nodeJs事件循環(huán)運行代碼解析

更新時間：2023年04月14日 15:19:27 作者：Tomarsh

這篇文章主要為大家介紹了nodeJs事件循環(huán)運行代碼解析，有需要的朋友可以借鑒參考下，希望能夠有所幫助，祝大家多多進步，早日升職加薪

Nodejs運行時

JS語言是同步，阻塞，單線程的，但是nodejs不是。Nodejs由三個主要組件：

外部依賴例如 v8,libuv,crypto
提供文件和網(wǎng)絡(luò)服務(wù)的c++模塊
基于c++模塊上層封裝的JS庫

nodejs的異步特性主要由libuv提供。libuv是跨平臺的使用c語言寫的庫，它主要提供對異步操作的支持。

node運行時代碼運行

當我們在Nodejs中執(zhí)行JS代碼時，是由v8引擎處理代碼執(zhí)行，v8包括一塊內(nèi)存區(qū)域（堆）和調(diào)用棧。當定義函數(shù)，變量時，從堆中分配內(nèi)存，當執(zhí)行代碼時將函數(shù)入棧，函數(shù)返回時出棧。

當執(zhí)行異步操作時，libuv將接管該任務(wù)，然后使用操作系統(tǒng)的異步機制運行任務(wù)。如果缺乏系統(tǒng)級的異步機制，就使用線程池運行任務(wù)，保證主線程不被阻塞。

Event Loop

事件循環(huán)是一個nodejs應(yīng)用運行后一直存在的循環(huán)。存在著六個不同的隊列，每個都存儲著不同的回調(diào)。

Timer queue（定時器隊列），最小堆，由setTimeout, setInterval創(chuàng)建
IO隊列：文件、網(wǎng)絡(luò)操作
check隊列，任務(wù)由setImmediate產(chǎn)生，node專有
close隊列，與異步任務(wù)的close事件相關(guān)
nextTick隊列
promise隊列

除了兩個微任務(wù)隊列，其他隊列都是libuv自帶的

如何工作？

同步代碼優(yōu)于異步代碼，事件循環(huán)是call stack為空后開始。事件循環(huán)遵循的優(yōu)先級規(guī)則：

微任務(wù)隊列有任務(wù)，先處理完。nextTick先于promise
定時器任務(wù)執(zhí)行
IO隊列
check隊列
close隊列

需要注意的是在定時器隊列，IO隊列，check隊列，close隊列執(zhí)行一個任務(wù)后都會檢查并運行微任務(wù)隊列。

實驗

實驗1

// index.js
console.log("console.log 1");
process.nextTick(() => console.log("this is process.nextTick 1"));
console.log("console.log 2");

輸出

console.log 1
console.log 2
this is process.nextTick 1

結(jié)論：同步先于異步

實驗2

// index.js
Promise.resolve().then(() => console.log("this is Promise.resolve 1"));
process.nextTick(() => console.log("this is process.nextTick 1"));

輸出

this is process.nextTick 1
this is Promise.resolve 1

結(jié)論: nextTick先于promise

實驗3

// index.js
process.nextTick(() => console.log("this is process.nextTick 1"));
process.nextTick(() => {
  console.log("this is process.nextTick 2");
  process.nextTick(() =>
    console.log("this is the inner next tick inside next tick")
  );
});
process.nextTick(() => console.log("this is process.nextTick 3"));
Promise.resolve().then(() => console.log("this is Promise.resolve 1"));
Promise.resolve().then(() => {
  console.log("this is Promise.resolve 2");
  process.nextTick(() =>
    console.log("this is the inner next tick inside Promise then block")
  );
});
Promise.resolve().then(() => console.log("this is Promise.resolve 3"));

實驗3

// index.js
process.nextTick(() => console.log("this is process.nextTick 1"));
process.nextTick(() => {
  console.log("this is process.nextTick 2");
  process.nextTick(() =>
    console.log("this is the inner next tick inside next tick")
  );
});
process.nextTick(() => console.log("this is process.nextTick 3"));
Promise.resolve().then(() => console.log("this is Promise.resolve 1"));
Promise.resolve().then(() => {
  console.log("this is Promise.resolve 2");
  process.nextTick(() =>
    console.log("this is the inner next tick inside Promise then block")
  );
});
Promise.resolve().then(() => console.log("this is Promise.resolve 3"));

輸出

this is process.nextTick 1
this is process.nextTick 2
this is process.nextTick 3
this is the inner next tick inside next tick
this is Promise.resolve 1
this is Promise.resolve 2
this is Promise.resolve 3
this is the inner next tick inside Promise then block

