golang channel讀取數(shù)據(jù)的幾種情況

更新時(shí)間：2023年02月01日 09:53:06 作者：-_-void

本文主要介紹了golang channel讀取數(shù)據(jù)的幾種情況，文中通過(guò)示例代碼介紹的非常詳細(xì)，對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值，需要的朋友們下面隨著小編來(lái)一起學(xué)習(xí)學(xué)習(xí)吧

用var定義channel且不make

wg := sync.WaitGroup{}
var ch chan string

read := func() {
?? ?fmt.Println("reading")
?? ?s := <-ch
?? ?fmt.Println("read:", s)
?? ?wg.Done()
}

write := func() {
?? ?fmt.Println("writing")
?? ?s := "t"
?? ?ch <- s
?? ?fmt.Println("write:", s)
?? ?wg.Done()
}

wg.Add(2)
go read()
go write()

fmt.Println("waiting")
wg.Wait()

輸出：

waiting
writing
reading
fatal error: all goroutines are asleep - deadlock!

這種情況并不是報(bào)錯(cuò)空指針，而是死鎖。加上make看看

用var定義channel且make

wg := sync.WaitGroup{}
var ch = make(chan string)

read := func() {
?? ?fmt.Println("reading")
?? ?s := <-ch
?? ?fmt.Println("read:", s)
?? ?wg.Done()
}

write := func() {
?? ?fmt.Println("writing")
?? ?s := "t"
?? ?ch <- s
?? ?fmt.Println("write:", s)
?? ?wg.Done()
}

wg.Add(2)
go read()
go write()

輸出

waiting
writing
reading
read: t
write: t

這種情況沒(méi)什么毛病，之所以先輸出的read，是因?yàn)镮O機(jī)制。下面給寫加上for

直給寫操作加for

wg := sync.WaitGroup{}
var ch = make(chan string)

read := func() {
?? ?fmt.Println("reading")
?? ?s := <-ch
?? ?fmt.Println("read:", s)
?? ?wg.Done()
}

write := func() {
?? ?for {
?? ??? ?fmt.Println("writing")
?? ??? ?s := "t"
?? ??? ?ch <- s
?? ??? ?fmt.Println("write:", s)
?? ?}
?? ?wg.Done()
}

wg.Add(2)
go read()
go write()

fmt.Println("waiting")
wg.Wait()
fmt.Println("finish")

輸出

waiting
reading
writing
write: t
writing
read: t
fatal error: all goroutines are asleep - deadlock!

報(bào)錯(cuò)說(shuō)所有的協(xié)程都睡著，意思就是runtime發(fā)現(xiàn)沒(méi)有能拿來(lái)調(diào)度的協(xié)程了，報(bào)錯(cuò)退出。如果是在大項(xiàng)目中，這里則會(huì)阻塞，runtime會(huì)調(diào)度其他可運(yùn)行的協(xié)程。下面把for移到讀操作上。

直給讀操作加for

wg := sync.WaitGroup{}
var ch = make(chan string)

read := func() {
?? ?for {
?? ??? ?fmt.Println("reading")
?? ??? ?s := <-ch
?? ??? ?fmt.Println("read:", s)
?? ?}
?? ?wg.Done()
}

write := func() {
?? ?fmt.Println("writing")
?? ?s := "t"
?? ?ch <- s
?? ?fmt.Println("write:", s)
?? ?wg.Done()
}

wg.Add(2)
go read()
go write()

fmt.Println("waiting")
wg.Wait()
fmt.Println("finish")

輸出

waiting
reading
writing
write: t
read: t
reading
fatal error: all goroutines are asleep - deadlock!

跟上面現(xiàn)象基本一樣，不再贅述，然后給倆操作都加上for

讀寫都加for

wg := sync.WaitGroup{}
var ch = make(chan string)

read := func() {
?? ?for {
?? ??? ?fmt.Println("reading")
?? ??? ?s := <-ch
?? ??? ?fmt.Println("read:", s)
?? ?}
?? ?wg.Done()
}

write := func() {
?? ?for {
?? ??? ?fmt.Println("writing")
?? ??? ?s := "t"
?? ??? ?ch <- s
?? ??? ?fmt.Println("write:", s)
?? ?}
?? ?wg.Done()
}

wg.Add(2)
go read()
go write()

fmt.Println("waiting")
wg.Wait()
fmt.Println("finish")

輸出

waiting
writing
reading
read: t
write: t
writing
reading
read: t
reading
write: t
writing
write: t
writing
...

