Java多线程下载文件实现案例详解

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：

代码如下

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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