java多线程断点续传下载

实现多线程断点续传下载需要以下步骤：

1. 获取文件大小和已下载的大小，计算出剩余未下载的文件大小。

2. 开启多个线程，每个线程负责下载文件的一部分内容。每个线程要知道自己需要下载的文件范围，即从哪里开始下载，下载多少字节。

3. 每个线程要在自己的范围内不断地读取数据，并写入文件。同时要记录已经下载的字节数。

4. 如果下载过程中出现错误，可以将已下载的字节数保存到本地文件中，下次启动程序时读取该文件，接着下载。

5. 下载完成后，合并所有下载的数据，得到完整的文件。

下面是一个示例代码：

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

public class MultiThreadDownloader {
    private static final String DOWNLOAD_DIR = "C:/download/";
    private static final int THREAD_COUNT = 4;

    public static void main(String[] args) {
        String url = "http://example.com/file.zip";
        String fileName = url.substring(url.lastIndexOf("/") + 1);

        try {
            URL fileUrl = new URL(url);
            HttpURLConnection conn = (HttpURLConnection) fileUrl.openConnection();

            int fileSize = conn.getContentLength();
            System.out.println("File size: " + fileSize);

            List<DownloadThread> threads = new ArrayList<>();
            int blockSize = fileSize / THREAD_COUNT;

            for (int i = 0; i < THREAD_COUNT; i++) {
                int start = i * blockSize;
                int end = (i + 1) * blockSize - 1;

                if (i == THREAD_COUNT - 1) {
                    end = fileSize - 1;
                }

                DownloadThread t = new DownloadThread(url, DOWNLOAD_DIR + fileName, start, end);
                t.start();
                threads.add(t);
            }

            for (DownloadThread t : threads) {
                t.join();
            }

            mergeFile(DOWNLOAD_DIR + fileName, THREAD_COUNT);

            System.out.println("Download complete");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static void mergeFile(String fileName, int threadCount) throws IOException {
        BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(fileName));

        for (int i = 0; i < threadCount; i++) {
            BufferedInputStream in = new BufferedInputStream(new FileInputStream(fileName + "." + i));
            byte[] buffer = new byte[1024];
            int len;

            while ((len = in.read(buffer)) != -1) {
                out.write(buffer, 0, len);
            }

            in.close();
            File file = new File(fileName + "." + i);
            file.delete();
        }

        out.close();
    }

    private static class DownloadThread extends Thread {
        private String url;
        private String fileName;
        private int start;
        private int end;

        public DownloadThread(String url, String fileName, int start, int end) {
            this.url = url;
            this.fileName = fileName;
            this.start = start;
            this.end = end;
        }

        @Override
        public void run() {
            try {
                File file = new File(fileName + "." + getId());
                RandomAccessFile raf = new RandomAccessFile(file, "rw");
                HttpURLConnection conn = (HttpURLConnection) new URL(url).openConnection();
                conn.setRequestProperty("Range", "bytes=" + start + "-" + end);

                BufferedInputStream in = new BufferedInputStream(conn.getInputStream());
                byte[] buffer = new byte[1024];
                int len, progress = 0;

                while ((len = in.read(buffer)) != -1) {
                    raf.write(buffer, 0, len);
                    progress += len;
                }

                raf.close();
                in.close();

                System.out.println("Thread " + getId() + " download complete (" + progress + " bytes)");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

在这个示例代码中，我们使用了4个线程来下载文件。每个线程下载的范围计算方式为：第i个线程下载的范围是[start, end]，其中start=i*blockSize，end=(i+1)*blockSize-1。最后一个线程的end是文件的总大小减1。每个线程下载完后，将下载的数据保存到本地文件中，文件名为原文件名加上线程ID。下载完成后，我们将所有下载的文件合并成一个完整的文件。