 89                     byte[] buffer = new byte[1024*10];
 90                     int length = -1;
 91                     int total = 0;//记录下载的总量
 92                     System.err.println("实际线程:"+threadId+",开始位置:"+startThred+",结束位置:"+endThread);
 93                     while((length = inputStream.read(buffer)) != -1) {
 94                         randomAccessFile.write(buffer, 0, length);
 95                         total += length;
 96                         int currentThreadPostion = startThred + total;
 97                         RandomAccessFile randomAccessFile2 = new RandomAccessFile(file, "rwd");
 98                         randomAccessFile2.write(String.valueOf(currentThreadPostion).getBytes());
 99                         randomAccessFile2.close();
100                     }
101                     randomAccessFile.close();
102                     inputStream.close();
103                     System.err.println("线程:"+threadId+"下载完毕");
104                     synchronized (DownloadThread.class) {
105                         currentRunThreadCount -= 1;
106                         if(currentRunThreadCount == 0){
107                             for(int i = 0; i < threadCount; i ++) {
108                                 File file2 = new File(i+".txt");
109                                 file2.delete();
110                             }
111                         }
112                     }
113                 }
114                 
115             } catch (Exception e) {
116                 e.printStackTrace();
117             }
118             
119              
120             super.run();
121         }
122     }
123     
124     public static String getFileName(String path) {
125         return path.substring(path.lastIndexOf("/")+1);
126     }
127 
128 }

View Code