本篇文章主要介绍了浅谈express 中间件机制及实现原理，小编觉得挺不错的，现在分享给大家，也给大家做个参考。一起跟随小编过来看看吧
简介
中间件机制可以让我们在一个给定的流程中添加一个处理步骤，从而对这个流程的输入或者输出产生影响，或者产生一些中作用、状态，或者拦截这个流程。中间件机制和tomcat的过滤器类似，这两者都属于责任链模式的具体实现。
express 中间件使用案例
let express = require('express')
let app = express()
//解析request 的body
app.use(bodyParser.json())
//解析 cookie
app.use(cookieParser())
//拦截
app.get('/hello', function (req, res) {
res.send('Hello World!');
});模拟中间件机制并且模拟实现解析request的中间件
首先模拟一个request
request = { //模拟的request
requestLine: 'POST /iven_ HTTP/1.1',
headers: 'Host:www.php.cn\r\nCookie:BAIDUID=E063E9B2690116090FE24E01ACDDF4AD:FG=1;BD_HOME=0',
requestBody: 'key1=value1&key2=value2&key3=value3',
}一个http请求分为请求行、请求头、和请求体，这三者之间通过\r\n\r\n即一个空行来分割，这里假设已经将这三者分开，requestLine（请求行）中有方法类型，请求url，http版本号，这三者通过空格来区分，headers（请求头）中的各部分通过\r\n来分割，requestBody（请求体）中通过 & 来区分参数
模拟中间件机制
约定 中间件一定是一个函数并且接受 request, response, next三个参数
function App() {
if (!(this instanceof App))
return new App();
this.init();
}
App.prototype = {
constructor: App,
init: function() {
this.request = { //模拟的request
requestLine: 'POST /iven_ HTTP/1.1',
headers: 'Host:www.php.cn\r\nCookie:BAIDUID=E063E9B2690116090FE24E01ACDDF4AD:FG=1;BD_HOME=0',
requestBody: 'key1=value1&key2=value2&key3=value3',
};
this.response = {}; //模拟的response
this.chain = []; //存放中间件的一个数组
this.index = 0; //当前执行的中间件在chain中的位置
},
use: function(handle) { //这里默认 handle 是函数，并且这里不做判断
this.chain.push(handle);
},
next: function() { //当调用next时执行index所指向的中间件
if (this.index >= this.chain.length)
return;
let middleware = this.chain[this.index];
this.index++;
middleware(this.request, this.response, this.next.bind(this));
},
}对 request 处理的中间件
function lineParser(req, res, next) {
let items = req.requestLine.split(' ');
req.methond = items[0];
req.url = items[1];
req.version = items[2];
next(); //执行下一个中间件
}
function headersParser(req, res, next) {
let items = req.headers.split('\r\n');
let header = {}
for(let i in items) {
let item = items[i].split(':');
let key = item[0];
let value = item[1];
header[key] = value;
}
req.header = header;
next(); //执行下一个中间件
}
function bodyParser(req, res, next) {
let bodyStr = req.requestBody;
let body = {};
let items = bodyStr.split('&');
for(let i in items) {
let item = items[i].split('=');
let key = item[0];
let value = item[1];
body[key] = value;
}
req.body = body;
next(); //执行下一个中间件
}
function middleware3(req, res, next) {
console.log('url: '+req.url);
console.log('methond: '+req.methond);
console.log('version: '+req.version);
console.log(req.body);
console.log(req.header);
next(); //执行下一个中间件
}测试代码
let app = App(); app.use(lineParser); app.use(headersParser); app.use(bodyParser); app.use(middleware3); app.next();
整体代码
function App() {
if (!(this instanceof App))
return new App();
this.init();
}
App.prototype = {
constructor: App,
init: function() {
this.request = { //模拟的request
requestLine: 'POST /iven_ HTTP/1.1',
headers: 'Host:www.php.cn\r\nCookie:BAIDUID=E063E9B2690116090FE24E01ACDDF4AD:FG=1;BD_HOME=0',
requestBody: 'key1=value1&key2=value2&key3=value3',
};
this.response = {}; //模拟的response
this.chain = []; //存放中间件的一个数组
this.index = 0; //当前执行的中间件在chain中的位置
},
use: function(handle) { //这里默认 handle 是函数，并且这里不做判断
this.chain.push(handle);
},
next: function() { //当调用next时执行index所指向的中间件
if (this.index >= this.chain.length)
return;
let middleware = this.chain[this.index];
this.index++;
middleware(this.request, this.response, this.next.bind(this));
},
}
function lineParser(req, res, next) {
let items = req.requestLine.split(' ');
req.methond = items[0];
req.url = items[1];
req.version = items[2];
next(); //执行下一个中间件
}
function headersParser(req, res, next) {
let items = req.headers.split('\r\n');
let header = {}
for(let i in items) {
let item = items[i].split(':');
let key = item[0];
let value = item[1];
header[key] = value;
}
req.header = header;
next(); //执行下一个中间件
}
function bodyParser(req, res, next) {
let bodyStr = req.requestBody;
let body = {};
let items = bodyStr.split('&');
for(let i in items) {
let item = items[i].split('=');
let key = item[0];
let value = item[1];
body[key] = value;
}
req.body = body;
next(); //执行下一个中间件
}
function middleware3(req, res, next) {
console.log('url: '+req.url);
console.log('methond: '+req.methond);
console.log('version: '+req.version);
console.log(req.body);
console.log(req.header);
next(); //执行下一个中间件
}
let app = App();
app.use(lineParser);
app.use(headersParser);
app.use(bodyParser);
app.use(middleware3);
app.next();运行结果
将以上整体代码运行后将打印以下信息
url: /iven_
methond: POST
version: HTTP/1.1
{key1: "value1", key2: "value2", key3: "value3"}
{Host: "www.php.cn", Cookie: "BAIDUID=E063E9B2690116090FE24E01ACDDF4AD"} 
