核心

先发送请求探测是否支持分片下载，同时从响应头中获取Content-Range文件总大小
然后分片，获取到每页的开始结束位置
提交分片任务到线程池中，await等待所有分片任务下载完成，进行合并任务

实现

基础bean：

@Data
public class SlicePageInfo {

    private CopyOnWriteArrayList<SliceInfo> sliceInfoList;

    private Long page;
}

@Data
@AllArgsConstructor
public class SliceInfo {
    private long start;
    private long end;
    private long page;
}

@Slf4j
public class DownLoadSliceUtil {

    /**
     * 文件分片大小,大文件可以调整
     */
    public final static long PER_PAGE = (long) 1024 * 1024;

    private static final RestTemplate REST_TEMPLATE = new RestTemplate();

    public static ResponseEntity<byte[]> getFileContentByUrlAndPosition(String downloadUrl, long start, long end) {
        HttpHeaders httpHeaders = new HttpHeaders();
        httpHeaders.set("Range", "bytes=" + start + "-" + end);

        org.springframework.http.HttpEntity<Object> httpEntity = new org.springframework.http.HttpEntity<>(httpHeaders);
        return REST_TEMPLATE.exchange(downloadUrl, HttpMethod.GET, httpEntity, byte[].class);
    }

    public static void download(String tempPath, String downloadUrl, SliceInfo sliceInfo, String fName) {
        log.info("下载分片文件：{}，分片序号 {}", fName, sliceInfo.getPage());

        // 创建一个分片文件对象
        File file = new File(tempPath, sliceInfo.getPage() + "-" + fName);

        if (file.exists() && file.length() == PER_PAGE) {
            log.info("此分片文件 {} 已存在", sliceInfo.getPage());
            return;
        }

        try (FileOutputStream fos = new FileOutputStream(file);) {
            ResponseEntity<byte[]> responseEntity = DownLoadSliceUtil.getFileContentByUrlAndPosition(downloadUrl, sliceInfo.getStart(), sliceInfo.getEnd());

            byte[] body = responseEntity.getBody();
            if (body != null && body.length == 0) {
                log.warn("分片文件：{},没有内容", file.getName());
                return;
            }
            // 将分片内容写入临时存储分片文件
            fos.write(body);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }


    public static void mergeFileTranTo(String tempPath, String fName, long page) {

        try (FileChannel channel = new FileOutputStream(new File(tempPath, fName)).getChannel()) {
            for (long i = 1; i <= page; i++) {
                File file = new File(tempPath, i + "-" + fName);
                FileChannel fileChannel = new FileInputStream(file).getChannel();
                long size = fileChannel.size();
                for (long left = size; left > 0; ) {
                    left -= fileChannel.transferTo((size - left), left, channel);
                }
                fileChannel.close();
                file.delete();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

@Slf4j
public class DownLoadEngine {
    private static final ExecutorService executorService = ExecutorFactory.newFixedExecutorService(5);


    /**
     * 分片下载
     *
     * @param downloadUrl 下载链接
     * @param tempPath    临时文件路径
     * @param fileName    文件名称
     */
    public static void downloadSlice(String downloadUrl, String tempPath, String fileName) {
        //大小探测
        ResponseEntity<byte[]> responseEntity = DownLoadSliceUtil.getFileContentByUrlAndPosition(downloadUrl, 0, 1);
        HttpHeaders headers = responseEntity.getHeaders();
        String rangeBytes = headers.getFirst("Content-Range");

        if (Objects.isNull(rangeBytes)) {
            log.error("url:{},不支持分片下载", downloadUrl);
            return;
        }

        long allBytes = Long.parseLong(rangeBytes.split("/")[1]);
        log.info("文件总大小：{}M", allBytes / 1024.0 / 1024.0);

        //分页
        SlicePageInfo slicePageInfo = splitPage(allBytes);

        CountDownLatch countDownLatch = new CountDownLatch(Math.toIntExact(slicePageInfo.getPage()));
        CountDownLatch mainLatch = new CountDownLatch(1);

        executorService.execute(() -> {
            try {
                countDownLatch.await();
                DownLoadSliceUtil.mergeFileTranTo(tempPath, fileName, slicePageInfo.getPage());
            } catch (InterruptedException e) {
                e.printStackTrace();    
            } finally {
                mainLatch.countDown();
            }
        });

        for (SliceInfo sliceInfo : slicePageInfo.getSliceInfoList()) {
            executorService.submit(() -> {
           try {
                    DownLoadSliceUtil.download(tempPath, downloadUrl, sliceInfo, fileName);
                } catch (Exception e) {
                    e.printStackTrace();
                } finally {
                    countDownLatch.countDown();
                }
            });
        }

        try {
            mainLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }


    }

    /**
     * 文件分片下载
     * @param allBytes 文件总大小
     * @return /
     */
    public static SlicePageInfo splitPage(long allBytes) {
        CopyOnWriteArrayList<SliceInfo> list = new CopyOnWriteArrayList<>();
        long size = allBytes;
        long left = 0;
        long page = 0;
        while (size > 0) {
            long start = 0;
            long end;
            start = left;
            //分页
            if (size < PER_PAGE) {
                end = left + size;
            } else {
                end = left += PER_PAGE;
            }
            size -= PER_PAGE;
            page++;
            if (start != 0) {
                start++;
            }
            log.info("页码：{}，开始位置：{}，结束位置：{}", page, start, end);
            final SliceInfo sliceInfo = new SliceInfo(start, end, page);
            list.add(sliceInfo);
        }
        SlicePageInfo slicePageInfo = new SlicePageInfo();
        slicePageInfo.setSliceInfoList(list);
        slicePageInfo.setPage(page);
        return slicePageInfo;
    }

}

测试

    @Test
    public void downloadFile() {
        DownLoadEngine.downloadSlice("https://dldir1.qq.com/qqfile/qq/PCQQ9.6.1/QQ9.6.1.28732.exe", "D:\temp", "qq.exe");
    }