Java实现多线程断点下载

Java实现多线程断点下载功能可以用于大文件的下载，可以提高下载速度，增加用户体验。以下是实现的完整攻略：

1. 分析

• 当前文件大小：文件已下载的长度
• 总文件大小：文件在服务器上的长度
• 当前已下载部分的起点和终点
• 每条线程要下载的文件块大小

2. 算法流程

1. 获得URL连接对象，获取文件大小
2. 计算出每条线程要下载的大小
3. 检查下载目录是否存在，若不存在则创建
4. 创建多条线程，每条线程下载一段文件块
5. 等待所有线程下载完成后进行合并

3. 示例说明

示例一：多线程下载一个.txt文件

假设需要下载一个大小为500MB的txt文件，以下为示例代码：

public class MultiThreadDownloadDemo {
    private static final int THREAD_COUNT = 4;   //线程数
    private static final String URL_PATH = "http://example.com/file.txt";  //文件下载路径
    private static final String FILE_PATH = "D:/downloads/file.txt";  //文件保存路径
    private static final int BUFFER_SIZE = 1024*1024;   //缓冲区大小
    private static final long BLOCK_SIZE = 1024*1024*50; //每条线程下载文件块大小

    public static void main(String[] args) {
        try {
            URL url = new URL(URL_PATH);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            long totalSize = conn.getContentLength();
            conn.disconnect();
            File parent = new File(FILE_PATH).getParentFile();
            if (!parent.exists()) {
                parent.mkdirs();
            }
            RandomAccessFile raf = new RandomAccessFile(FILE_PATH, "rw");
            raf.setLength(totalSize);
            long blockSize = (totalSize % THREAD_COUNT == 0) ? (totalSize / THREAD_COUNT) :
                                  ((totalSize / THREAD_COUNT) + 1);
            List<DownloadThread> threads = new ArrayList<>();
            for (int i = 0; i < THREAD_COUNT; i++) {
                long start = i * blockSize;
                long end = (i+1) * blockSize - 1;
                if (end >= totalSize) {
                    end = totalSize - 1;
                }
                DownloadThread thread = new DownloadThread(i+1, URL_PATH, FILE_PATH,
                                                             start, end, BLOCK_SIZE, BUFFER_SIZE);
                threads.add(thread);
                thread.start();
            }
            for (DownloadThread thread : threads) {
                thread.join();
            }
            raf.close();
            System.out.println("下载完成");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static class DownloadThread extends Thread {
        private int index;  //线程编号
        private String urlPath;   //文件下载路径
        private String filePath;   //保存路径
        private long start;   //开始位置
        private long end;    //结束位置
        private long blockSize;   //每条线程下载文件块大小
        private int bufferSize;   //缓冲区大小

        public DownloadThread(int index, String urlPath, String filePath,
                                   long start, long end, long blockSize, int bufferSize) {
            this.index = index;
            this.urlPath = urlPath;
            this.filePath = filePath;
            this.start = start;
            this.end = end;
            this.blockSize = blockSize;
            this.bufferSize = bufferSize;
        }

        @Override
        public void run() {
            try {
                URL url = new URL(urlPath);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                conn.setRequestProperty("Connection", "Keep-Alive");
                conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
                InputStream is = conn.getInputStream();
                RandomAccessFile raf = new RandomAccessFile(filePath, "rw");
                raf.seek(start);
                byte[] buffer = new byte[bufferSize];
                int len;
                while ((len = is.read(buffer)) != -1) {
                    raf.write(buffer, 0, len);
                    synchronized (DownloadThread.class) {
                        start += len;
                        if (start > end) {
                            System.out.println("线程" + index + "下载完成");
                            return;
                        }
                    }
                }
                raf.close();
                is.close();
                conn.disconnect();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

