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2023-06-26 17:30:52 +00:00
< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface-zh.html" > 译者序< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< /
2022-08-04 06:59:33 +00:00
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2024-06-12 07:20:07 +00:00
< li > < em > Go语言圣经读书笔记(挖坑中...): < a href = "https://github.com/chai2010/gopl-notes-zh" > https://github.com/chai2010/gopl-notes-zh< / a > < / em > < / li >
< li > < em > 凹语言(Go实现, 面向WASM设计): < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2023-06-26 17:46:27 +00:00
< / ul >
2022-08-04 06:59:33 +00:00
< hr >
< h2 id = "84-channels" > < a class = "header" href = "#84-channels" > 8.4. Channels< / a > < / h2 >
< p > 如果说goroutine是Go语言程序的并发体的话， ， ， < / p >
< p > 使用内置的make函数， ： < / p >
< pre > < code class = "language-Go" > ch := make(chan int) // ch has type 'chan int'
< / code > < / pre >
< p > 和map类似， ， ， ， < / p >
< p > 两个相同类型的channel可以使用==运算符比较。如果两个channel引用的是相同的对象， < / p >
< p > 一个channel有发送和接受两个主要操作， < code > < -< / code > 运算符。在发送语句中，< code > < -< / code > 运算符分割channel和要发送的值。在接收语句中， < code > < -< / code > 运算符写在channel对象之前。一个不使用接收结果的接收操作也是合法的。< / p >
< pre > < code class = "language-Go" > ch < - x // a send statement
x = < -ch // a receive expression in an assignment statement
< -ch // a receive statement; result is discarded
< / code > < / pre >
< p > Channel还支持close操作， ， ； < / p >
< p > 使用内置的close函数就可以关闭一个channel： < / p >
< pre > < code class = "language-Go" > close(ch)
< / code > < / pre >
< p > 以最简单方式调用make函数创建的是一个无缓存的channel， ， ， < / p >
< pre > < code class = "language-Go" > ch = make(chan int) // unbuffered channel
ch = make(chan int, 0) // unbuffered channel
ch = make(chan int, 3) // buffered channel with capacity 3
< / code > < / pre >
< p > 我们将先讨论无缓存的channel， < / p >
< h3 id = "841-不带缓存的channels" > < a class = "header" href = "#841-不带缓存的channels" > 8.4.1. 不带缓存的Channels< / a > < / h3 >
< p > 一个基于无缓存Channels的发送操作将导致发送者goroutine阻塞， ， ， ， ， ， < / p >
< p > 基于无缓存Channels的发送和接收操作将导致两个goroutine做一次同步操作。因为这个原因， ， （ ： < em > happens before< / em > ， ！ ） < / p >
< p > 在讨论并发编程时， （ < em > happens before< / em > ） ， ； ， ， < / p >
< p > 当我们说x事件既不是在y事件之前发生也不是在y事件之后发生， ， ， ， ， < / p >
< p > 在8.3节的客户端程序， （ ： ） ， ， ， ： < / p >
< p > < u > < i > gopl.io/ch8/netcat3< / i > < / u > < / p >
< pre > < code class = "language-Go" > func main() {
conn, err := net.Dial(" tcp" , " localhost:8000" )
if err != nil {
log.Fatal(err)
}
done := make(chan struct{})
go func() {
io.Copy(os.Stdout, conn) // NOTE: ignoring errors
log.Println(" done" )
done < - struct{}{} // signal the main goroutine
}()
mustCopy(conn, os.Stdin)
conn.Close()
< -done // wait for background goroutine to finish
}
< / code > < / pre >
< p > 当用户关闭了标准输入， ， （ ） （ ） ， ； （ ， ） < / p >
< p > 在后台goroutine返回之前， ， ， < / p >
