Nodejs运行时

JS语言是同步，阻塞，单线程的，但是nodejs不是。Nodejs由三个主要组件：

  • 外部依赖例如 v8,libuv,crypto
  • 提供文件和网络服务的c++模块
  • 基于c++模块上层封装的JS库

nodejs的异步特性主要由libuv提供。libuv是跨平台的使用c语言写的库，它主要提供对异步操作的支持。

node运行时代码运行

当我们在Nodejs中执行JS代码时，是由v8引擎处理代码执行，v8包括一块内存区域（堆）和调用栈。当定义函数，变量时，从堆中分配内存，当执行代码时将函数入栈，函数返回时出栈。

当执行异步操作时，libuv将接管该任务，然后使用操作系统的异步机制运行任务。如果缺乏系统级的异步机制，就使用线程池运行任务，保证主线程不被阻塞。

Event Loop

事件循环是一个nodejs应用运行后一直存在的循环。存在着六个不同的队列，每个都存储着不同的回调。

  • Timer queue（定时器队列），最小堆，由setTimeout, setInterval创建
  • IO队列：文件、网络操作
  • check队列，任务由setImmediate产生，node专有
  • close队列， 与异步任务的close事件相关
  • nextTick队列
  • promise队列

除了两个微任务队列，其他队列都是libuv自带的

如何工作？

同步代码优于异步代码，事件循环是call stack为空后开始。事件循环遵循的优先级规则：

  • 微任务队列有任务，先处理完。nextTick先于promise
  • 定时器任务执行
  • IO队列
  • check队列
  • close队列

需要注意的是在定时器队列，IO队列，check队列，close队列执行一个任务后都会检查并运行微任务队列。

实验

实验1

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

输出

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

结论： 同步先于异步

实验2

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

输出

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

结论: nextTick先于promise

实验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"));

实验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"));

输出

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内部增加的nextTick任务还是先于promise，因为nexttick队列清完后才会执行promise队列的任务。

promise里增加的nextTick任务晚于其他的promise，因为此时是在执行promise阶段，需要清空promise才会检查nextTick队列。

实验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"));

输出

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

结论：

nextTick先于promise；微任务先于setTimeout；每个Timer任务后会检查执行微任务。

实验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);

输出

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

结论： Timer队列是按时间排序的

实验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"));

输出

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

结论：微任务先于io任务

实验8

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

输出

不确定

解析：setTimeout 0通常内部会取1ms，也就是1ms后执行Timer任务，而cpu进入事件循环的时机不定，所以有可能进入事件循环时已经过了1ms，那么先执行timer任务，也可能进入时定时任务没到时间，会先执行IO任务。

实验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++) {}

输出

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

解析：

代码最后加了循环保证进入事件循环时定时器任务已经到期，所以先执行Timer任务

实验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++) {}

输出

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

解析： 按理说IO任务先于check任务，但是第一次事件循环时IO任务的callback并不在队列里。在两个队列之间会通过IO polling的方式去查看io任务是否完成，完成了就将callback加到队列里，然后下一轮循环时会调用

I/O events are polled and callback functions are added to the I/O queue only after the I/O is complete

www.builder.io/blog/visual…