示例二：多线程下载图片文件并显示

假设需要下载一个图片文件并在Swing程序中展示图片，以下为示例代码：

public class MultiThreadDownloadAndDisplayDemo extends JFrame {
    private static final int THREAD_COUNT = 4;  //线程数
    private static final String URL_PATH = "http://example.com/image.jpg";  //文件下载路径
    private static final String FILE_PATH = "D:/downloads/image.jpg";  //文件保存路径
    private static final int BUFFER_SIZE = 1024*1024;  //缓冲区大小
    private static final long BLOCK_SIZE = 1024*1024*50;  //每条线程下载文件块大小
    private static final int WIDTH = 800;  //窗口宽度
    private static final int HEIGHT = 600;  //窗口高度

    public MultiThreadDownloadAndDisplayDemo() {
        setLayout(new BorderLayout());
        setSize(WIDTH, HEIGHT);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        ImageIcon icon = downloadImage();
        JLabel label = new JLabel(icon);
        add(label);
        setLocationRelativeTo(null);
        setVisible(true);
    }

    private ImageIcon downloadImage() {
        ImageIcon icon = null;
        try {
            URL url = new URL(URL_PATH);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            long totalSize = conn.getContentLength();
            conn.disconnect();
            File parent = new File(FILE_PATH).getParentFile();
            if (!parent.exists()) {
                parent.mkdirs();
            }
            RandomAccessFile raf = new RandomAccessFile(FILE_PATH, "rw");
            raf.setLength(totalSize);
            long blockSize = (totalSize % THREAD_COUNT == 0) ? (totalSize / THREAD_COUNT) :
                                  ((totalSize / THREAD_COUNT) + 1);
            List<DownloadThread> threads = new ArrayList<>();
            for (int i = 0; i < THREAD_COUNT; i++) {
                long start = i * blockSize;
                long end = (i+1) * blockSize - 1;
                if (end >= totalSize) {
                    end = totalSize - 1;
                }
                DownloadThread thread = new DownloadThread(i+1, URL_PATH, FILE_PATH,
                                                             start, end, BLOCK_SIZE, BUFFER_SIZE);
                threads.add(thread);
                thread.start();
            }
            for (DownloadThread thread : threads) {
                thread.join();
            }
            raf.close();
            InputStream is = new FileInputStream(FILE_PATH);
            byte[] bytes = new byte[is.available()];
            is.read(bytes);
            is.close();
            icon = new ImageIcon(bytes);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return icon;
    }

    public static class DownloadThread extends Thread {
        private int index;  //线程编号
        private String urlPath;  //文件下载路径
        private String filePath;   //保存路径
        private long start;   //开始位置
        private long end;    //结束位置
        private long blockSize;   //每条线程下载文件块大小
        private int bufferSize;   //缓冲区大小

        public DownloadThread(int index, String urlPath, String filePath,
                                   long start, long end, long blockSize, int bufferSize) {
            this.index = index;
            this.urlPath = urlPath;
            this.filePath = filePath;
            this.start = start;
            this.end = end;
            this.blockSize = blockSize;
            this.bufferSize = bufferSize;
        }

        @Override
        public void run() {
            try {
                URL url = new URL(urlPath);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                conn.setRequestProperty("Connection", "Keep-Alive");
                conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
                InputStream is = conn.getInputStream();
                RandomAccessFile raf = new RandomAccessFile(filePath, "rw");
                raf.seek(start);
                byte[] buffer = new byte[bufferSize];
                int len;
                while ((len = is.read(buffer)) != -1) {
                    raf.write(buffer, 0, len);
                    synchronized (DownloadThread.class) {
                        start += len;
                        if (start > end) {
                            System.out.println("线程" + index + "下载完成");
                            return;
                        }
                    }
                }
                raf.close();
                is.close();
                conn.disconnect();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        new MultiThreadDownloadAndDisplayDemo();
    }
}

以上是多线程断点下载的完整攻略，其中示例一演示了在控制台中下载文件的过程，示例二演示了在Swing程序中下载并展示图片的过程。