< p > 基于channels发送消息有两个重要方面。首先每个消息都有一个值， ， < strong > 消息事件< / strong > 。有些消息事件并不携带额外的信息， ， < code > struct{}< / code > 空结构体作为channels元素的类型， ， < code > done < - 1< / code > 语句也比< code > done < - struct{}{}< / code > 更短。< / p >
< p > < strong > 练习 8.3： < / strong > 在netcat3例子中， ， < code > *net.TCPConn< / code > ， ， ， ， （ ； < strong > 练习 8.4< / strong > 。）< / p >
< h3 id = "842-串联的channelspipeline" > < a class = "header" href = "#842-串联的channelspipeline" > 8.4.2. 串联的Channels（ ） < / a > < / h3 >
< p > Channels也可以用于将多个goroutine连接在一起， （ ） ， < / p >
< p > < img src = "../images/ch8-01.png" alt = "" / > < / p >
< p > 第一个goroutine是一个计数器， ， ； ， ， ； ， ， ， ， < / p >
< p > < u > < i > gopl.io/ch8/pipeline1< / i > < / u > < / p >
< pre > < code class = "language-Go" > func main() {
naturals := make(chan int)
squares := make(chan int)
// Counter
go func() {
for x := 0; ; x++ {
naturals < - x
}
}()
// Squarer
go func() {
for {
x := < -naturals
squares < - x * x
}
}()
// Printer (in main goroutine)
for {
fmt.Println(< -squares)
}
}
< / code > < / pre >
< p > 如您所料， （ ） ， ， ， ？ < / p >
< p > 如果发送者知道， ， ， ： < / p >
< pre > < code class = "language-Go" > close(naturals)
< / code > < / pre >
< p > 当一个channel被关闭后， ， ， ， ， < / p >
< p > 没有办法直接测试一个channel是否被关闭， ： ， ， ， ， ， ， < / p >
< pre > < code class = "language-Go" > // Squarer
go func() {
for {
x, ok := < -naturals
if !ok {
break // channel was closed and drained
}
squares < - x * x
}
close(squares)
}()
< / code > < / pre >
< p > 因为上面的语法是笨拙的， ， ， ， < / p >
< p > 在下面的改进中， ， ， （ ， ） ， < / p >
< p > < u > < i > gopl.io/ch8/pipeline2< / i > < / u > < / p >
< pre > < code class = "language-Go" > func main() {
naturals := make(chan int)
squares := make(chan int)
// Counter
go func() {
for x := 0; x < 100; x++ {
naturals < - x
}
close(naturals)
}()
// Squarer
go func() {
for x := range naturals {
squares < - x * x
}
close(squares)
}()
// Printer (in main goroutine)
for x := range squares {
fmt.Println(x)
}
}
< / code > < / pre >
< p > 其实你并不需要关闭每一个channel。只有当需要告诉接收者goroutine， ， （ ， ） < / p >
< p > 试图重复关闭一个channel将导致panic异常， ， < / p >
< h3 id = "843-单方向的channel" > < a class = "header" href = "#843-单方向的channel" > 8.4.3. 单方向的Channel< / a > < / h3 >
< p > 随着程序的增长， ， ， ： < / p >
< pre > < code class = "language-Go" > func counter(out chan int)
func squarer(out, in chan int)
func printer(in chan int)
< / code > < / pre >
< p > 其中计算平方的squarer函数在两个串联Channels的中间， ， ， ： ， ， < / p >
< p > 这种场景是典型的。当一个channel作为一个函数参数时， < / p >
< p > 为了表明这种意图并防止被滥用， ， < code > chan< - int< / code > 表示一个只发送int的channel， ， < code > < -chan int< / code > 表示一个只接收int的channel， （ < code > < -< / code > 和关键字chan的相对位置表明了channel的方向。） < / p >
< p > 因为关闭操作只用于断言不再向channel发送新的数据， ， < / p >
< p > 这是改进的版本， ： < / p >
< p > < u > < i > gopl.io/ch8/pipeline3< / i > < / u > < / p >
< pre > < code class = "language-Go" > func counter(out chan< - int) {
for x := 0; x < 100; x++ {
out < - x
}
close(out)
