Java multi-thread download implementation, java multi-thread implementation

Java multi-thread download implementation, java multi-thread implementation

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Package cn. me. test;

Import java. io. InputStream;

Import java. io. RandomAccessFile;

Import java.net. HttpURLConnection;

Import java.net. URL;

/**

* Multi-thread download

* 1: Use RandomAccessFile to write data anywhere.

* 2: calculate the volume of data downloaded in the first thread, which can be evenly allocated. If the average is not enough,

* The last thread processes a relatively small amount of data.

* 3: files of the same size must be prepared and obtained through the file header before downloading.

*/

Public class MultiThreadDownload {

Public static void main (String [] args) throws Exception {

// 1: declare the 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 METHOD

Con. setRequestMethod ("GET ");

// 5: Get the request header, that is, the object Length

Int length = con. getContentLength (); // obtain the length of the downloaded file to calculate the amount of data that 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 placeholder

File. setLength (length); // sets the file size.

 

File. close ();

// 8: defines the number of threads

Int size = 3;

// 9: calculate the number of bytes of data to be downloaded by each thread. It is best to divide the data exactly; otherwise, add 1.

Int block = length/size = 0? Length/size: length/size + 1; // calculate the data volume that each thread should download.

System. err. println ("Each thread should download:" + block );

// 10: run the three threads and calculate the start byte and end of the byte.

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 the file name

Private String fileName;

// Define where to download

Private int start;

// Define which byte to download

Private int end;

Private 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 download

HttpURLConnection con =

(HttpURLConnection) url. openConnection ();

Con. setRequestMethod ("GET ");

// 12: Set the multipart download request header.

Con. setRequestProperty ("Range", "bytes =" + start + "-" + end); // sets the file block read from the server.

 

// 13: start downloading. You need to determine the value 206.

If (con. getResponseCode () = 206) {// if the access is successful, status code 206 is returned.

InputStream in = con. getInputStream ();

// 14: declare random write object. Note that rwd refers to writing data to the file in real time without using the cache area.

RandomAccessFile out = new RandomAccessFile ("d:/a/" + fileName, "rwd ");

Out. seek (start); // set to write data from a certain position 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 () + "execution completed ");

} Catch (Exception e ){

Throw new RuntimeException (e );

}

}

}

}