java中使用FileUtils.copyURLToFile下载文件时，如何判断文件下载完毕

可以通过判断下载后的文件大小是否和预期相等来判断文件是否下载完毕。代码示例如下：

URL url = new URL("http://example.com/file.zip");
File file = new File("file.zip");
long expectedFileSize = url.openConnection().getContentLengthLong();
FileUtils.copyURLToFile(url, file);
long actualFileSize = file.length();
if (actualFileSize == expectedFileSize) {
    System.out.println("文件下载完毕");
} else {
    System.out.println("文件下载未完成");
}

其中，`url.openConnection().getContentLengthLong()`可以获取到文件的预期大小，`file.length()`可以获取到下载后文件的实际大小。如果两者相等，则说明文件下载完毕。

相关问题

用FileUtils.copyURLToFile下载网图

好的，这个问题可以回答。你可以使用FileUtils类的copyURLToFile方法来下载网图。通过将图片的URL作为参数传递给该方法，可以将其下载到本地文件中。以下是示例代码：

URL imageUrl = new URL("https://example.com/image.jpg");
File destinationFile = new File("image.jpg");
FileUtils.copyURLToFile(imageUrl, destinationFile);

请确保你拥有网图的下载权限和合法使用权限。

多线程用FileUtils.copyURLToFile下载网图代码

可以实现多线程下载，提高下载速度。具体代码实现可以参考以下示例：

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class MultiThreadDownloader {
    private static final int THREAD_SIZE = 3; // 线程数
    private static final String URL_PATH = "https://example.com/image.jpg"; // 要下载的文件URL路径
    private static final String FILE_NAME = "image.jpg"; // 下载后保存的文件名
    private static final String FILE_PATH = "D:\\"; // 下载后保存的路径

    public static void main(String[] args) {
        ExecutorService threadPool = Executors.newFixedThreadPool(THREAD_SIZE);
        try {
            URL url = new URL(URL_PATH);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            conn.setConnectTimeout(5000);
            int len = conn.getContentLength();
            int block = len % THREAD_SIZE == 0 ? len / THREAD_SIZE : len / THREAD_SIZE + 1;

            for (int i = 0; i < THREAD_SIZE; i++) {
                int start = i * block;
                int end = (i + 1) * block - 1;
                if (i == THREAD_SIZE - 1) {
                    end = len - 1;
                }
                Runnable task = new DownloadTask(url, start, end, FILE_PATH + FILE_NAME);
                threadPool.execute(task);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            threadPool.shutdown();
        }
    }

    static class DownloadTask implements Runnable {
        private URL url;
        private int start;
        private int end;
        private String filePath;

        public DownloadTask(URL url, int start, int end, String filePath) {
            this.url = url;
            this.start = start;
            this.end = end;
            this.filePath = filePath;
        }

        @Override
        public void run() {
            try {
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                conn.setConnectTimeout(5000);
                conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
                InputStream is = conn.getInputStream();
                RandomAccessFile raf = new RandomAccessFile(new File(filePath), "rwd");
                raf.seek(start);
                byte[] buffer = new byte[1024];
                int len;
                while ((len = is.read(buffer)) != -1) {
                    raf.write(buffer, 0, len);
                }
                raf.close();
                is.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}