}
func squarer(out chan< - int, in < -chan int) {
for v := range in {
out < - v * v
}
close(out)
}
func printer(in < -chan int) {
for v := range in {
fmt.Println(v)
}
}
func main() {
naturals := make(chan int)
squares := make(chan int)
go counter(naturals)
go squarer(squares, naturals)
printer(squares)
}
< / code > < / pre >
< p > 调用counter（ ） ， < - int。调用printer(squares)也会导致相似的隐式转换，这一次是转换为< code > < -chan int< / code > 类型只接收型的channel。任何双向channel向单向channel变量的赋值操作都将导致该隐式转换。这里并没有反向转换的语法： < code > chan< - int< / code > 类型的单向型的channel转换为< code > chan int< / code > 类型的双向型的channel。< / p >
< h3 id = "844-带缓存的channels" > < a class = "header" href = "#844-带缓存的channels" > 8.4.4. 带缓存的Channels< / a > < / h3 >
< p > 带缓存的Channel内部持有一个元素队列。队列的最大容量是在调用make函数创建channel时通过第二个参数指定的。下面的语句创建了一个可以持有三个字符串元素的带缓存Channel。图8.2是ch变量对应的channel的图形表示形式。< / p >
< pre > < code class = "language-Go" > ch = make(chan string, 3)
< / code > < / pre >
< p > < img src = "../images/ch8-02.png" alt = "" / > < / p >
< p > 向缓存Channel的发送操作就是向内部缓存队列的尾部插入元素， ， ， ， < / p >
< p > 我们可以在无阻塞的情况下连续向新创建的channel发送三个值： < / p >
< pre > < code class = "language-Go" > ch < - " A"
ch < - " B"
ch < - " C"
< / code > < / pre >
< p > 此刻， （ < / p >
< p > < img src = "../images/ch8-03.png" alt = "" / > < / p >
< p > 如果我们接收一个值，< / p >
< pre > < code class = "language-Go" > fmt.Println(< -ch) // " A"
< / code > < / pre >
< p > 那么channel的缓存队列将不是满的也不是空的（ ） ， ， < / p >
< p > < img src = "../images/ch8-04.png" alt = "" / > < / p >
< p > 在某些特殊情况下， ， ： < / p >
< pre > < code class = "language-Go" > fmt.Println(cap(ch)) // " 3"
< / code > < / pre >
< p > 同样， ， ， ， < / p >
< pre > < code class = "language-Go" > fmt.Println(len(ch)) // " 2"
< / code > < / pre >
< p > 在继续执行两次接收操作后channel内部的缓存队列将又成为空的， ： < / p >
< pre > < code class = "language-Go" > fmt.Println(< -ch) // " B"
fmt.Println(< -ch) // " C"
< / code > < / pre >
< p > 在这个例子中， ， ， ， ， ， ， < / p >
< p > 下面的例子展示了一个使用了带缓存channel的应用。它并发地向三个镜像站点发出请求， ， ， （ ， ， ） < / p >
< pre > < code class = "language-Go" > func mirroredQuery() string {
responses := make(chan string, 3)
go func() { responses < - request(" asia.gopl.io" ) }()
go func() { responses < - request(" europe.gopl.io" ) }()
go func() { responses < - request(" americas.gopl.io" ) }()
return < -responses // return the quickest response
}
func request(hostname string) (response string) { /* ... */ }
< / code > < / pre >
< p > 如果我们使用了无缓存的channel， ， ， ， ， < / p >
< p > 关于无缓存或带缓存channels之间的选择， ， ； ， ， ， ， < / p >
< p > Channel的缓存也可能影响程序的性能。想象一家蛋糕店有三个厨师， ， ， ， ； < / p >
< p > 如果在每个厨师之间有一个放置一个蛋糕的额外空间， ； ， ， ， ， < / p >
< p > 另一方面，如果生产线的前期阶段一直快于后续阶段，那么它们之间的缓存在大部分时间都将是满的。相反，如果后续阶段比前期阶段更快，那么它们之间的缓存在大部分时间都将是空的。对于这类场景，额外的缓存并没有带来任何好处。< / p >
< p > 生产线的隐喻对于理解channels和goroutines的工作机制是很有帮助的。例如， ， ， ， ， < / p >
< p > 我们没有太多的空间展示全部细节， ， （ ） < / p >
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