結(jié)果當(dāng)然就是死循環(huán)了，這個(gè)很好理解。接下來(lái)才是本文的重點(diǎn)：讀數(shù)據(jù)的第二個(gè)參數(shù)。我們先保持其他的都不動(dòng)，在讀的時(shí)候接收第二個(gè)返回值。

讀channel的第二個(gè)返回值

wg := sync.WaitGroup{}
var ch = make(chan string)

read := func() {
?? ?for {
?? ??? ?fmt.Println("reading")
?? ??? ?s, ok := <-ch
?? ??? ?fmt.Println("read:", s, ok)
?? ?}
?? ?wg.Done()
}

write := func() {
?? ?for {
?? ??? ?fmt.Println("writing")
?? ??? ?s := "t"
?? ??? ?ch <- s
?? ??? ?fmt.Println("write:", s)
?? ?}
?? ?wg.Done()
}

wg.Add(2)
go read()
go write()

fmt.Println("waiting")
wg.Wait()
fmt.Println("finish")

輸出

waiting
writing
reading
read: t true
reading
write: t
writing
write: t
writing
read: t true
reading
read: t true
reading
write: t
...

可以看出來(lái)，這第二個(gè)返回值是個(gè)bool類型，目前全都是true。那么什么時(shí)候會(huì)是false呢，把channel關(guān)上試試。為了更直觀，把字符串的長(zhǎng)度一起輸出

關(guān)閉channel繼續(xù)讀

wg := sync.WaitGroup{}
var ch = make(chan string)

read := func() {
?? ?for {
?? ??? ?fmt.Println("reading")
?? ??? ?s, ok := <-ch
?? ??? ?fmt.Println("read:", len(s), s, ok)
?? ?}
?? ?wg.Done()
}

write := func() {
?? ?for i := 0; i < 5; i++ {
?? ??? ?fmt.Println("writing")
?? ??? ?s := "t"
?? ??? ?ch <- s
?? ??? ?fmt.Println("write:", s)
?? ?}
?? ?wg.Done()
?? ?close(ch)
}

wg.Add(2)
go read()
go write()

fmt.Println("waiting")
wg.Wait()
fmt.Println("finish")

輸出

waiting
writing
reading
read: 1 t true
reading
write: t
writing
write: t
writing
read: 1 t true
reading
read: 1 t true
reading
write: t
writing
write: t
writing
read: 1 t true
reading
read: 1 t true
reading
write: t
read: 0 false
reading
read: 0 false
reading
read: 0 false
...

接下來(lái)就是很規(guī)律的死循環(huán)了。這樣是不是可以猜測(cè)，從已經(jīng)close的channle讀數(shù)據(jù)，會(huì)讀到該數(shù)據(jù)類型的零值，且第二個(gè)返回值為false？再試試給channel加個(gè)buffer，先寫完關(guān)上再開始讀

寫完然后關(guān)閉channel再開始讀

wg := sync.WaitGroup{}
var ch = make(chan string, 5)

read := func() {
?? ?for {
?? ??? ?fmt.Println("reading")
?? ??? ?s, ok := <-ch
?? ??? ?fmt.Println("read:", len(s), s, ok)
?? ?}
?? ?wg.Done()
}

write := func() {
?? ?for i := 0; i < 5; i++ {
?? ??? ?fmt.Println("writing")
?? ??? ?s := "t"
?? ??? ?ch <- s
?? ??? ?fmt.Println("write:", s)
?? ?}
?? ?wg.Done()
?? ?close(ch)
?? ?fmt.Println("closed")
}

wg.Add(2)
write()
go read()

fmt.Println("waiting")
wg.Wait()
fmt.Println("finish")

輸出

writing
write: t
writing
write: t
writing
write: t
writing
write: t
writing
write: t
closed
waiting
reading
read: 1 t true
reading
read: 1 t true
reading
read: 1 t true
reading
read: 1 t true
reading
read: 1 t true
reading
read: 0 false
reading
read: 0 false
reading
read: 0 false
...

