java 如何下载文件

使用 URL 和 HttpURLConnection 下载文件

通过 java.net.URL 和 HttpURLConnection 建立连接，读取输入流并写入本地文件。需要处理 HTTP 响应码和异常。

URL url = new URL("http://example.com/file.zip");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");

int responseCode = connection.getResponseCode();
if (responseCode == HttpURLConnection.HTTP_OK) {
    try (InputStream inputStream = connection.getInputStream();
         FileOutputStream outputStream = new FileOutputStream("local_file.zip")) {
        byte[] buffer = new byte[4096];
        int bytesRead;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
    }
} else {
    throw new IOException("Server returned non-OK status: " + responseCode);
}

使用 Apache HttpClient 下载文件

引入 Apache HttpClient 依赖（如 org.apache.httpcomponents:httpclient），通过 CloseableHttpClient 发送请求并处理响应流。

CloseableHttpClient client = HttpClients.createDefault();
HttpGet request = new HttpGet("http://example.com/file.zip");

try (CloseableHttpResponse response = client.execute(request);
     InputStream inputStream = response.getEntity().getContent();
     FileOutputStream outputStream = new FileOutputStream("local_file.zip")) {
    byte[] buffer = new byte[4096];
    int bytesRead;
    while ((bytesRead = inputStream.read(buffer)) != -1) {
        outputStream.write(buffer, 0, bytesRead);
    }
}

使用 Java NIO 的 Files.copy 方法

Java 7+ 的 Files.copy 方法可直接将输入流复制到目标路径，简化代码。

URL url = new URL("http://example.com/file.zip");
try (InputStream inputStream = url.openStream()) {
    Files.copy(inputStream, Paths.get("local_file.zip"), StandardCopyOption.REPLACE_EXISTING);
}

处理大文件下载的进度监控

通过自定义 InputStream 包装类或回调接口实现下载进度跟踪。

try (InputStream inputStream = connection.getInputStream();
     FileOutputStream outputStream = new FileOutputStream("local_file.zip")) {
    byte[] buffer = new byte[4096];
    int totalBytesRead = 0;
    int bytesRead;
    while ((bytesRead = inputStream.read(buffer)) != -1) {
        outputStream.write(buffer, 0, bytesRead);
        totalBytesRead += bytesRead;
        System.out.println("Downloaded: " + totalBytesRead + " bytes");
    }
}

异步下载文件

结合 CompletableFuture 或线程池实现异步下载，避免阻塞主线程。

ExecutorService executor = Executors.newFixedThreadPool(1);
Future<?> future = executor.submit(() -> {
    URL url = new URL("http://example.com/file.zip");
    try (InputStream in = url.openStream()) {
        Files.copy(in, Paths.get("local_file.zip"));
    }
});

// 可调用 future.get() 等待下载完成

注意事项

• 添加超时设置：通过 connection.setConnectTimeout(5000) 防止网络阻塞。
• 关闭资源：确保 InputStream、OutputStream 和连接在 try-with-resources 中自动关闭。
• 错误处理：检查 HTTP 状态码（如 404、503）并重试或终止。
• 文件名处理：从 Content-Disposition 头或 URL 路径解析原始文件名。

以上方法覆盖了从基础到进阶的文件下载需求，可根据项目依赖和复杂度选择合适方案。