原理解析：利用RandomAccessFile在本地创建一个随机访问文件，文件大小和服务器要下载的文件大小相同。根据线程的数量（假设有三个线程），服务器的文件三等分，并把我们在本地创建的文件同样三等分，每个线程下载自己负责的部分，到相应的位置即可。

示例图：

示例demo

import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.URL;public class MutilDownload { private static String path = "http://192.168.80.85:8080/test.doc"; private static final int threadCount = 3; public static void main(String[] args) { try { URL url = new URL(path); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setRequestMethod("GET"); conn.setConnectTimeout(5000); int responseCode = conn.getResponseCode(); if (responseCode == 200) { int contentLength = conn.getContentLength(); System.out.println("length" + contentLength); RandomAccessFile rafAccessFile = new RandomAccessFile("test.doc", "rw"); rafAccessFile.setLength(contentLength); int blockSize = contentLength / threadCount; for (int i = 0; i < threadCount; i++) { int startIndex = i * blockSize; //每个现成下载的开始位置 int endIndex = (i + 1) * blockSize - 1;// 每个线程的结束位置 if (i == threadCount - 1) { //最后一个线程 endIndex = contentLength - 1; } new DownloadThread(startIndex, endIndex, i).start(); } } } catch (Exception e) { } } private static class DownloadThread extends Thread { private int startIndex; private int endIndex; private int threadId; public DownloadThread(int startIndex, int endIndex, int threadId) { this.startIndex = startIndex; this.endIndex = endIndex; this.threadId = threadId; } @Override public void run() { try { URL url = new URL(path); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setRequestMethod("GET"); conn.setConnectTimeout(5000); conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex); //固定写法，请求部分资源 int responseCode = conn.getResponseCode(); // 206表示请求部分资源 if (responseCode == 206) { RandomAccessFile rafAccessFile = new RandomAccessFile("test.doc", "rw"); rafAccessFile.seek(startIndex); InputStream is = conn.getInputStream(); int len = -1; byte[] buffer = new byte[1024]; while ((len = is.read(buffer)) != -1) { rafAccessFile.write(buffer, 0, len); } rafAccessFile.close(); System.out.println("线程" + threadId + "下载完成"); } } catch (Exception e) { } } }}

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