摘要：簡(jiǎn)介中間件機(jī)制可以讓我們?cè)谝粋€(gè)給定的流程中添加一個(gè)處理步驟，從而對(duì)這個(gè)流程的輸入或者輸出產(chǎn)生影響，或者產(chǎn)生一些中作用狀態(tài)，或者攔截這個(gè)流程。中間件機(jī)制和的過濾器類似，這兩者都屬于責(zé)任鏈模式的具體實(shí)現(xiàn)。

中間件機(jī)制可以讓我們?cè)谝粋€(gè)給定的流程中添加一個(gè)處理步驟，從而對(duì)這個(gè)流程的輸入或者輸出產(chǎn)生影響，或者產(chǎn)生一些中作用、狀態(tài)，或者攔截這個(gè)流程。中間件機(jī)制和tomcat的過濾器類似，這兩者都屬于責(zé)任鏈模式的具體實(shí)現(xiàn)。

let express = require("express")
let app = express()
//解析request 的body
app.use(bodyParser.json())
//解析 cookie
app.use(cookieParser())
//攔截
app.get("/hello", function (req, res) {
  res.send("Hello World!");
});

首先模擬一個(gè)request

request = { //模擬的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",
}

一個(gè)http請(qǐng)求分為請(qǐng)求行、請(qǐng)求頭、和請(qǐng)求體，這三者之間通過 即一個(gè)空行來分割，這里假設(shè)已經(jīng)將這三者分開，requestLine（請(qǐng)求行）中有方法類型，請(qǐng)求url，http版本號(hào)，這三者通過空格來區(qū)分，headers（請(qǐng)求頭）中的各部分通過 來分割，requestBody（請(qǐng)求體）中通過 & 來區(qū)分參數(shù)

模擬中間件機(jī)制

約定 中間件一定是一個(gè)函數(shù)并且接受 request, response, next三個(gè)參數(shù)

function App() {
    if (!(this instanceof App))
        return new App();
    this.init();
}
App.prototype = {
    constructor: App,
    init: function() {
        this.request = { //模擬的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 = {}; //模擬的response
        this.chain = []; //存放中間件的一個(gè)數(shù)組
        this.index = 0; //當(dāng)前執(zhí)行的中間件在chain中的位置
    },
    use: function(handle) { //這里默認(rèn) handle 是函數(shù)，并且這里不做判斷
        this.chain.push(handle);
    },
    next: function() { //當(dāng)調(diào)用next時(shí)執(zhí)行index所指向的中間件
        if (this.index >= this.chain.length)
            return;
        let middleware = this.chain[this.index];
        this.index++;
        middleware(this.request, this.response, this.next.bind(this));
    },
}

對(duì) request 處理的中間件

 function lineParser(req, res, next) {
        let items = req.requestLine.split(" ");
        req.methond = items[0];
        req.url = items[1];
        req.version = items[2];
        next(); //執(zhí)行下一個(gè)中間件
    }

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(); //執(zhí)行下一個(gè)中間件
}

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(); //執(zhí)行下一個(gè)中間件
}

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(); //執(zhí)行下一個(gè)中間件
}

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 = { //模擬的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 = {}; //模擬的response
        this.chain = []; //存放中間件的一個(gè)數(shù)組
        this.index = 0; //當(dāng)前執(zhí)行的中間件在chain中的位置
    },
    use: function(handle) { //這里默認(rèn) handle 是函數(shù)，并且這里不做判斷
        this.chain.push(handle);
    },
    next: function() { //當(dāng)調(diào)用next時(shí)執(zhí)行index所指向的中間件
        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(); //執(zhí)行下一個(gè)中間件
    }

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(); //執(zhí)行下一個(gè)中間件
}

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(); //執(zhí)行下一個(gè)中間件
}

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(); //執(zhí)行下一個(gè)中間件
}
let app = App();
app.use(lineParser);
app.use(headersParser);
app.use(bodyParser);
app.use(middleware3);
app.next();

將以上整體代碼運(yùn)行后將打印以下信息

url: /iven_
methond: POST
version: HTTP/1.1
{key1: "value1", key2: "value2", key3: "value3"}
{Host: "www.baidu.com", Cookie: "BAIDUID=E063E9B2690116090FE24E01ACDDF4AD"}