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< ol class = "chapter" >
< li class = "chapter-item expanded " >
< a href = "../index.html" > Go语言圣经< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../preface-zh.html" > 译者序< / a >
< / li >
< li class = "chapter-item expanded " >
< 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 >
< 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 class = "chapter-item expanded " >
< a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a >
< / 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 class = "chapter-item expanded " >
< a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a >
< / 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 class = "chapter-item expanded " >
< a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a >
< / 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< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch4/ch4-06.html" > < strong aria-hidden = "true" > 4.6.< / strong > 文本和HTML模板< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5.html" > < strong aria-hidden = "true" > 5.< / strong > 函数< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-01.html" > < strong aria-hidden = "true" > 5.1.< / strong > 函数声明< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-02.html" > < strong aria-hidden = "true" > 5.2.< / strong > 递归< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-03.html" > < strong aria-hidden = "true" > 5.3.< / strong > 多返回值< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-04.html" > < strong aria-hidden = "true" > 5.4.< / strong > 错误< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-05.html" > < strong aria-hidden = "true" > 5.5.< / strong > 函数值< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-06.html" > < strong aria-hidden = "true" > 5.6.< / strong > 匿名函数< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-07.html" > < strong aria-hidden = "true" > 5.7.< / strong > 可变参数< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-08.html" > < strong aria-hidden = "true" > 5.8.< / strong > Deferred函数< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-09.html" > < strong aria-hidden = "true" > 5.9.< / strong > Panic异常< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-10.html" > < strong aria-hidden = "true" > 5.10.< / strong > Recover捕获异常< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6.html" > < strong aria-hidden = "true" > 6.< / strong > 方法< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-01.html" > < strong aria-hidden = "true" > 6.1.< / strong > 方法声明< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-02.html" > < strong aria-hidden = "true" > 6.2.< / strong > 基于指针对象的方法< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-03.html" > < strong aria-hidden = "true" > 6.3.< / strong > 通过嵌入结构体来扩展类型< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-04.html" > < strong aria-hidden = "true" > 6.4.< / strong > 方法值和方法表达式< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-05.html" > < strong aria-hidden = "true" > 6.5.< / strong > 示例: Bit数组< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-06.html" > < strong aria-hidden = "true" > 6.6.< / strong > 封装< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7.html" > < strong aria-hidden = "true" > 7.< / strong > 接口< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-01.html" > < strong aria-hidden = "true" > 7.1.< / strong > 接口是合约< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-02.html" > < strong aria-hidden = "true" > 7.2.< / strong > 接口类型< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-03.html" > < strong aria-hidden = "true" > 7.3.< / strong > 实现接口的条件< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-04.html" > < strong aria-hidden = "true" > 7.4.< / strong > flag.Value接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-05.html" > < strong aria-hidden = "true" > 7.5.< / strong > 接口值< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-06.html" > < strong aria-hidden = "true" > 7.6.< / strong > sort.Interface接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-07.html" > < strong aria-hidden = "true" > 7.7.< / strong > http.Handler接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-08.html" > < strong aria-hidden = "true" > 7.8.< / strong > error接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-09.html" > < strong aria-hidden = "true" > 7.9.< / strong > 示例: 表达式求值< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-10.html" > < strong aria-hidden = "true" > 7.10.< / strong > 类型断言< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-11.html" > < strong aria-hidden = "true" > 7.11.< / strong > 基于类型断言识别错误类型< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-12.html" > < strong aria-hidden = "true" > 7.12.< / strong > 通过类型断言查询接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-13.html" > < strong aria-hidden = "true" > 7.13.< / strong > 类型分支< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-14.html" > < strong aria-hidden = "true" > 7.14.< / strong > 示例: 基于标记的XML解码< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-15.html" > < strong aria-hidden = "true" > 7.15.< / strong > 补充几点< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8.html" > < strong aria-hidden = "true" > 8.< / strong > Goroutines和Channels< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-01.html" > < strong aria-hidden = "true" > 8.1.< / strong > Goroutines< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-02.html" > < strong aria-hidden = "true" > 8.2.< / strong > 示例: 并发的Clock服务< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-03.html" > < strong aria-hidden = "true" > 8.3.< / strong > 示例: 并发的Echo服务< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-04.html" class = "active" > < strong aria-hidden = "true" > 8.4.< / strong > Channels< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-05.html" > < strong aria-hidden = "true" > 8.5.< / strong > 并发的循环< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-06.html" > < strong aria-hidden = "true" > 8.6.< / strong > 示例: 并发的Web爬虫< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-07.html" > < strong aria-hidden = "true" > 8.7.