java实现多线程下载

Java实现多线程下载主要有以下步骤：

1. 创建一个URL对象，指定下载文件的URL地址。

2. 使用URLConnection类打开URL的连接，获取文件的大小。

3. 使用RandomAccessFile类创建一个指定大小的空文件，用于存储下载的文件内容。

4. 创建多个线程，每个线程负责下载文件的一部分内容。可以使用HttpURLConnection类，设置请求头Range，指定下载内容的范围。

5. 启动多个线程，进行下载。

6. 下载完成后，合并所有下载的文件内容，生成完整的文件。

下面是一个简单的Java多线程下载实现代码示例：

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;

public class MultiThreadDownloader {
    private int threadNum; // 线程数
    private String downloadUrl; // 下载链接
    private String savePath; // 保存路径
    private int fileSize; // 文件大小

    public MultiThreadDownloader(String downloadUrl, String savePath, int threadNum) {
        this.downloadUrl = downloadUrl;
        this.savePath = savePath;
        this.threadNum = threadNum;
    }

    public void download() throws Exception {
        URL url = new URL(downloadUrl);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setConnectTimeout(5000);
        conn.setReadTimeout(5000);
        fileSize = conn.getContentLength();

        RandomAccessFile raf = new RandomAccessFile(savePath, "rw");
        raf.setLength(fileSize);
        raf.close();

        int block = fileSize % threadNum == 0 ? fileSize / threadNum : fileSize / threadNum + 1;
        for (int i = 0; i < threadNum; i++) {
            int start = i * block;
            int end = (i + 1) * block - 1 < fileSize ? (i + 1) * block - 1 : fileSize - 1;
            new DownloadThread(downloadUrl, savePath, i, start, end).start();
        }
    }

    private class DownloadThread extends Thread {
        private String downloadUrl;
        private String savePath;
        private int threadId;
        private int start;
        private int end;

        public DownloadThread(String downloadUrl, String savePath, int threadId, int start, int end) {
            this.downloadUrl = downloadUrl;
            this.savePath = savePath;
            this.threadId = threadId;
            this.start = start;
            this.end = end;
        }

        public void run() {
            try {
                RandomAccessFile raf = new RandomAccessFile(savePath, "rw");
                raf.seek(start);

                URL url = new URL(downloadUrl);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                conn.setConnectTimeout(5000);
                conn.setReadTimeout(5000);
                conn.setRequestProperty("Range", "bytes=" + start + "-" + end);

                InputStream is = conn.getInputStream();
                byte[] buffer = new byte[1024];
                int len;
                while ((len = is.read(buffer)) != -1) {
                    raf.write(buffer, 0, len);
                }

                is.close();
                raf.close();
                System.out.println("Thread " + threadId + " download finished");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

使用示例：

MultiThreadDownloader downloader = new MultiThreadDownloader("http://example.com/file.zip", "C:/file.zip", 3);
downloader.download();

其中，第一个参数是下载链接，第二个参数是保存路径，第三个参数是线程数。