如何解决java.net.ProtocolException:服务器重定向次数过多问题？

出现java.net.ProtocolException: Server redirected too many times (20)的核心原因是你的请求未通过USPS的认证流程，服务器反复将请求重定向到登录页面，最终触发重定向次数上限。USPS的InformedDelivery不支持直接用Basic Auth认证，你需要模拟浏览器的完整登录流程：

关键修复点 #

• 先发起GET请求获取登录页面的Cookie和必要认证参数（如CSRF令牌）
• 提交登录表单时使用POST，携带正确的表单数据（而非Basic Auth请求头）
• 确保Cookie在整个会话中持续传递

修改后的代码示例 #

import java.io.*;
import java.net.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class GetMailStatus {

    public static void main(String[] args) {
        CookieManager cookieManager = new CookieManager(null, CookiePolicy.ACCEPT_ALL);
        CookieHandler.setDefault(cookieManager);

        try {
            // 第一步：获取登录页面，提取Cookie和CSRF令牌
            URL loginPageUrl = new URL("https://informeddelivery.usps.com/login");
            HttpURLConnection loginPageConn = (HttpURLConnection) loginPageUrl.openConnection();
            loginPageConn.setRequestMethod("GET");
            loginPageConn.connect();

            // 读取页面内容，提取CSRF令牌（需根据实际页面结构调整正则）
            String loginPageContent = readInputStream(loginPageConn.getInputStream());
            Pattern csrfPattern = Pattern.compile("<input type=\"hidden\" name=\"csrfToken\" value=\"([^\"]+)\"");
            Matcher matcher = csrfPattern.matcher(loginPageContent);
            String csrfToken = "";
            if (matcher.find()) {
                csrfToken = matcher.group(1);
            }

            loginPageConn.disconnect();

            // 第二步：提交登录表单
            URL loginPostUrl = new URL("https://informeddelivery.usps.com/login/submit");
            HttpURLConnection loginPostConn = (HttpURLConnection) loginPostUrl.openConnection();
            loginPostConn.setRequestMethod("POST");
            loginPostConn.setDoOutput(true);
            loginPostConn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
            loginPostConn.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36");

            // 构造表单数据（替换为你的实际用户名和密码）
            String formData = "username=xxxx&password=yyyy&csrfToken=" + csrfToken + "&rememberMe=false";
            try (OutputStream os = loginPostConn.getOutputStream()) {
                byte[] input = formData.getBytes("utf-8");
                os.write(input, 0, input.length);
            }

            // 处理登录后的重定向，访问目标页面
            int responseCode = loginPostConn.getResponseCode();
            if (responseCode == HttpURLConnection.HTTP_MOVED_TEMP || responseCode == HttpURLConnection.HTTP_MOVED_PERM) {
                String redirectUrl = loginPostConn.getHeaderField("Location");
                URL targetUrl = new URL(redirectUrl);
                HttpURLConnection targetConn = (HttpURLConnection) targetUrl.openConnection();
                targetConn.setRequestMethod("GET");
                targetConn.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36");
                
                String targetContent = readInputStream(targetConn.getInputStream());
                System.out.println(targetContent);
                targetConn.disconnect();
            }

            loginPostConn.disconnect();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static String readInputStream(InputStream is) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(is));
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = br.readLine()) != null) {
            sb.append(line).append("\n");
        }
        br.close();
        return sb.toString();
    }
}

额外注意事项 #

• CSRF令牌适配：实际登录页面的CSRF令牌字段名可能不同，需用浏览器开发者工具查看表单结构，调整正则表达式
• HTTPS证书：若遇到证书信任问题，可临时添加信任所有证书的逻辑（生产环境不建议）
• 用户代理模拟：添加User-Agent请求头模拟浏览器行为，避免被服务器识别为爬虫
• 会话验证：登录后可通过检查响应内容或状态码，确认是否成功进入目标页面

内容的提问来源于stack exchange，提问作者Cardsfan