express 中间件机制及实现原理

摘要：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。中间件机制和的过滤器类似，这两者都属于责任链模式的具体实现。

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。

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!");
});

首先模拟一个request

request = { //模拟的request
    requestLine: "POST /iven_ HTTP/1.1",
    headers: "Host:www.baidu.com
Cookie: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 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 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
Cookie: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("
");
    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();

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
Cookie: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("
");
    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();

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.baidu.com", Cookie: "BAIDUID=E063E9B2690116090FE24E01ACDDF4AD"}