java多线程下载的实现示例-优快云博客

本文介绍了一种使用Java实现多线程下载大文件的方法，通过利用HttpURLConnection和RandomAccessFile，实现高效地分块下载，并且支持断点续传，确保即使在下载过程中发生中断也能继续从上次中断的位置继续下载。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

import java.io.File;
import java.io.FileNotFoundException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;


public class Main {

	/**
	 * @param args
	 * @throws Exception 
	 */
	public static void main(String[] args) throws Exception {
		new Main().download("http://gdown.baidu.com/data/wisegame/a3d96bf8e8a5102c/baiduliulanqi_25.apk", 5);
	}
	
	public void download(String downPath, int threadNum) throws Exception{
		URL url = new URL(downPath);
		HttpURLConnection conn = (HttpURLConnection) url.openConnection();
		conn.setConnectTimeout(5000);
		conn.setRequestMethod("GET");
		//InputStream in = conn.getInputStream();
		
		if(conn.getResponseCode() == 200){
			int size = conn.getContentLength();
			File file = new File(downPath.substring(downPath.lastIndexOf("/")+1));
			
			//"rwd" 会立刻把数据同步到设备上,防止手机没电数据丢失
			RandomAccessFile raf = new RandomAccessFile(file, "rwd");
			raf.setLength(size); //暂时还没数据
			raf.close();
			
			int block = size%threadNum == 0 ? size/threadNum : size/threadNum + 1;
			for(int i=0; i<threadNum; i++){
				new DownThread(i, block, url, file).start();
			}
			
		}else{
			System.out.println("下载失败");
		}
		
	}
	
	//下载线程类
	private class DownThread extends Thread{
		private int id;
		private int block; //每个线程下载块大小
		private URL url; //地址
		private File file; //下载保存在哪个文件
		
		public DownThread(int i, int block, URL url, File file) {
			this.id = i;
			this.block = block;
			this.url = url;
			this.file = file;
		}
		
		public void run(){
			int start = id * block;
			int end = (id+1) * block - 1;
			
			try {
				RandomAccessFile raf = new RandomAccessFile(file, "rwd");
				raf.seek(start);
				HttpURLConnection conn = (HttpURLConnection) url.openConnection();
				conn.setConnectTimeout(5000);
				conn.setRequestMethod("GET");
				
				//通过头字段，指定获取文件的位置 bytes="1-1000"
				conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
				
				//分段请求文件时，返回码是206
				if(conn.getResponseCode() == 206){
					byte[] buffer = new byte[1024];
					InputStream in = conn.getInputStream();
					int len = 0;
					while( (len=in.read(buffer)) != -1){
						raf.write(buffer, 0, len);
					}
					raf.close();
					in.close();

					System.out.println("线程:" + id + "下载完成");
				}else{
					System.out.println("失败");
				}
				
			} catch (Exception e) {
				e.printStackTrace();
			}
			System.out.println("");
		}
		
	}

}