我們把寫操作前的go關(guān)鍵字去了，并且在關(guān)閉channel之后加了log?？梢院芮逦目吹?，先往channel里寫了5次，然后close了，之后才有wait及read的log。并且前5個(gè)ok是true，后面循環(huán)輸出false?，F(xiàn)在我們可以得出結(jié)論當(dāng)channel關(guān)閉且數(shù)據(jù)都讀完了，再讀數(shù)據(jù)會(huì)讀到該數(shù)據(jù)類型的零值，且第二個(gè)返回值為false。下面再套上select

加個(gè)select

wg := sync.WaitGroup{}
var ch = make(chan string, 5)

read := func() {
?? ?for {
?? ??? ?fmt.Println("reading")
?? ??? ?select {
?? ??? ?case s, ok := <-ch:
?? ??? ??? ?fmt.Println("read:", len(s), s, ok)
?? ??? ?}

?? ?}
?? ?wg.Done()
}

write := func() {
?? ?for i := 0; i < 5; i++ {
?? ??? ?fmt.Println("writing")
?? ??? ?s := "t"
?? ??? ?ch <- s
?? ??? ?fmt.Println("write:", s)
?? ?}
?? ?wg.Done()
?? ?close(ch)
?? ?fmt.Println("closed")
}

wg.Add(2)
write()
go read()

fmt.Println("waiting")
wg.Wait()
fmt.Println("finish")

輸出

writing
write: t
writing
write: t
writing
write: t
writing
write: t
writing
write: t
closed
waiting
reading
read: 1 t true
reading
read: 1 t true
reading
read: 1 t true
reading
read: 1 t true
reading
read: 1 t true
reading
read: 0 false
reading
read: 0 false
reading
read: 0 false
...

很明顯跟上面現(xiàn)象一致，如果忘了關(guān)閉channel呢？

channel未及時(shí)關(guān)閉

wg := sync.WaitGroup{}
var ch = make(chan string, 5)

read := func() {
?? ?for {
?? ??? ?fmt.Println("reading")
?? ??? ?select {
?? ??? ?case s, ok := <-ch:
?? ??? ??? ?fmt.Println("read:", len(s), s, ok)
?? ??? ?}

?? ?}
?? ?wg.Done()
}

write := func() {
?? ?for i := 0; i < 5; i++ {
?? ??? ?fmt.Println("writing")
?? ??? ?s := "t"
?? ??? ?ch <- s
?? ??? ?fmt.Println("write:", s)
?? ?}
?? ?wg.Done()
?? ?//close(ch)
?? ?//fmt.Println("closed")
}

wg.Add(2)
write()
go read()

fmt.Println("waiting")
wg.Wait()
fmt.Println("finish")

輸出

writing
write: t
writing
write: t
writing
write: t
writing
write: t
writing
write: t
waiting
reading
read: 1 t true
reading
read: 1 t true
reading
read: 1 t true
reading
read: 1 t true
reading
read: 1 t true
reading
fatal error: all goroutines are asleep - deadlock!

睡著了，然后報(bào)錯(cuò)。跟上面情況一樣，如果是在大項(xiàng)目中，runtime會(huì)調(diào)度其他可運(yùn)行的協(xié)程。最后來(lái)總結(jié)一下怎么操作才算優(yōu)(sao)雅(qi)。

總結(jié)

對(duì)寫的一方來(lái)說(shuō)，一定記著及時(shí)關(guān)閉channel，避免出現(xiàn)協(xié)程泄露。雖然它占得資源少，省點(diǎn)電不香么。
對(duì)讀的一方來(lái)說(shuō)，除非十分確定數(shù)據(jù)的個(gè)數(shù)，最好是用for來(lái)讀數(shù)據(jù)，省的在“管兒”里有“野數(shù)據(jù)”造成內(nèi)存泄露。同時(shí)根據(jù)第二個(gè)返回值的真假來(lái)控制for循環(huán)，避免出現(xiàn)“無(wú)效工作量”

到此這篇關(guān)于golang channel讀取數(shù)據(jù)的幾種情況的文章就介紹到這了,更多相關(guān)golang channel讀取內(nèi)容請(qǐng)搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！

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