题干描述：
现有如下koa2代码，请根据"任务要求"执行下列代码，并结合洋葱圈模型图解释next的执行过程！


任务要求：
1、执行代码，在命令窗口查看打印结果
2、去掉"222"函数的next()，再在命令窗口查看打印结果

const Koa = require('koa')
const app = new Koa()
app.use(async (ctx, next)=>{
  let startTime = new Date().getTime()
  await next()
  let endTime = new Date().getTime()
  console.log(`此次的响应时间为：${endTime - startTime}ms`)
})
app.use(async (ctx, next) => {
  console.log('111, 然后doSomething')
  await next()
  console.log('111 end')
})
app.use(async (ctx, next) => {
  console.log('222, 然后doSomething')
  await next()
  console.log('222 end')
})
app.use(async (ctx, next) => {
  console.log('333, 然后doSomething')
  await next()
  console.log('333 end')
})
app.listen(3333, ()=>{
    // 可修改打印内容
  console.log('server is running at http://localhost:8080')
})