我们先看下下面这段代码:
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package main
import (
"fmt"
"time"
)
func write(c chan int) {
for i := 0; i < 10; i++ {
c <- i
time.Sleep(time.Second)
}
}
func main() {
var c = make(chan int)
go write(c)
for e := range c {
fmt.Println("from channel get ", e)
}
}
程序执行结果:
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from channel get 0
from channel get 1
from channel get 2
from channel get 3
from channel get 4
from channel get 5
from channel get 6
from channel get 7
from channel get 8
from channel get 9
fatal error: all goroutines are asleep - deadlock!
goroutine 1 [chan receive]:
main.main()
channel.go:20 +0xe5
exit status 2
goroutine 死锁了,因为 all goroutines are asleep
。为什么会 asleep 呢?
我们仔细看看上面的代码会发现,write
向 channel 写完数据后,又从 channel 遍历读取完了数据。 然后读操作会阻塞等待channel 写操作,即 asleep;而写操作在等待channel读操作,也在阻塞,也即 asleep。 双方都等待对方执行,于是就死锁了。
要解决这个问题,需要从写的这一方解决,写完数据后关闭 channel:
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func write(c chan int) {
for i := 0; i < 10; i++ {
c <- 1
time.Sleep(time.Second)
}
close(c)
}
对于未关闭的 channel,for 循环读取不到数据后就会阻塞,直到写操作执行;但是对于已关闭的 channel,for 循环读取不到数据后就会结束退出循环。
另外补充,for range
语句遍历 channel 是一种阻塞读的方式,同样阻塞读的还有 value:=<- somechan
,<- somechan
语句,读不到数据时就会一直等待 channel 的写操作,不过 for range
语句是不定次数的读,后两者是一次性读。 除此之外有一种非阻塞读,即 select case
语句,读不到数据时就会退出。
我们将最上面的代码改造下:
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package main
import (
"fmt"
"time"
)
func write(c chan int) {
for i := 0; i < 10; i++ {
c <- i
time.Sleep(time.Second)
}
}
func main() {
var c = make(chan int)
go write(c)
for i := 0; i < 10; i++ {
val := <-c
fmt.Println("from channel get ", val)
}
select {
case val := <-c:
fmt.Println("select get ", val)
default:
fmt.Println("select nothing")
}
}
结果就不会 panic:
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from channel get 0
from channel get 1
from channel get 2
from channel get 3
from channel get 4
from channel get 5
from channel get 6
from channel get 7
from channel get 8
from channel get 9
select nothing
或者这样改造,读次数与写次数相同,也能避免死锁:
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package main
import (
"fmt"
"time"
)
func write(c chan int) {
for i := 0; i < 10; i++ {
c <- i
time.Sleep(time.Second)
}
}
func main() {
var c = make(chan int)
go write(c)
for i := 0; i < 10; i++ {
val := <-c
fmt.Println("from channel get ", val)
}
}
总之读者要理解死锁的本质,根据实际需要编写合适的代码。