我们输入www.baidu.com 来访问百度的主页，浏览器会自动转换为 http://www.baidu.com:80/(省略一些参数)。 http://代表我们同服务器连接使用的是http协议，www.baidu.com 代表的是服务器的主机地址，会被我们的pc通过DNS解析为IP地址。80是默认的应用层端口。/ 即为我们访问的服务器(www.baidu.com)的路径，服务器要对我们访问的这个路径做出响应，采取一定的动作。我们可以把这一过程看做一个路由。

An Express application is essentially a stack of middleware which are executed serially.(express应用其实就是由一系列顺序执行的Middleware组成。)
A middleware is a function with access to the request object (req), the response object (res), and the next middleware in line in the request-response cycle of an Express application. It is commonly denoted by a variable named next. Each middleware has the capacity to execute any code, make changes to the request and the reponse object, end the request-response cycle, and call the next middleware in the stack. Since middleware are execute serially, their order of inclusion is important.(中间件其实就是一个访问express应用串入的req，res，nex参数的函数，这个函数可以访问任何通过req，res传入的资源。)
If the current middleware is not ending the request-response cycle, it is important to call next() to pass on the control to the next middleware, else the request will be left hanging.(如果当前中间件没有完成对网页的res响应 ，还可以通过next把router 留给下一个middleware继续执行)
With an optional mount path, middleware can be loaded at the application level or at the router level. Also, a series of middleware functions can be loaded together, creating a sub-stack of middleware system at a mount point.

1 app.use(function (req, res, next) {
2   console.log('Time: %d', Date.now());
3   next();
4 })

1 app.get('/', function(req, res){
2   res.send('hello world');
3 });
4
5
6 app.post('/', function(req, res){
7   res.send('hello world');
8 });

 1 var express = require('express');
 2 var app = express();
 3 var router = express.Router();
 4
 5 // simple logger for this router's requests
 6 // all requests to this router will first hit this middleware
 7 router.use(function(req, res, next) {
 8   console.log('%s %s %s', req.method, req.url, req.path);
 9   next();
10 });
11
12 // this will only be invoked if the path ends in /bar
13 router.use('/bar', function(req, res, next) {
14   // ... maybe some additional /bar logging ...
15   next();
16 });
17
18 // always invoked
19 router.use(function(req, res, next) {
20   res.send('Hello World');
21 });
22
23 app.use('/foo', router);
24
25 app.listen(3000);

 1 var express = require('express');
 2
 3 var app = express();
 4 var router = express.Router();
 5
 6 app.get('/', function(req, res){
 7      console.log('test1');
 8 });
 9
10 app.use('/', function(req, res){
11      console.log('test2');
12 });
13
14 router.get('/', function(req, res){
15      console.log('test3');
16 });
17
18 app.listen(4000);

View Code

1 app.use([path], [function...], function)
2 Mount the middleware function(s) at the path. If path is not specified, it defaults to "/".
3
4 Mounting a middleware at a path will cause the middleware function to be executed whenever the base of the requested path matches the path.