Go语言中的select语句怎么使用?用法示例解析

2021年4月5日16:04:43 发表评论 850 次浏览

Go语言中, select语句就像switch语句, 但在select语句中, case语句指的是通信, 即通道上的发送或接收操作。

语法如下:

select{

case SendOrReceive1: // Statement
case SendOrReceive2: // Statement
case SendOrReceive3: // Statement
.......
default: // Statement

要点:

Select语句等待, 直到准备开始进行某些通信(发送或接收操作)为止。

例子:

// Go program to illustrate the
// concept of select statement
package main
   
import( "fmt"
  "time" )
       
     // function 1
     func portal1(channel1 chan string) {
  
         time .Sleep(3* time .Second)
         channel1 <- "Welcome to channel 1"
     }
       
     // function 2
      func portal2(channel2 chan string) {
  
         time .Sleep(9* time .Second)
         channel2 <- "Welcome to channel 2"
     }
   
// main function
func main(){
       
     // Creating channels
    R1:= make(chan string)
    R2:= make(chan string)
      
    // calling function 1 and 
    // function 2 in goroutine
    go portal1(R1)
    go portal2(R2)
  
    select{
  
         // case 1 for portal 1
        case op1:= <- R1:
        fmt.Println(op1)
   
        // case 2 for portal 2
        case op2:= <- R2:
        fmt.Println(op2)
    }
      
}

输出如下:

Welcome to channel 1

说明:在上面的程序中, 门户网站1的睡眠时间为3秒钟, 门户网站2的睡眠时间为9秒钟, 经过它们的睡眠时间后, 他们将准备继续。现在, select语句一直等到他们的睡眠时间, 当门户2唤醒时, 它选择情况2并打印" Welcome to channel 1"。如果门户1在门户2之前醒来, 则输出为"欢迎使用通道2"。

如果select语句不包含任何case语句, 则该select语句将永远等待。

语法如下:

select{}

例子:

// Go program to illustrate the
// concept of select statement
package main
  
// main function
func main() {
      
     // Select statement 
    // without any case
    select{ }
  
     
}

输出如下:

fatal error: all goroutines are asleep - deadlock!

goroutine 1 [select (no cases)]:
main.main()
    /home/runner/main.go:9 +0x20
exit status 2

select语句中的默认语句用于保护select语句不被阻塞。当没有case语句准备进行时, 将执行此语句。

例子:

// Go program to illustrate the
// concept of select statement
package main
  
import "fmt"
  
// main function
func main() {
      
     // creating channel
     mychannel:= make(chan int )
    select{
      case <- mychannel: 
       
      default :fmt.Println( "Not found" )
}   
}

输出如下:

Not found

select语句的阻塞意味着当没有case语句准备好并且select语句不包含任何默认语句时, 则select语句将阻塞, 直到至少一个case语句或通信可以继续。

例子:

// Go program to illustrate the
// concept of select statement
package main
  
// main function
func main() {
      
     // creating channel
     mychannel:= make(chan int )
      
     // channel is not ready 
    // and no default case
    select{
        case <- mychannel:
         
    }
}

输出如下:

fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.main()
    /home/runner/main.go:14 +0x52
exit status 2

在select语句中, 如果准备好处理多种情况, 则可以随机选择其中一种。

例子:

// Go program to illustrate the
// concept of select statement
package main
  
import "fmt"
      
      
     // function 1
     func portal1(channel1 chan string){
         for i := 0; i <= 3; i++{
             channel1 <- "Welcome to channel 1"
         }
          
     }
      
     // function 2
      func portal2(channel2 chan string){
         channel2 <- "Welcome to channel 2"
     }
  
// main function
func main() {
      
     // Creating channels
    R1:= make(chan string)
    R2:= make(chan string)
     
    // calling function 1 and 
    // function 2 in goroutine
    go portal1(R1)
    go portal2(R2)
     
    // the choice of selection 
    // of case is random
    select{
        case op1:= <- R1:
        fmt.Println(op1)
        case op2:= <- R2:
        fmt.Println(op2)
    }
}

输出如下:

Welcome to channel 2

木子山

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