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golang channel读取数据的几种情况

2023-02-01 10:37:29

目录用var定义channel且不make用var定义channel且make直给写操作加for直给读操作加for读写都加for读channel的第二个返回值关闭channel继续读写完然后关闭cha...

目录
用var定义channel且不make
用var定义channel且make
直给写操作加for
直给读操作加for
读写都加for
读channel的第二个返回值
关闭channel继续读
写完然后关闭channel再开始读
加个select
channel未及时关闭
总结

用var定义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()

输出：

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

这种情况并不是报错空指针，而是死锁。加上make看看

用var定义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()

输出

waiting
writing
reading
read: t
write: t

这种情况没什么毛病，之所以先输出的read，是因为IO机制。下面给写加上for

直给写操作加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")

输出

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

报错说所有的协程都睡着，意思就是runtime发现没有能拿来调度的协程了，报错退出。如果是在大项目中，这里则会阻塞，runtime会调度其他可运行的协程。下面把for移到读操作上。

直给读操作加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")

输出

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

跟上面现象基本一样，不再赘述，然后给俩操作都加上for

读写都加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")

输出

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

结果当然就是死循环了，这个很好理解。接下来才是本文的重点：读数据的第二个参数。我们先保持其他的都不动，在读的时候接收第二个返回值。

读channel的第二个返回值

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")

输出

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

可以看出来，这第二个返回值是个bool类型，目前全都是true。那么什么时候会是false呢，把channel关上试试。为了更直观，把字符串的长度一起输出

关闭channel继续读

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")

输出

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
...

接下来就是很规律的死循环了。这样是不是可以猜测，从已经close的channle读数据，会读到该数据类型的零值，且第二个返回值为false？再试试给channel加个buffer，先写完关上再开始读

写完然后关闭channel再开始读

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")

输出

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
...

我们把写操作前的go关键字去了，并且在关闭channel之后加了log。可以很清晰的看到，先往channel里写了5次，然后close了，之后才有wait及read的log。并且前5个ok是true，后面循环输出false。现在我们可以得出结论当channel关闭且数据都读完了，再读数据会读到该数据类型的零值，且第二个返回值为false。下面再套上select

加个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")

输出

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
...

很明显跟上面现象一致，如果忘了关闭channel呢？

channel未及时关闭

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")

输出

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!

睡着了，然后报错。跟上面情况一样，如果是在大项目中，runtime会调度其他可运行的协程。最后来总结一下怎么操作才算优(sao)雅(qi)。

总结

对写的一方来说，一定记着及时关闭channel，避免出现协程泄露。虽然它占得资源少，省点电不香么。
对读的一方来说，除非十分确定数据的个数，最好是用for来读数据，省的在“管儿”里有“野数据”造成内存泄露。同时根据第二个返回值的真假来控制for循环，避免出现“无效工作量”

到此这篇关于golang channel读取数据的几种情况的文章就介绍到这了,更多相关golang channel读取内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！

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