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) {

}
   }
 }
}

来源：https://www.cnblogs.com/myseries/p/12554084.html