1. 什么是router路径，什么是middleware？
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。
访问的路径‘/’即为router的路径，服务器采取的动作即为middleware，即为一个个特殊的函数。
2. router路径
www.baidu.com/test: 路径为 /test
www.baidu.com/test?name=1&number=2: 路径同样为/test, ?后面会被服务器理解传给路径的参数。
3. Middleware
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.
路由的产生是通过HTTP的各种方法(GET, POST)产生的，Middleware可以跟router路径跟特定的HTTP方法绑定，也可以跟所有的方法绑定。
3.1 通过express应用的use（all），把Middleware同router路径上的所有HTTP方法绑定：
app.use(function (req, res, next) {
console.log('Time: %d', Date.now());
next();
})
3.2 通过express应用的http.verb，把Middleware同router路径上的特定的HTTP方法绑定：
app.get('/', function(req, res){
res.send('hello world');
});
app.post('/', function(req, res){
res.send('hello world');
});
4. Express的Router对象
当express实例的路由越来越多的时候，最好把路由分类独立出去，express的实例(app) 能更好的处理其他逻辑流程。Express的Router对象是一个简化的 app实例，只具有路由相关的功能，包括use, http verbs等等。最后这个Router再通过app的use挂载到app的相关路径下。
var express = require('express');
var app = express();
var router = express.Router();
// simple logger for this router's requests
// all requests to this router will first hit this middleware
router.use(function(req, res, next) {
console.log('%s %s %s', req.method, req.url, req.path);
next();
});
// this will only be invoked if the path ends in /bar
router.use('/bar', function(req, res, next) {
// ... maybe some additional /bar logging ...
next();
});
// always invoked
router.use(function(req, res, next) {
res.send('Hello World');
});
app.use('/foo', router);
app.listen(3000);
router的路由必须通过app.use和app.verbs 挂载到app上才能被响应。所以上述代码，只有在app捕捉到 /foo路径上的路由时，才能router中定义的路由，虽然router中有针对 '/' 的路由，但是被app中的路由给覆盖了。
附：app.verbs和app.use的路由路径区别：
先看一段测试代码：
var express = require('express');
var app = express();
var router = express.Router();
app.get('/', function(req, res){
console.log('test1');
});
app.use('/', function(req, res){
console.log('test2');
});
router.get('/', function(req, res){
console.log('test3');
});
app.listen(4000);
输入url： localhost:4000
app.use([path], [function...], function)
Mount the middleware function(s) at the path. If path is not specified, it defaults to "/". Mounting a middleware at a path will cause the middleware function to be executed whenever the base of the requested path matches the path.