golang 的 select 就是監(jiān)聽 IO 操作，當 IO 操作發(fā)生時，觸發(fā)相應的動作。?

在執(zhí)行select語句的時候，運行時系統(tǒng)會自上而下地判斷每個case中的發(fā)送或接收操作是否可以被立即執(zhí)行(立即執(zhí)行：意思是當前Goroutine不會因此操作而被阻塞)

select的用法與switch非常類似，由select開始一個新的選擇塊，每個選擇條件由case語句來描述。與switch語句可以選擇任何可使用相等比較的條件相比，select有比較多的限制，其中最大的一條限制就是每個case語句里必須是一個IO操作，確切的說，應該是一個面向channel的IO操作。

下面這段話來自官方文檔：

A "select" statement chooses which of a set of possible?send?or?receive?operations will proceed. It looks similar to a?"switch"?statement but with the cases all referring to communication operations.

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語法格式如下：

select {

case?SendStmt:

? //statements

case?RecvStmt:

? //statements

default:

? //statements

}

其中，

SendStmt : channelVariable <- value

RecvStmt : variable <-channelVariable

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A case with a RecvStmt may assign the result of a RecvExpr to one or two variables, which may be declared using a?short variable declaration(IdentifierList := value). The RecvExpr must be a (possibly parenthesized) receive operation(<-channelVariable). There can be at most one default case and it may appear anywhere in the list of cases.

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示例：

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????ch1 := make(chan int, 1) ????ch2 := make(chan int, 1) ????ch1 <- 1 ????select { ????case e1 := <-ch1: ????????//如果ch1通道成功讀取數(shù)據(jù)，則執(zhí)行該case處理語句 ????????fmt.Printf("1th case is selected. e1=%v", e1) ????case e2 := <-ch2: ????????//如果ch2通道成功讀取數(shù)據(jù)，則執(zhí)行該case處理語句 ????????fmt.Printf("2th case is selected. e2=%v", e2) ????default: ????????//如果上面case都沒有成功，則進入default處理流程 ????????fmt.Println("default!.") ????}

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Execution of a "select" statement proceeds in several steps:

1、For all the cases in the statement, the channel operands of receive operations and the channel and right-hand-side expressions of send statements are evaluated exactly once, in source order, upon entering the "select" statement.(所有channel表達式都會被求值、所有被發(fā)送的表達式都會被求值。求值順序：自上而下、從左到右)

2、If one or more of the communications can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection. Otherwise, if there is a default case, that case is chosen. If there is no default case, the "select" statement blocks until at least one of the communications can proceed.（如果有一個或多個IO操作可以完成，則Go運行時系統(tǒng)會隨機的選擇一個執(zhí)行，否則的話，如果有default分支，則執(zhí)行default分支語句，如果連default都沒有，則select語句會一直阻塞，直到至少有一個IO操作可以進行）

3、Unless the selected case is the default case, the respective communication operation is executed.

4、If the selected case is a RecvStmt with a short variable declaration or an assignment, the left-hand side expressions are evaluated and the received value (or values) are assigned.

5、The statement list of the selected case is executed.

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Since communication on?nil?channels can never proceed, a select with only?nil?channels and no default case blocks forever.

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可以使用break語句來終止select語句的執(zhí)行。

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示例1：select語句會一直等待，直到某個case里的IO操作可以進行

//main.go

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package main import "fmt" import "time" func f1(ch chan int) { ????time.Sleep(time.Second * 5) ????ch <- 1 } func f2(ch chan int) { ????time.Sleep(time.Second * 10) ????ch <- 1 } func main() { ????var ch1 = make(chan int) ????var ch2 = make(chan int) ????go f1(ch1) ????go f2(ch2) ????select { ????case <-ch1: ????????fmt.Println("The first case is selected.") ????case <-ch2: ????????fmt.Println("The second case is selected.") ????} }

編譯運行：

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C:/go/bin/go.exe run test14.go [E:/project/go/proj/src/test]

The first case is selected.

成功: 進程退出代碼 0.

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示例2：所有跟在case關鍵字右邊的發(fā)送語句或接收語句中的通道表達式和元素表達式都會先被求值。無論它們所在的case是否有可能被選擇都會這樣。?

