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golang?channel讀取數據的幾種情況_Golang

作者：-_-void ? 更新時間： 2023-03-29 編程語言

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!

這種情況并不是報錯空指針，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

這種情況沒什么毛病，之所以先輸出的read，是因為IO機制。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!

報錯說所有的協程都睡著，意思就是runtime發現沒有能拿來調度的協程了，報錯退出。如果是在大項目中，這里則會阻塞，runtime會調度其他可運行的協程。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
...

結果當然就是死循環了，這個很好理解。接下來才是本文的重點：讀數據的第二個參數。我們先保持其他的都不動，在讀的時候接收第二個返回值。

讀channel的第二個返回值

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，這第二個返回值是個bool類型，目前全都是true。那么什么時候會是false呢，把channel關上試試。為了更直觀，把字符串的長度一起輸出

關閉channel繼續讀

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
...

接下來就是很規律的死循環了。這樣是不是可以猜測，從已經close的channle讀數據，會讀到該數據類型的零值，且第二個返回值為false？再試試給channel加個buffer，先寫完關上再開始讀

寫完然后關閉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關鍵字去了，并且在關閉channel之后加了log。可以很清晰的看到，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ok是true，后面循環輸出false?，F在我們可以得出結論當channel關閉且數據都讀完了，再讀數據會讀到該數據類型的零值，且第二個返回值為false。下面再套上select

加個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
...

很明顯跟上面現象一致，如果忘了關閉channel呢？

channel未及時關閉

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，然后報錯。跟上面情況一樣，如果是在大項目中，runtime會調度其他可運行的協程。最后來總結一下怎么操作才算優(sao)雅(qi)。

總結

  • 對寫的一方來說，一定記著及時關閉channel，避免出現協程泄露。雖然它占得資源少，省點電不香么。
  • 對讀的一方來說，除非十分確定數據的個數，最好是用for來讀數據，省的在“管兒”里有“野數據”造成內存泄露。同時根據第二個返回值的真假來控制for循環，避免出現“無效工作量”

原文鏈接：https://blog.csdn.net/xmh19936688/article/details/106280737

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