原文链接：https://www.dubby.cn/detail.html?id=9090

1.基本原理

先使用head方法查询得到对应文件的Content-Length，然后拆分成多个部分，交由多个线程去处理，使用"Range", "bytes=" + start + "-" + end这个header来指定下载文件的哪个部分。

2.代码实现

为了方便展示，我是用了一个类来实现，其余都是内部类

/**
 * Created by yangzheng03 on 2018/1/16.
 * https://www.dubby.cn/
 */
public class DownloadTool {
    private static String prefix = String.valueOf(System.currentTimeMillis());
    private static String path = "";
    public static void main(String[] args) {
        long startTimestamp = System.currentTimeMillis();
        if (args == null || args.length < 1) {
            System.out.println("please input the file url.");
            return;
        }
        final String urlString = args[0];
        if (args.length >= 2) {
            path = args[1];
        }
        int number = 10;
        if (args.length >= 3) {
            number = Integer.parseInt(args[2]);
        }
        System.out.println("Download started, url is \"" + urlString + "\"");
        ExecutorService threadPool = Executors.newFixedThreadPool(number);
        try {
            URL url = new URL(urlString);
            HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
            httpURLConnection.setRequestMethod("HEAD");
            String contentLengthStr = httpURLConnection.getHeaderField("Content-Length");
            long contentLength = Long.parseLong(contentLengthStr);
            if (contentLength > 1024 * 1024) {
                System.out.println("Content-Length\t" + (contentLength / 1024 / 1024) + "MB");
            } else if (contentLength > 1024) {
                System.out.println("Content-Length\t" + (contentLength / 1024) + "KB");
            } else {
                System.out.println("Content-Length\t" + (contentLength) + "B");
            }
            long tempLength = contentLength / number;
            long start = 0, end = -1;
            Map<Integer, Future<DownloadTemp>> futureMap = new HashMap<>(number);
            for (int i = 0; i < number; ++i) {
                start = end + 1;
                end = end + tempLength;
                if (i == number - 1) {
                    end = contentLength;
                }
                System.out.println("start:\t" + start + "\tend:\t" + end);
                DownloadThread thread = new DownloadThread(i, start, end, urlString);
                futureMap.put(i, threadPool.submit(thread));
            }
            System.out.println();
            String filename = urlString.substring(urlString.lastIndexOf("/") + 1);
            if (!path.equals("")) {
                filename = path + "/" + filename;
            }
            RandomAccessFile resultFile = new RandomAccessFile(filename, "rw");
            for (int i = 0; i < number; ++i) {
                Future<DownloadTemp> future = futureMap.get(i);
                DownloadTemp temp = future.get();
                RandomAccessFile tempFile = new RandomAccessFile(temp.filename, "r");
                tempFile.getChannel().transferTo(0, tempFile.length(), resultFile.getChannel());
            }
            threadPool.shutdown();
        } catch (Exception e) {
            e.printStackTrace();
        }
        long completedTimestamp = System.currentTimeMillis();
        System.out.println();
        System.out.println("cost " + (completedTimestamp - startTimestamp) / 1000 + "s");
    }
    private static class DownloadThread implements Callable<DownloadTemp> {
        private int index;
        private String filename;
        private long start, end;
        private String urlString;
        DownloadThread(int index, long start, long end, String url) {
            this.urlString = url;
            this.index = index;
            this.start = start;
            this.end = end;
            if (path.equals("")) {
                this.filename = prefix + "-temp-" + index;
            } else {
                this.filename = path + "/" + prefix + "-temp-" + index;
            }
        }
        @Override
        public DownloadTemp call() throws Exception {
            URL url = new URL(urlString);
            HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
            httpURLConnection.setRequestMethod("GET");
            httpURLConnection.setRequestProperty("Range", "bytes=" + start + "-" + end);
            InputStream inputStream = null;
            OutputStream outputStream = null;
            try {
                inputStream = httpURLConnection.getInputStream();
                File file = new File(filename);
                outputStream = new FileOutputStream(file);
                while (true) {
                    byte[] bytes = new byte[10240];
                    int length = inputStream.read(bytes);
                    if (length <= 0) {
                        break;
                    }
                    outputStream.write(bytes, 0, length);
                }
                outputStream.flush();
            } finally {
                if (inputStream != null) {
                    inputStream.close();
                }
                if (outputStream != null) {
                    outputStream.close();
                }
            }
            DownloadTemp downloadTemp = new DownloadTemp();
            downloadTemp.index = index;
            downloadTemp.filename = filename;
            System.out.println("thread\t" + index + "\tcompleted.");
            return downloadTemp;
        }
    }
    private static class DownloadTemp {
        private int index;
        private String filename;
    }
}

3.使用Jar包

下载工具 download-1.0.jar

下载下来后，改名为download-1.0.jar，执行：

java -jar download-1.0.jar https://blog.dubby.cn/upload/2018-01-13/68b780a2-1a6c-49dc-914e-0469cb969471.zip  /Users/test/Desktop/download 15

其中有三个参数:

下载的URL，必填
下载到目标目录，选填，如果不指定，下载到当前目前下
开启的线程数，选填，默认是10个线程

测试结果:

➜  target java -jar download-1.0.jar https://blog.dubby.cn/upload/2018-01-13/68b780a2-1a6c-49dc-914e-0469cb969471.zip  /Users/test/Desktop/download 15 
Download started, url is "https://blog.dubby.cn/upload/2018-01-13/68b780a2-1a6c-49dc-914e-0469cb969471.zip"
Content-Length  1MB
start:  0       end:    83780
start:  83781   end:    167561
start:  167562  end:    251342
start:  251343  end:    335123
start:  335124  end:    418904
start:  418905  end:    502685
start:  502686  end:    586466
start:  586467  end:    670247
start:  670248  end:    754028
start:  754029  end:    837809
start:  837810  end:    921590
start:  921591  end:    1005371
start:  1005372 end:    1089152
start:  1089153 end:    1172933
start:  1172934 end:    1256726

thread  0       completed.
thread  10      completed.
thread  1       completed.
thread  9       completed.
thread  14      completed.
thread  11      completed.
thread  7       completed.
thread  6       completed.
thread  2       completed.
thread  4       completed.
thread  3       completed.
thread  8       completed.
thread  12      completed.
thread  13      completed.
thread  5       completed.

cost 9s