File Download Manager System in Java

package com.downloader;

import java.util.concurrent.Callable;

public class DownloadTask implements Callable<String> {
    private String fileUrl;
    private String destination;

    public DownloadTask(String fileUrl, String destination) {
        this.fileUrl = fileUrl;
        this.destination = destination;
    }

    @Override
    public String call() throws Exception {
        System.out.println("Starting download from: " + fileUrl);
        try {
            Thread.sleep(3000); // Simulating download time
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            return "Download interrupted for " + fileUrl;
        }
        System.out.println("Download completed for: " + fileUrl);
        return "Download successful for " + fileUrl;
    }
}

package com.downloader;

import java.io.*;
import java.net.*;
import java.nio.file.*;

public class DownloadUtils {

    /**
     * Validates the given URL to check if it's a valid and accessible URL.
     *
     * @param urlString The URL to validate.
     * @return true if the URL is valid, false otherwise.
     */
    public static boolean isValidUrl(String urlString) {
        try {
            URL url = new URL(urlString);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("HEAD"); // Only check the headers (no body)
            connection.setConnectTimeout(5000); // Set timeout to 5 seconds
            connection.setReadTimeout(5000);
            int responseCode = connection.getResponseCode();
            return (responseCode == HttpURLConnection.HTTP_OK);
        } catch (MalformedURLException e) {
            System.err.println("Invalid URL: " + urlString);
            return false;
        } catch (IOException e) {
            System.err.println("Error checking URL: " + e.getMessage());
            return false;
        }
    }

    /**
     * Downloads a file from a URL and saves it to the specified destination.
     *
     * @param fileUrl      The URL of the file to download.
     * @param destination  The file path to save the downloaded file.
     * @throws IOException if an I/O error occurs during downloading.
     */
    public static void downloadFile(String fileUrl, String destination) throws IOException {
        URL url = new URL(fileUrl);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");
        connection.setConnectTimeout(5000); // Set timeout to 5 seconds
        connection.setReadTimeout(5000);

        try (InputStream in = connection.getInputStream();
             FileOutputStream out = new FileOutputStream(destination)) {

            byte[] buffer = new byte[4096];
            int bytesRead;
            while ((bytesRead = in.read(buffer)) != -1) {
                out.write(buffer, 0, bytesRead);
            }
        }

        System.out.println("File downloaded to: " + destination);
    }

    /**
     * Verifies if the destination directory exists. If it doesn't, it creates the directory.
     *
     * @param directoryPath The path to the directory.
     * @return true if the directory exists or was successfully created, false otherwise.
     */
    public static boolean verifyDirectory(String directoryPath) {
        Path path = Paths.get(directoryPath);
        if (!Files.exists(path)) {
            try {
                Files.createDirectories(path);
                System.out.println("Directory created: " + directoryPath);
                return true;
            } catch (IOException e) {
                System.err.println("Failed to create directory: " + e.getMessage());
                return false;
            }
        }
        return true;
    }
}

package com.downloader;

import java.util.concurrent.*;
import java.util.*;

public class DownloadManager {

    private static final int NUM_THREADS = 4;
    private ExecutorService executorService;

    public DownloadManager() {
        executorService = Executors.newFixedThreadPool(NUM_THREADS);
    }

    public void startDownload(String[] fileUrls, String destination) {
        List<Callable<String>> tasks = new ArrayList<>();
        for (String fileUrl : fileUrls) {
            tasks.add(new DownloadTask(fileUrl, destination));
        }

        try {
            List<Future<String>> results = executorService.invokeAll(tasks);
            for (Future<String> result : results) {
                try {
                    System.out.println(result.get());
                } catch (ExecutionException | InterruptedException e) {
                    e.printStackTrace();
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            executorService.shutdown();
        }
    }

    public static void main(String[] args) {
        String[] fileUrls = {
                "http://example.com/file1.zip",
                "http://example.com/file2.zip",
                "http://example.com/file3.zip",
                "http://example.com/file4.zip"
        };

        String destination = "/path/to/save/files";

        DownloadManager downloadManager = new DownloadManager();
        downloadManager.startDownload(fileUrls, destination);
    }
}