简析go语言中select和switch
select类似于switch语句，但是只能用于channel操作，可以用于channel的数据接收也可以用于channel的数据发送。每个case必须是channel操作（接收或者发送数据）。select分支执行顺序如下：
如果select多个case的channel都准备好了，select随机选择一个case执行，其他case不会执行。
func main() {
c1 := make(chan int)
c2 := make(chan int)
c3 := make(chan int)
go func() {
c1 <- 1
}()
go func() {
c2 <- 1
}()
go func() {
c3 <- 1
}()
select {
case msg1 := <-c1:
if msg1 == 1 {
print("This is case msg1", "\n")
}
case msg2 := <-c2:
if msg2 == 1 {
print("This is case msg2", "\n")
}
case msg3 := <-c3:
if msg3 == 1 {
print("This is case msg3", "\n")
}
}
}
$ go run select_example.go
This is case msg1
如果select多个case任意channel准备好了，select选择先准备好的case执行，其他的case不会执行。
import "time"
func main() {
c1 := make(chan int)
c2 := make(chan int)
c3 := make(chan int)
go func() {
time.Sleep(time.Second * 5)
c1 <- 1
}()
go func() {
time.Sleep(time.Second * 5)
c2 <- 1
}()
go func() {
time.Sleep(time.Second * 1)
c3 <- 1
}()
select {
case msg1 := <-c1:
if msg1 == 1 {
print("This is case msg1", "\n")
}
case msg2 := <-c2:
if msg2 == 1 {
print("This is case msg2", "\n")
}
case msg3 := <-c3:
if msg3 == 1 {
print("This is case msg3", "\n")
}
}
}
$ go run select_example.go
This is case msg3
如果select多个case都没有channel准备好且有default分支，select选择default执行，其他的case不会执行。
import "time"
func main() {
c1 := make(chan int)
c2 := make(chan int)
c3 := make(chan int)
go func() {
time.Sleep(time.Second * 5)
c1 <- 1
}()
go func() {
time.Sleep(time.Second * 5)
c2 <- 1
}()
go func() {
time.Sleep(time.Second * 1)
c3 <- 1
}()
select {
case msg1 := <-c1:
if msg1 == 1 {
print("This is case msg1", "\n")
}
case msg2 := <-c2:
if msg2 == 1 {
print("This is case msg2", "\n")
}
case msg3 := <-c3:
if msg3 == 1 {
print("This is case msg3", "\n")
}
default:
print("There are no channel ready, so exit!")
}
}
$ go run select_example.go
There are no channel ready, so exit!
如果select多个case都没有channel准备好且没有default分支，select会阻塞，直到有channel准备好。
import "time"
func main() {
c1 := make(chan int)
c2 := make(chan int)
c3 := make(chan int)
go func() {
time.Sleep(time.Second * 1)
c1 <- 1
}()
go func() {
time.Sleep(time.Second * 1)
c2 <- 1
}()
go func() {
time.Sleep(time.Second * 1)
c3 <- 1
}()
select {
case msg1 := <-c1:
if msg1 == 1 {
print("This is case msg1", "\n")
}
case msg2 := <-c2:
if msg2 == 1 {
print("This is case msg2", "\n")
}
case msg3 := <-c3:
if msg3 == 1 {
print("This is case msg3", "\n")
}
}
}
$ go run select_example.go
This is case msg3
switch可以为各种类型进行分支操作，各个case分支是顺序求值的，且可以为接口类型进行分支判断（i.(type)，即类型断言）。
import "fmt"
func printArgs(args ...interface{}) { // ...interface{}在函数体内部呈现为[]interface{}
for _, v := range args {
switch v.(type) {
case int:
fmt.Println("receive int args", v)
case string:
fmt.Println("receive string args", v)
default:
fmt.Println("receive other types but not int or string")
}
}
}
func main() {
printArgs('a', "b", 1, 2, 3)
println("-----------------")
//s := []int{1, 2, 3, 4, 5}
//printArgs(s...) 此处会编译报错：panic: interface conversion: interface {} is string, not int32。这是需要注意的一个点，虽然空接口类型变量可以赋予任意类型的值，但是任意类型的切片并不能赋值给空接口类型的切片
s := []interface{}{1, 2, 3, 4, 5}
printArgs(s...)
}
$ go run switch_example.go
receive other types but not int or string
receive string args b
receive int args 1
receive int args 2
receive int args 3
-----------------
receive int args 1
receive int args 2
receive int args 3
receive int args 4
receive int args 5
关于类似于function(args ...T)中的...T写法，参见往期分析：https://www.nowcoder.com/issue/tutorial?zhuanlanId=0k81bm&uuid=070b83d4ce9a4c70be72931823bd4c01
关于select和switch中表达式的求值顺序，参见往期分析：https://www.nowcoder.com/issue/tutorial?zhuanlanId=0k81bm&uuid=cdc5bbfc7845458abc33519663ba72a2
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