Java HTTPURLConnection不跟随从HTTP重定向到HTTPS

Question

我不明白为什么Java HttpURLConnection不遵循从HTTP到HTTPS URL的HTTP重定向。我使用以下代码在https://httpstat.us/上获取页面：

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

public class Tester {

    public static void main(String argv[]) throws Exception{
        InputStream is = null;

        try {
            String httpUrl = "http://httpstat.us/301";
            URL resourceUrl = new URL(httpUrl);
            HttpURLConnection conn = (HttpURLConnection)resourceUrl.openConnection();
            conn.setConnectTimeout(15000);
            conn.setReadTimeout(15000);
            conn.connect();
            is = conn.getInputStream();
            System.out.println("Original URL: "+httpUrl);
            System.out.println("Connected to: "+conn.getURL());
            System.out.println("HTTP response code received: "+conn.getResponseCode());
            System.out.println("HTTP response message received: "+conn.getResponseMessage());
       } finally {
            if (is != null) is.close();
        }
    }
}

该程序的输出为：

Original URL: http://httpstat.us/301
Connected to: http://httpstat.us/301
HTTP response code received: 301
HTTP response message received: Moved Permanently

对http://httpstat.us/301的请求返回以下（缩短的）响应（这似乎绝对正确！）：

HTTP/1.1 301 Moved Permanently
Cache-Control: private
Content-Length: 21
Content-Type: text/plain; charset=utf-8
Location: https://httpstat.us

不幸的是，Java HttpURLConnection不遵循重定向！

Answer 1

仅当重定向使用相同的协议时，才遵循重定向。（请参见源代码中的followRedirect()方法。）无法禁用此检查。

即使我们知道它是HTTP的镜像，但从HTTP协议的角度来看，HTTPS还是其他完全不同的未知协议。未经用户批准而进行重定向是不安全的。

例如，假设将应用程序设置为自动执行客户端身份验证。用户期望匿名访问，因为他正在使用HTTP。但是，如果他的客户不经询问就遵循HTTPS，则他的身份将显示给服务器。