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java如何实现下载

2026-03-03 11:01:01Java

使用 Java 实现下载功能

Java 提供了多种方式实现文件下载功能，以下是几种常见的方法：

使用 URL 和 URLConnection 下载文件

通过 java.net.URL 和 URLConnection 可以建立网络连接并下载文件。

import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.URL;
import java.net.URLConnection;

public class Downloader {
    public static void downloadFile(String fileURL, String saveDir) throws IOException {
        URL url = new URL(fileURL);
        URLConnection connection = url.openConnection();
        try (BufferedInputStream in = new BufferedInputStream(connection.getInputStream());
             FileOutputStream out = new FileOutputStream(saveDir)) {
            byte[] buffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = in.read(buffer)) != -1) {
                out.write(buffer, 0, bytesRead);
            }
        }
    }
}

使用 HttpClient（Java 11+）

Java 11 引入了新的 HttpClient，提供了更现代的 HTTP 请求方式。

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.nio.file.Path;
import java.nio.file.Paths;

public class HttpClientDownloader {
    public static void downloadFile(String fileURL, String savePath) throws Exception {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create(fileURL))
                .build();
        Path path = Paths.get(savePath);
        client.send(request, HttpResponse.BodyHandlers.ofFile(path));
    }
}

使用 Apache Commons IO

Apache Commons IO 库提供了更简洁的文件下载方式。

import org.apache.commons.io.FileUtils;
import java.net.URL;

public class CommonsIODownloader {
    public static void downloadFile(String fileURL, String savePath) throws Exception {
        URL url = new URL(fileURL);
        FileUtils.copyURLToFile(url, new java.io.File(savePath));
    }
}

多线程下载

对于大文件，可以使用多线程分段下载以提高速度。

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

public class MultiThreadDownloader {
    public static void download(String fileURL, String savePath, int threadCount) throws Exception {
        URL url = new URL(fileURL);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        long fileSize = connection.getContentLengthLong();

        try (RandomAccessFile file = new RandomAccessFile(savePath, "rw")) {
            file.setLength(fileSize);

            long partSize = fileSize / threadCount;
            for (int i = 0; i < threadCount; i++) {
                long start = i * partSize;
                long end = (i == threadCount - 1) ? fileSize - 1 : start + partSize - 1;
                new DownloadThread(url, file, start, end).start();
            }
        }
    }
}

class DownloadThread extends Thread {
    private final URL url;
    private final RandomAccessFile file;
    private final long start;
    private final long end;

    public DownloadThread(URL url, RandomAccessFile file, long start, long end) {
        this.url = url;
        this.file = file;
        this.start = start;
        this.end = end;
    }

    @Override
    public void run() {
        try {
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestProperty("Range", "bytes=" + start + "-" + end);

            try (var in = connection.getInputStream()) {
                file.seek(start);
                byte[] buffer = new byte[1024];
                int bytesRead;
                while ((bytesRead = in.read(buffer)) != -1) {
                    file.write(buffer, 0, bytesRead);
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}