用Java实现一个多线程下载器

要用Java实现一个多线程下载器，需要以下步骤：

1. 获取要下载文件的大小。
2. 将文件分成多个块，每个线程负责下载一块。
3. 开启多个线程，每个线程负责下载它所分配的一块。
4. 每个线程在下载完成后，将它下载的数据写入文件中的对应位置。
5. 当所有线程都完成下载后，合并文件块，得到完整的文件。

代码实现：

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

public class MultiThreadDownloader {
    private static final int THREAD_COUNT = 4;

    public static void download(String url, String fileName) throws Exception {
        URL downloadURL = new URL(url);
        long contentLength = downloadURL.openConnection().getContentLengthLong();
        ExecutorService executor = Executors.newFixedThreadPool(THREAD_COUNT);
        CountDownLatch latch = new CountDownLatch(THREAD_COUNT);
        long blockSize = contentLength / THREAD_COUNT;
        for (int i = 0; i < THREAD_COUNT; i++) {
            long startIndex = i * blockSize;
            long endIndex = (i == THREAD_COUNT - 1) ? contentLength : startIndex + blockSize - 1;
            executor.submit(new DownloadTask(downloadURL, startIndex, endIndex, fileName, latch));
        }
        latch.await();
        executor.shutdown();
    }

    private static class DownloadTask implements Runnable {
        private URL downloadURL;
        private long startIndex;
        private long endIndex;
        private String fileName;
        private CountDownLatch latch;

        public DownloadTask(URL downloadURL, long startIndex, long endIndex, String fileName, CountDownLatch latch) {
            this.downloadURL = downloadURL;
            this.startIndex = startIndex;
            this.endIndex = endIndex;
            this.fileName = fileName;
            this.latch = latch;
        }

        @Override
        public void run() {
            try {
                RandomAccessFile file = new RandomAccessFile(fileName, "rw");
                file.seek(startIndex);
                InputStream inputStream = downloadURL.openConnection().