原理

先获取文件大小，然后分段分配任务给线程下载

在开始多线程下载前得先得知下载文件的大小，如果在之前的流程中并没有告知文件大小则可以使用HTTP请求方法 HEAD，这个请求方法类似于 GET 请求，只不过返回的响应中没有具体的内容，用于获取报头，在头部中可以找到字段content-length就是文件大小了。

得知文件长度后应分割需要下载的起止位置以便之后使用。
有具体位置后就可以启动多线程发送网络请求，在网络请求中使用HTTP协议的头部标志Range，这个标志的使用方法是Range:bytes=start-end。

实现

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;/*** Created with IntelliJ IDEA.** @Auther: zlf* @Date: 2021/09/14/1:01* @Description:*/
public class MutiThreadDownload {private int threadCount = 5; // 下载线程数private long blocksize; // 每线程下载区块大小private int runningThreadCount; // 正运行线程数public int getThreadCount() {return threadCount;}public void setThreadCount(int threadCount) {this.threadCount = threadCount;}public long getBlocksize() {return blocksize;}public void setBlocksize(long blocksize) {this.blocksize = blocksize;}public int getRunningThreadCount() {return runningThreadCount;}public void setRunningThreadCount(int runningThreadCount) {this.runningThreadCount = runningThreadCount;}/*** @Description: 多线程分段下载* @Param: [url, filePath]* @return: void* @Author: zlf* @Date: 2021/9/14*/public void download(String url, String filePath) throws IOException {HttpURLConnection conn = (HttpURLConnection) new URL(url).openConnection();int code = conn.getResponseCode();if (code == 200) {long size = conn.getContentLength();// 文件大小blocksize = size / threadCount;// 1.创建RandomAccessFile文件(可以随机访问)RandomAccessFile raf = new RandomAccessFile(new File(filePath), "rw");raf.setLength(size);// 2.多线程下载对应区块runningThreadCount = threadCount;for (int i = 1; i <= threadCount; i++) {long startIndex = (i - 1) * blocksize;long endIndex = i * blocksize - 1;if (i == threadCount) {endIndex = size - 1;}new DownloadThread(i, url, filePath,startIndex, endIndex).start();}}conn.disconnect();}/**** @Description: 分段下载的线程* @Param: * @return: * @Author: zlf* @Date: 2021/9/14*/private class DownloadThread extends Thread {private int id;private String url;private long startIndex;private long endIndex;private String filePath;public DownloadThread(int id, String url, String filePath, long startIndex, long endIndex) {this.id = id;this.url = url;this.startIndex = startIndex;this.filePath = filePath;this.endIndex = endIndex;}@Overridepublic void run() {try {HttpURLConnection conn = (HttpURLConnection) new URL(url).openConnection();conn.setRequestMethod("GET");// 1.读取文件断点位置curIndexint curIndex = 0;File indexFile = new File("indexFile_"+id);if (indexFile.exists() && indexFile.length() > 0) {FileInputStream fis = new FileInputStream(indexFile);BufferedReader br = new BufferedReader(new InputStreamReader(fis));curIndex = Integer.valueOf(br.readLine());startIndex += curIndex;fis.close();}// 2.从startIndex位置下载文件, 并从startIndex位置写入raf文件conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);InputStream is = conn.getInputStream();RandomAccessFile raf = new RandomAccessFile(new File(filePath), "rw");raf.seek(startIndex);int len = 0;byte[] buffer = new byte[1024*1024];while ((len = is.read(buffer)) != -1) {raf.write(buffer, 0, len);// 更新断点位置RandomAccessFile indexRaf = new RandomAccessFile(indexFile, "rwd");curIndex += len;indexRaf.write(String.valueOf(curIndex).getBytes());indexRaf.close();}is.close();raf.close();} catch (Exception e) {e.printStackTrace();} finally {// 3.所有线程下载完,删除断点位置文件indexFilesynchronized (MutiThreadDownload.class){if (--runningThreadCount == 0) {for (int i = 1; i <= threadCount; i++) new File("indexFile_"+i).delete();}}}}}public static void main(String[] args) throws IOException {MutiThreadDownload mutiThreadDownload = new MutiThreadDownload();mutiThreadDownload.setRunningThreadCount(5);mutiThreadDownload.download("http://img.netbian.com/file/2020/0904/de2f77ed1090735b441ba5e4c2b460ca.jpg","D:/a.jpg");}}