package cn.me.test;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
/**
* Multithreaded download
* 1 Use: RandomAccessFile Writes data to any location.
* 2 : the amount of data to be downloaded by the first thread needs to be calculated and can be distributed equally. If it's not equal,
* The last thread handles relatively little data directly
* 3 : files of the same size must be prepared before downloading
*/
public class MultiThreadDownload {
public static void main(String[] args) throws Exception {
//1: declare file name and download address
String fileName = "aa.rar";
String urlStr = "http://localhost:7777/day18";
//2: declare the Url
URL url = new URL(urlStr+"/"+fileName);
//3: get the connection
HttpURLConnection con =
(HttpURLConnection) url.openConnection();
//4: set the request mode
con.setRequestMethod("GET");
//5: gets the request header, that is, the length of the file
int length = con.getContentLength();//Gets the length of the downloaded file to calculate how much data each thread should download.
//6: create a file of the same size in the specified directory
RandomAccessFile file = new RandomAccessFile("d:/a/"+fileName, "rw");//Create a file of the same size.
//7: set the file size and hold the space
file.setLength(length);//Set the file size.
file.close();
//8: define the number of threads
int size = 3;
//9: calculate how many bytes of data each thread should download, preferably if it is exactly divisible, otherwise add 1
int block = length/size==0?length/size:length/size+1;//Calculates the amount of data that each thread should download.
System.err.println(" Each thread should download: "+block);
//10: run three threads and calculate which byte to start and which byte to end
for(int i=0;i<size;i++){
int start = i*block;
int end = start+(block-1);//Calculates the start and end bytes of each thread.
System.err.println(i+"="+start+","+end);
new MyDownThread(fileName, start, end,url).start();
}
}
static class MyDownThread extends Thread{
//Define file name
private String fileName;
//Define where to start downloading
private int start;
//Defines which bytes to download to
private int end;
private URL url;
public MyDownThread(String fileName,int start,int end,URL url){
this.fileName=fileName;
this.start=start;
this.end=end;
this.url=url;
}
@Override
public void run() {
try{
//11: start downloading
HttpURLConnection con =
(HttpURLConnection) url.openConnection();
con.setRequestMethod("GET");
//12: sets the request header for segmented downloads
con.setRequestProperty("Range","bytes="+start+"-"+end);//Sets the file blocks to be read from the server.
//13: start downloading, need to judge 206
if(con.getResponseCode()==206){//If the access is successful, the status code returned is 206.
InputStream in = con.getInputStream();
//14: declare a random write file object. Note that RWD means writing data to a file immediately without using a cache
RandomAccessFile out = new RandomAccessFile("d:/a/"+fileName,"rwd");
out.seek(start);//Set to start writing data from somewhere in the file.
byte[] b=new byte[1024];
int len = 0;
while((len=in.read(b))!=-1){
out.write(b,0,len);
}
out.close();
in.close();
}
System.err.println(this.getName()+" completes ");
}catch(Exception e){
throw new RuntimeException(e);
}
}
}
}