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求值順序：自上而下、從左到右?

此示例使用空值channel進行驗證。

//main.go

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package main import ( ????"fmt" ) //定義幾個變量，其中chs和numbers分別代表通道列表和整數(shù)列表 var ch1 chan int var ch2 chan int var chs = []chan int{ch1, ch2} var numbers = []int{1, 2, 3, 4, 5} func main() { ????select { ????case getChan(0) <- getNumber(2): ????????fmt.Println("1th case is selected.") ????case getChan(1) <- getNumber(3): ????????fmt.Println("2th case is selected.") ????default: ????????fmt.Println("default!.") ????} } func getNumber(i int) int { ????fmt.Printf("numbers[%d]\n", i) ????return numbers[i] } func getChan(i int) chan int { ????fmt.Printf("chs[%d]\n", i) ????return chs[i] }

編譯運行：

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C:/go/bin/go.exe run test4.go [E:/project/go/proj/src/test]

chs[0]

numbers[2]

chs[1]

numbers[3]

default!.

成功: 進程退出代碼 0.

上面的案例，之所以輸出default!.是因為chs[0]和chs[1]都是空值channel，和空值channel通信永遠都不會成功。

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示例3：所有跟在case關鍵字右邊的發(fā)送語句或接收語句中的通道表達式和元素表達式都會先被求值。無論它們所在的case是否有可能被選擇都會這樣。?

求值順序：自上而下、從左到右?

此示例使用非空值channel進行驗證。

//main.go

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package main import ( ????"fmt" ) //定義幾個變量，其中chs和numbers分別代表通道列表和整數(shù)列表 var ch1 chan int = make(chan int, 1) //聲明并初始化channel變量 var ch2 chan int = make(chan int, 1) //聲明并初始化channel變量 var chs = []chan int{ch1, ch2} var numbers = []int{1, 2, 3, 4, 5} func main() { ????select { ????case getChan(0) <- getNumber(2): ????????fmt.Println("1th case is selected.") ????case getChan(1) <- getNumber(3): ????????fmt.Println("2th case is selected.") ????default: ????????fmt.Println("default!.") ????} } func getNumber(i int) int { ????fmt.Printf("numbers[%d]\n", i) ????return numbers[i] } func getChan(i int) chan int { ????fmt.Printf("chs[%d]\n", i) ????return chs[i] } 編譯運行：

C:/go/bin/go.exe run test4.go [E:/project/go/proj/src/test]

chs[0]

numbers[2]

chs[1]

numbers[3]

1th case is selected.

成功: 進程退出代碼 0.

此示例，使用非空值channel進行IO操作，所以可以成功，沒有走default分支。

示例4：如果有多個case同時可以運行，go會隨機選擇一個case執(zhí)行 //main.go package main import ( ????"fmt" ) func main() { ????chanCap := 5 ????ch := make(chan int, chanCap) //創(chuàng)建channel，容量為5 ????for i := 0; i < chanCap; i++ { //通過for循環(huán)，向channel里填滿數(shù)據(jù) ????????select { //通過select隨機的向channel里追加數(shù)據(jù) ????????case ch <- 1: ????????case ch <- 2: ????????case ch <- 3: ????????} ????} ????for i := 0; i < chanCap; i++ { ????????fmt.Printf("%v\n", <-ch) ????} }

編譯運行：

C:/go/bin/go.exe run test5.go [E:/project/go/proj/src/test]

2

1

2

1

1

成功: 進程退出代碼 0.

注意：上面的案例每次運行結果都不一樣。

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示例5：使用break終止select語句的執(zhí)行

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package main import "fmt" func main() { ????var ch = make(chan int, 1) ????ch <- 1 ????select { ????case <-ch: ????????fmt.Println("This case is selected.") ????????break //The following statement in this case will not execute. ????????fmt.Println("After break statement") ????default: ????????fmt.Println("This is the default case.") ????} ????fmt.Println("After select statement.") }

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編譯運行：

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C:/go/bin/go.exe run test15.go [E:/project/go/proj/src/test]

This case is selected.

After select statement.

成功: 進程退出代碼 0.

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