浅谈express 中间件机制及实现原理

2019-11-19 15:35:48

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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bodyapp.use(bodyParser.json())//解析 cookieapp.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.baidu.com/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.baidu.com/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.baidu.com/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: POSTversion: HTTP/1.1{key1: "value1", key2: "value2", key3: "value3"}{Host: "www.baidu.com", Cookie: "BAIDUID=E063E9B2690116090FE24E01ACDDF4AD"}

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