Java – How to download a file from the Internet

参考链接：https://www.mkyong.com/java/java-how-to-download-a-file-from-the-internet/

1、Apache Commons IO

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.5</version>
</dependency>

 String fromFile = "ftp://ftp.arin.net/pub/stats/arin/delegated-arin-extended-latest";
        String toFile = "F:\\arin.txt";
        try {

            // connectionTimeout, readTimeout = 10 seconds
            FileUtils.copyURLToFile(new URL(fromFile), new File(toFile), 10000, 10000);

        } catch (IOException e) {
            e.printStackTrace();
        }

2、Java NIO

String fromFile = "https://download-cf.jetbrains.com/idea/ideaIU-2018.1.6.win.zip";
        String toFile = "F:\\IDEA\\ideaIU-2018.1.6.win.zip";       

        try {

            URL website = new URL(fromFile);
            ReadableByteChannel rbc = Channels.newChannel(website.openStream());
            FileOutputStream fos = new FileOutputStream(toFile);
            fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
            fos.close();
            rbc.close();
            System.out.println("---------success----------------");
        } catch (IOException e) {
            e.printStackTrace();
        }

    static final String FILE_URL  = "https://download-cf.jetbrains.com/idea/ideaIU-2018.1.6.exe";

    static final String FILE_NAME = "F:\\IDEA\\ideaIU-2018.1.6.exe";

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

        javaIO();

    }

    static void javaIO() {
        try (BufferedInputStream in = new BufferedInputStream(new URL(FILE_URL).openStream());
                FileOutputStream fileOutputStream = new FileOutputStream(FILE_NAME)) {
            byte dataBuffer[] = new byte[1024];
            int bytesRead;
            while ((bytesRead = in.read(dataBuffer, 0, 1024)) != -1) {
                System.out.println("read >" + bytesRead);
                fileOutputStream.write(dataBuffer, 0, bytesRead);
            }
        } catch (IOException e) {
            // handle exception
        }
    }

3、多线程下载文件

package net.liuzidong.download;

import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

/**
 * 多线程下载文件
 */
public class DownloadUtil {

    // 定义成员变量
    private String           path;             // 远程资源路径
    private String           targetPath; // 本地存储路径
    private DownFileThread[] threads;       // 线程list
    private int              threadNum;   // 线程数量
    private long             length;         // 下载的文件大小

    // 构造初始化
    public DownloadUtil(String path, String targetPath, int threadNum) {
        super();
        this.path = path;
        this.targetPath = targetPath;
        this.threads = new DownFileThread[threadNum];
        this.threadNum = threadNum;
    }

    // 多线程下载文件资源
    public void download() {
        URL url;
        try {
            url = new URL(path);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setReadTimeout(5 * 1000); // 设置超时时间为5秒
            conn.setRequestMethod("GET");
            conn.setRequestProperty("connection", "keep-alive");
            conn.setRequestProperty("accept", "*/*");

            // 获取远程文件的大小
            length = conn.getContentLength();
            conn.disconnect();

            // 设置本地文件大小
            RandomAccessFile targetFile = new RandomAccessFile(targetPath, "rw");
            targetFile.setLength(length);

            // 每个线程下载大小
            long avgPart = length / threadNum + 1;
            // 下载文件
            for (int i = 0; i < threadNum; i++) {
                long startPos = avgPart * i;
                RandomAccessFile targetTmp = new RandomAccessFile(targetPath, "rw");
                targetTmp.seek(startPos); // 分段下载
                threads[i] = new DownFileThread(startPos, targetTmp, avgPart);
                threads[i].start();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    // 监控下载进度
    public double getDownRate() {
        int currentSize = 0;
        for (int i = 0; i < threadNum; i++) {
            currentSize += threads[i].length;
        }
        return currentSize * 1.0 / length;
    }

    // 定义线程类
    class DownFileThread extends Thread {

        private long             startPos;
        private RandomAccessFile raf;
        private long             size;
        private long             length;

        public DownFileThread(long startPos, RandomAccessFile raf, long size) {
            super();
            this.startPos = startPos;
            this.raf = raf;
            this.size = size;
        }

        public void run() {
            URL url;
            try {
                url = new URL(path);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setReadTimeout(5 * 1000); // 设置超时时间为5秒
                conn.setRequestMethod("GET");
                conn.setRequestProperty("connection", "keep-alive");
                conn.setRequestProperty("accept", "*/*");

                InputStream in = conn.getInputStream();
                in.skip(this.startPos);
                byte[] buf = new byte[1024];
                int hasRead = 0;
                while (length < size && (hasRead = in.read(buf)) != -1) {
                    raf.write(buf, 0, hasRead);
                    length += hasRead;
                }
                raf.close();
                in.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    // 测试
    public static void main(String[] args) {
        String path = "http://ftp.jaist.ac.jp/pub/eclipse/technology/epp/downloads/release/photon/R/eclipse-jee-photon-R-win32-x86_64.zip";
        String targetPath = "F:\\download\\eclipse\\eclipse-jee-photon-R-win32-x86_64.zip";
        final DownloadUtil download = new DownloadUtil(path, targetPath, 3);
        download.download();
        System.out.println("开始下载...");
        // 主线程负责下载文件，在启动一个线程负责监控下载的进度
        new Thread(new Runnable() {

            @Override
            public void run() {
                while (download.getDownRate() < 1) {
                    try {
                        Thread.sleep(2000); // 200毫秒扫描一次
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("下载完成...");
            }

        }).start();
    }
}