java多线程下载文件原理解析

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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