java 文件下载添加header属性Content-Length

在Java中下载文件并添加HTTP header属性`Content-Length`通常涉及到使用HttpURLConnection、HttpClient或者其他支持HTTP请求的库。这里是一个基本的例子，使用`HttpURLConnection`：

import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public void downloadFileWithLength(String urlString, String destinationPath) throws Exception {
    URL url = new URL(urlString);
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();

    // 添加Content-Length头信息
    long fileSize = getFileSize(url); // 获取文件大小
    connection.setRequestProperty("Content-Length", Long.toString(fileSize));

    // 设置GET请求
    connection.setRequestMethod("GET");
    
    // 打开连接并获取响应码
    int responseCode = connection.getResponseCode();
    if (responseCode == HttpURLConnection.HTTP_OK) {
        InputStream inputStream = connection.getInputStream();
        
        // 创建目标文件并写入数据
        OutputStream outputStream = new FileOutputStream(destinationPath);
        byte[] buffer = new byte[4096];
        int bytesRead;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.close();
        inputStream.close();
        System.out.println("Downloaded file to " + destinationPath);
    } else {
        throw new RuntimeException("Failed to download file with status code: " + responseCode);
    }

    connection.disconnect();
}

private long getFileSize(URL url) throws Exception {
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    connection.setRequestMethod("HEAD"); // 使用HEAD请求只获取头部信息，不会下载文件内容
    long fileSize = connection.getContentLengthLong(); // 获取头信息中的Content-Length字段
    connection.disconnect();
    return fileSize;
}

在这个例子中，首先我们打开一个到URL的连接，并设置`Content-Length`头，然后发送一个GET请求来下载文件。如果服务器返回状态码为200（表示成功），我们就读取输入流并将其写入到指定的本地文件。