Golang中Timer的陷阱

Comments(2217)


Posted on 2014-10-04 14:08:31 golang


Golang的Timer类,是一个普遍意义上的定时器,它有着普通定时器的一些特性,例如:

  • 给定一个到期时间,和一个回调函数,到期后会调用回调函数
  • 重置定时器的超时时间
  • 停止定时器

Golang的Timer在源码中,实现的方式是以一个小顶堆来维护所有的Timer集合。接着启动一个独立的goroutine,循环从小顶堆中的检测最近一个到期的Timer的到期时间,接着它睡眠到最近一个定时器到期的时间。最后会执行开始时设定的回调函数。Timer到期之后,会被Golang的runtime从小项堆中删除,并等待GC回收资源。

下面给出实际的代码:

package main

import (
    "time"
    "fmt"
)


func main() {
    timer := time.NewTimer(3 * time.Second)

    go func() {
        <-timer.C
        fmt.Println("Timer has expired.")
    }()

    timer.Stop()
    time.Sleep(60 * time.Second)
}

timer.NewTimer()会启动一个新的Timer实例,并开始计时。 我们启动一个新的goroutine,来以阻塞的方式从Timer的C这个channel中,等待接收一个值,这个值是到期的时间。并打印”Timer has expired.”

到现在看起来似乎没什么问题,但是当我们执行timer.Stop()之后,3秒钟过去了,程序却没有打印那句话。说明执行timer.Stop()之后,Timer自带的channel并没有关闭,而且这个Timer已经从runtime中删除了,所以这个Timer永远不会到期。

这会导致程序逻辑错误,或者更严重的导致goroutine和内存泄露。解决的办法是,使用timer.Reset()代替timer.Stop()来停止定时器。

package main

import (
    "time"
    "fmt"
)


func main() {
    timer := time.NewTimer(3 * time.Second)

    go func() {
        <-timer.C
        fmt.Println("Timer has expired.")
    }()

    //timer.Stop()
    timer.Reset(0  * time.Second)
    time.Sleep(60 * time.Second)
}

这样做就相当于给Timer一个0秒的超时时间,让Timer立刻过期。

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