golangchannel读取数据的几种情况

golang channel读取 golang channel数据 2023-02-01 12:02:11 330人浏览安东尼

摘要

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

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!