Multithreaded File Downloader in java

               Multiple small chunks of a large file are downloaded here. this program explores the concurrency, file input output operations and how it transferred.

Let us explore the steps.

Steps:

• First, initialize the URL.
• The content length is retrieved.
• Split the download task.
• By the use of Fixed Thread Pool, the parallel downloads are done.
•  Finally, the chunks are written.

Built-in Packages used for this process:

ExecutorService ,HttpURLConnection and RandomAccessFile.

Program implementation:

import java.io.*;

import java.net.HttpURLConnection;

import java.net.URL;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class MThreadedDownloader {

    private static final int T_COUNT = 3;

    private final String fURL;

    private final String opFile;

    public MThreadedDownloader(String fURL, String opFile) {

        this.fURL = fURL;

        this.opFile = opFile;

    }

    public void downloadIt() throws Exception {

        URL url1 = new URL(fURL);

 

        HttpURLConnection con1 = (HttpURLConnection) url1.openConnection();

        int cLength = con1.getContentLength();

        con1.disconnect();

        int chunk_Size = cLength / T_COUNT;

        ExecutorService exe1 = Executors.newFixedThreadPool(T_COUNT);

        for (int i = 0; i < T_COUNT; i++) {

            int start_Byte = i * chunk_Size;

            int end_Byte = (i == T_COUNT - 1) ? cLength - 1 : (start_Byte + chunk_Size - 1);

            exe1.execute(new DThread(fURL, opFile, start_Byte, end_Byte, i));

        }

        exe1.shutdown();

        while (!exe1.isTerminated()) {

            Thread.sleep(100);

        }

        System.out.println("Download complete.");

    }

    private static class DThread implements Runnable {

        private final String fURL;

        private final String opFile;

        private final int start_Byte;

        private final int end_Byte;

        private final int thread_Id;

        public DThread(String fURL, String opFile, int start_Byte, int end_Byte, int thread_Id) {

            this.fURL = fURL;

            this.opFile = opFile;

            this.start_Byte = start_Byte;

            this.end_Byte = end_Byte;

            this.thread_Id = thread_Id;

        }

        @Override

        public void run() {

            try {

                HttpURLConnection con1 = (HttpURLConnection) new URL(fURL).openConnection();

                String bRange = "bytes=" + start_Byte + "-" + end_Byte;

                con1.setRequestProperty("Range", bRange);

                con1.connect();

                try (InputStream in1 = con1.getInputStream();

                     RandomAccessFile raf1 = new RandomAccessFile(opFile, "rw")) {

                    raf1.seek(start_Byte);

                    byte[] buffer1 = new byte[4096];

                    int bytes_Read;

                    while ((bytes_Read = in1.read(buffer1)) != -1) {

                        raf1.write(buffer1, 0, bytes_Read);

                    }

                }

                System.out.printf("Thread is %d completed downloading bytes %d to %d%n", thread_Id, start_Byte, end_Byte);

            } catch (IOException e) {

                System.err.printf("Thread is %d encountered an error: %s%n", thread_Id, e.getMessage());

            }

        }

    }

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

        String url1 = "C:\\raji\\blog\\keywords.zip";

        String output1 = "largefile.zip";

        new MThreadedDownloader(url1, output1).downloadIt();

    }

}

This is the sample implementation of Multithreaded File Downloader. Hope, this code is useful to you.