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首先，我们要下载一个文件，可以通过多线程的方式快速下载！！！

多线程下载文件的步骤：

1、首先要知道请求下载的服务器支持断点下载，即支持request头信息中的Range的设置

2、然后通过对请求头设置

httpConnection.setRequestProperty("Range","bytes="+startIndex+"-"+endIndex);

3、然后获取整个文件的大小

4、在本地创建一个一样大的文件，然后根据线程数进行分配startIndex和endIndex

5、线程下载的同时，使用RandomAccessFile对所下载的内容随机对应写入文件

这里要注意，你所访问的文件在服务器端必须吧能够返回Content-Length这个参数才行！！！

完整的示例代码：

package cn.ljxwtl.plugin.common.util;

import com.alibaba.fastjson.JSON;
import cn.ljxwtl.plugin.engine.EngineAssistant;
import org.apache.commons.io.IOUtils;

import java.io.*;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
import java.util.RandomAccess;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ThreadPoolExecutor;

/**
 * @author: wtl
 * @License: (C) Copyright 2022, wtl Corporation Limited.
 * @Contact: 1050100468@qq.com
 * @Date: 2022/4/1 10:50
 * @Version: 1.0
 * @Description:
 */
public class DownloadFileUtil {

    public static void downloadFileWithMultiThread(String url,String filePath,Runnable runnable) throws Exception {
        List<long[]> perThreadDownloadFileOffset = getPerThreadDownloadFileOffset(url, 1024* 1024);
        ThreadPoolExecutor threadPoolExecutor = EngineAssistant.THREAD_POOL_EXECUTOR;

        int size = Objects.requireNonNull(perThreadDownloadFileOffset).size();

        CyclicBarrier cyclicBarrier = new CyclicBarrier(size, ()->{
            if (null != runnable){
                runnable.run();
            }
        });

        URL openUrl = new URL(url);

        for (int i = 0; i< size; i++){
            int finalI = i;
            new Thread(()->{
                try {
                    HttpURLConnection urlConnection = (HttpURLConnection) openUrl.openConnection();
                    urlConnection.setRequestProperty("Range","bytes="+perThreadDownloadFileOffset.get(finalI)[0]+"-"+perThreadDownloadFileOffset.get(finalI)[1]);
                    urlConnection.setDoInput(true);
                    BufferedInputStream bufferedInputStream = new BufferedInputStream(urlConnection.getInputStream());
                    int length = -1;
                    byte [] buffer = new byte[102400];
                    RandomAccessFile randomAccessFile = new RandomAccessFile(filePath,"rw");
                    System.out.println(perThreadDownloadFileOffset.get(finalI)[1] - perThreadDownloadFileOffset.get(finalI)[0]);
                    randomAccessFile.seek(perThreadDownloadFileOffset.get(finalI)[0]);
                    int perSize = 0;
                    while ((length = bufferedInputStream.read(buffer))!=-1){
                        randomAccessFile.write(buffer,0,length);
                        perSize +=length;
                    }
                    randomAccessFile.close();
                    System.out.println(perThreadDownloadFileOffset.get(finalI)[0] + "------------------" +(perThreadDownloadFileOffset.get(finalI)[0] +  perSize));
                } catch (Exception e) {
                    e.printStackTrace();
                }
                finally {
                    try {
                        cyclicBarrier.await();
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }).start();
        }

    }


    /**
     * 获取每个线程所需要的File的offset
     * @param url url地址
     * @param perThreadDealFileSize 每个线程处理的文件下载大小
     * @return List<int []>
     */
    private static List<long []> getPerThreadDownloadFileOffset(String url,long perThreadDealFileSize){
        URLConnection urlConnection = null;
        try {
            urlConnection = new URL(url).openConnection();
            long contentLengthLong = urlConnection.getContentLengthLong();
            List<long[]> threadOffsetDatas = new ArrayList<>();
            long size = (long) Math.ceil(contentLengthLong * 1.0 / perThreadDealFileSize);
            for (int i = 0; i < size; i++) {
                long start = i * perThreadDealFileSize;
                long end = (i + 1) * perThreadDealFileSize - 1;

                if (i == size - 1) {
                    start = i * perThreadDealFileSize;
                    end = contentLengthLong;
                }

                threadOffsetDatas.add(new long[]{start,end});
            }

            System.out.println(JSON.toJSONString(threadOffsetDatas));
            return threadOffsetDatas;
        }
        catch (Exception e){
            e.printStackTrace();
        }
        return null;
    }
}