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這篇文章主要講解了“nodeJs事件循環(huán)運(yùn)行代碼怎么寫”,文中的講解內(nèi)容簡單清晰,易于學(xué)習(xí)與理解,下面請大家跟著小編的思路慢慢深入,一起來研究和學(xué)習(xí)“nodeJs事件循環(huán)運(yùn)行代碼怎么寫”吧!
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?yán)?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。
當(dāng)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(dāng)定義函數(shù),變量時,從堆中分配內(nèi)存,當(dāng)執(zhí)行代碼時將函數(shù)入棧,函數(shù)返回時出棧。
當(dāng)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(jī)制運(yùn)行任務(wù)。如果缺乏系統(tǒng)級的異步機(jī)制,就使用線程池運(yùn)行任務(wù),保證主線程不被阻塞。
事件循環(huán)是一個nodejs應(yīng)用運(yùn)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
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(yùn)行微任務(wù)隊列。
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(jìn)入事件循環(huán)的時機(jī)不定,所以有可能進(jìn)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(jìn)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(jìn)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
this is process.nextTick 1
this is Promise.resolve 1
this is setTimeout 1
this is setImmediate 1
this is readFile 1
解析: 按理說IO任務(wù)先于check任務(wù),但是第一次事件循環(huán)時IO任務(wù)的callback并不在隊列里。在兩個隊列之間會通過IO polling的方式去查看io任務(wù)是否完成,完成了就將callback加到隊列里,然后下一輪循環(huán)時會調(diào)用
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