后台怎么获取请求的域名

const express = require('express');
const app = express();

app.get('/', (req, res) => {
  const domain = req.headers.host;
  res.send(`Domain: ${domain}`);
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

from flask import Flask, request

app = Flask(__name__)

@app.route('/')
def index():
    domain = request.headers.get('Host')
    return f'Domain: {domain}'

if __name__ == '__main__':
    app.run(port=3000)

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class DomainController {

    @GetMapping("/")
    public String getDomain(HttpServletRequest request) {
        String domain = request.getHeader("Host");
        return "Domain: " + domain;
    }
}