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