java多线程下载一个大文件示例

在Java中，我们可以使用多线程来并行下载大文件，提高下载速度。这里提供一个简单的示例，我们将使用`java.util.concurrent.ExecutorService`和`Callable`接口。首先确保已导入必要的库：

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

public class MultiThreadedDownload {
    private static final int THREAD_POOL_SIZE = 4; // 设置线程池大小

    public static void main(String[] args) throws IOException, InterruptedException {
        String url = "http://example.com/largefile.zip"; // 替换为你想要下载的文件URL
        String outputFile = "largefile.zip"; // 输出文件名

        ExecutorService executor = Executors.newFixedThreadPool(THREAD_POOL_SIZE);
        DownloadTask downloadTask = new DownloadTask(url, outputFile);
        Future<?> future = executor.submit(downloadTask);

        future.get(); // 等待任务完成

        executor.shutdown(); // 关闭线程池
    }

    static class DownloadTask implements Callable<Void> {
        private final URL sourceUrl;
        private final File outputFile;

        public DownloadTask(String sourceUrlStr, String outputPath) {
            this.sourceUrl = new URL(sourceUrlStr);
            this.outputFile = new File(outputPath);
        }

        @Override
        public Void call() {
            try (BufferedInputStream in = new BufferedInputStream(sourceUrl.openStream());
                 FileOutputStream out = new FileOutputStream(outputFile)) {

                byte[] buffer = new byte[1024];
                int bytesRead;
                while ((bytesRead = in.read(buffer)) != -1) {
                    out.write(buffer, 0, bytesRead);
                }
                System.out.println("Thread " + Thread.currentThread().getName() + " downloaded file.");
            } catch (IOException e) {
                System.err.println("Error downloading file: " + e.getMessage());
            } finally {
                try {
                    in.close();
                    out.close();
                } catch (IOException e) {
                    System.err.println("Error closing streams: " + e.getMessage());
                }
            }
            return null;
        }
    }
}

在这个例子中，我们创建了一个固定大小的线程池，并为每个下载任务创建一个`DownloadTask`实例。每个任务负责从给定URL读取数据并写入到指定的本地文件。`call()`方法运行在一个单独的线程中。