text: //express_demo.js 文件
  var express = require('express');
  var app = express();
   
  app.get('/', function (req, res) {
     res.send('Hello World');
  })
   
  var server = app.listen(8081, function () {
   
    var host = server.address().address
    var port = server.address().port
   
    console.log("应用实例，访问地址为 http://%s:%s", host, port)
   
  })