用java代码写一个多线程下载文件接口

好的，以下是一个简单的多线程下载文件接口的 Java 代码：

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

public class MultiThreadedDownloader {
    private static final int BUFFER_SIZE = 4096;
    private static final int NUM_THREADS = 4;

    public static void download(String fileUrl, String saveDir) throws IOException {
        URL url = new URL(fileUrl);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        int responseCode = conn.getResponseCode();
        if (responseCode != HttpURLConnection.HTTP_OK) {
            throw new IOException("Server returned HTTP response code: " + responseCode);
        }

        String fileName = getFileName(conn);
        String filePath = saveDir + File.separator + fileName;

        ExecutorService executor = Executors.newFixedThreadPool(NUM_THREADS);

        int fileSize = conn.getContentLength();
        int numParts = fileSize / BUFFER_SIZE + 1;

        for (int i = 0; i < numParts; i++) {
            int start = i * BUFFER_SIZE;
            int end = Math.min((i + 1) * BUFFER_SIZE, fileSize);
            executor.execute(new DownloadTask(url, filePath, start, end));
        }

        executor.shutdown();
        while (!executor.isTerminated()) {
        }

        System.out.println("File downloaded to: " + filePath);
    }

    private static String getFileName(HttpURLConnection conn) {
        String fileName = "";
        String disposition = conn.getHeaderField("Content-Disposition");
        String contentType = conn.getContentType();
        int contentLength = conn.getContentLength();

        if (disposition != null) {
            int index = disposition.indexOf("filename=");
            if (index > 0) {
                fileName = disposition.substring(index + 10, disposition.length() - 1);
            }
        } else {
            fileName = conn.getURL().getFile();
            fileName = fileName.substring(fileName.lastIndexOf("/") + 1);
        }

        return fileName;
    }

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

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

        @Override
        public void run() {
            try {
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
                InputStream inputStream = conn.getInputStream();
                RandomAccessFile file = new RandomAccessFile(filePath, "rw");
                file.seek(start);

                byte[] buffer = new byte[BUFFER_SIZE];
                int bytesRead;
                while ((bytesRead = inputStream.read(buffer)) != -1) {
                    file.write(buffer, 0, bytesRead);
                }

                file.close();
                inputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

你可以调用 `MultiThreadedDownloader.download(fileUrl, saveDir)` 方法来下载文件，其中 `fileUrl` 是文件的 URL，`saveDir` 是保存文件的目录。这个方法会启动多个线程来下载文件，提高下载速度。