解析：

nextTick內(nèi)部增加的nextTick任務(wù)還是先于promise，因為nexttick隊列清完后才會執(zhí)行promise隊列的任務(wù)。

promise里增加的nextTick任務(wù)晚于其他的promise，因為此時是在執(zhí)行promise階段，需要清空promise才會檢查nextTick隊列。

實驗4

// index.js
setTimeout(() => console.log("this is setTimeout 1"), 0);
setTimeout(() => {
  console.log("this is setTimeout 2");
  process.nextTick(() =>
    console.log("this is inner nextTick inside setTimeout")
  );
}, 0);
setTimeout(() => console.log("this is setTimeout 3"), 0);
process.nextTick(() => console.log("this is process.nextTick 1"));
process.nextTick(() => {
  console.log("this is process.nextTick 2");
  process.nextTick(() =>
    console.log("this is the inner next tick inside next tick")
  );
});
process.nextTick(() => console.log("this is process.nextTick 3"));
Promise.resolve().then(() => console.log("this is Promise.resolve 1"));
Promise.resolve().then(() => {
  console.log("this is Promise.resolve 2");
  process.nextTick(() =>
    console.log("this is the inner next tick inside Promise then block")
  );
});
Promise.resolve().then(() => console.log("this is Promise.resolve 3"));

輸出

this is process.nextTick 1
this is process.nextTick 2
this is process.nextTick 3
his is the inner next tick inside next tick
this is Promise.resolve 1
this is Promise.resolve 2
this is Promise.resolve 3
this is the inner next tick inside Promise then block
this is setTimeout 1
this is setTimeout 2
this is inner nextTick inside setTimeout
this is setTimeout 3

結(jié)論：

nextTick先于promise；微任務(wù)先于setTimeout；每個Timer任務(wù)后會檢查執(zhí)行微任務(wù)。

實驗6

// index.js
setTimeout(() => console.log("this is setTimeout 1"), 1000);
setTimeout(() => console.log("this is setTimeout 2"), 500);
setTimeout(() => console.log("this is setTimeout 3"), 0);

輸出

this is setTimeout 3
this is setTimeout 2
this is setTimeout 1

結(jié)論： Timer隊列是按時間排序的

實驗7

// index.js
const fs = require("fs");
fs.readFile(__filename, () => {
  console.log("this is readFile 1");
});
process.nextTick(() => console.log("this is process.nextTick 1"));
Promise.resolve().then(() => console.log("this is Promise.resolve 1"));

輸出

this is process.nextTick 1
this is Promise.resolve 1

結(jié)論：微任務(wù)先于io任務(wù)

實驗8

// index.js
const fs = require("fs");
setTimeout(() => console.log("this is setTimeout 1"), 0);
fs.readFile(__filename, () => {
  console.log("this is readFile 1");
});

輸出

不確定

解析：setTimeout 0通常內(nèi)部會取1ms，也就是1ms后執(zhí)行Timer任務(wù)，而cpu進入事件循環(huán)的時機不定，所以有可能進入事件循環(huán)時已經(jīng)過了1ms，那么先執(zhí)行timer任務(wù)，也可能進入時定時任務(wù)沒到時間，會先執(zhí)行IO任務(wù)。

實驗9

// index.js
const fs = require("fs");
fs.readFile(__filename, () => {
  console.log("this is readFile 1");
});
process.nextTick(() => console.log("this is process.nextTick 1"));
Promise.resolve().then(() => console.log("this is Promise.resolve 1"));
setTimeout(() => console.log("this is setTimeout 1"), 0);
for (let i = 0; i < 2000000000; i++) {}

輸出

this is process.nextTick 1
this is Promise.resolve 1
this is setTimeout 1
this is readFile 1

解析：

代碼最后加了循環(huán)保證進入事件循環(huán)時定時器任務(wù)已經(jīng)到期，所以先執(zhí)行Timer任務(wù)

實驗10

// index.js
const fs = require("fs");
fs.readFile(__filename, () => {
  console.log("this is readFile 1");
});
process.nextTick(() => console.log("this is process.nextTick 1"));
Promise.resolve().then(() => console.log("this is Promise.resolve 1"));
setTimeout(() => console.log("this is setTimeout 1"), 0);
setImmediate(() => console.log("this is setImmediate 1"));
for (let i = 0; i < 2000000000; i++) {}

輸出