< / strong > 基于select的多路复用< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-08.html" > < strong aria-hidden = "true" > 8.8.< / strong > 示例: 并发的目录遍历< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-09.html" > < strong aria-hidden = "true" > 8.9.< / strong > 并发的退出< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-10.html" > < strong aria-hidden = "true" > 8.10.< / strong > 示例: 聊天服务< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9.html" > < strong aria-hidden = "true" > 9.< / strong > 基于共享变量的并发< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-01.html" > < strong aria-hidden = "true" > 9.1.< / strong > 竞争条件< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-02.html" > < strong aria-hidden = "true" > 9.2.< / strong > sync.Mutex互斥锁< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-03.html" > < strong aria-hidden = "true" > 9.3.< / strong > sync.RWMutex读写锁< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-04.html" > < strong aria-hidden = "true" > 9.4.< / strong > 内存同步< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-05.html" > < strong aria-hidden = "true" > 9.5.< / strong > sync.Once惰性初始化< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-06.html" > < strong aria-hidden = "true" > 9.6.< / strong > 竞争条件检测< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-07.html" > < strong aria-hidden = "true" > 9.7.< / strong > 示例: 并发的非阻塞缓存< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-08.html" > < strong aria-hidden = "true" > 9.8.< / strong > Goroutines和线程< / a >
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< / ol >
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< a href = "../ch10/ch10.html" > < strong aria-hidden = "true" > 10.< / strong > 包和工具< / a >
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< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-01.html" > < strong aria-hidden = "true" > 10.1.< / strong > 包简介< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-02.html" > < strong aria-hidden = "true" > 10.2.< / strong > 导入路径< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-03.html" > < strong aria-hidden = "true" > 10.3.< / strong > 包声明< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-04.html" > < strong aria-hidden = "true" > 10.4.< / strong > 导入声明< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-05.html" > < strong aria-hidden = "true" > 10.5.< / strong > 包的匿名导入< / a >
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< a href = "../ch10/ch10-06.html" > < strong aria-hidden = "true" > 10.6.< / strong > 包和命名< / a >
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< a href = "../ch10/ch10-07.html" > < strong aria-hidden = "true" > 10.7.< / strong > 工具< / a >
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< a href = "../ch11/ch11.html" > < strong aria-hidden = "true" > 11.< / strong > 测试< / a >
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< a href = "../ch11/ch11-01.html" > < strong aria-hidden = "true" > 11.1.< / strong > go test< / a >
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< a href = "../ch11/ch11-02.html" > < strong aria-hidden = "true" > 11.2.< / strong > 测试函数< / a >
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< a href = "../ch11/ch11-03.html" > < strong aria-hidden = "true" > 11.3.< / strong > 测试覆盖率< / a >
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< a href = "../ch11/ch11-04.html" > < strong aria-hidden = "true" > 11.4.< / strong > 基准测试< / a >
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< a href = "../ch11/ch11-05.html" > < strong aria-hidden = "true" > 11.5.< / strong > 剖析< / a >
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< a href = "../ch11/ch11-06.html" > < strong aria-hidden = "true" > 11.6.< / strong > 示例函数< / a >
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< main >
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< ul dir = "auto" > < 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 > < li > < em > MnBook(Go语言实现的MD电子书构建工具): < a href = "https://github.com/wa-lang/mnbook" > https://github.com/wa-lang/mnbook< / a > < / em > < / li > < / ul > < hr >
< h2 > 8.4. Channels< / h2 >
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< 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 >
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< h3 > 8.4.1. 不带缓存的Channels< / h3 >
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< 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 >
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< p > <!-- raw HTML omitted --> <!-- raw HTML omitted --> gopl.io/ch8/netcat3<!-- raw HTML omitted --> <!-- raw HTML omitted --> < / p >
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< 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 >
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< h3 > 8.4.2. 串联的Channels（ ） < / h3 >
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< p > Channels也可以用于将多个goroutine连接在一起， （ ） ， < / p >
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< p > < img src = "../images/ch8-01.png" alt = "" > < / p >
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< p > 第一个goroutine是一个计数器， ， ； ， ， ； ， ， ， ， < / p >
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< p > <!-- raw HTML omitted --> <!-- raw HTML omitted --> gopl.io/ch8/pipeline1<!-- raw HTML omitted --> <!-- raw HTML omitted --> < / p >
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< 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 >
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< p > <!-- raw HTML omitted --> <!-- raw HTML omitted --> gopl.io/ch8/pipeline2<!-- raw HTML omitted --> <!-- raw HTML omitted --> < / p >
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< 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 >
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< h3 > 8.4.3. 单方向的Channel< / h3 >
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< 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 >
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< p > <!-- raw HTML omitted --> <!-- raw HTML omitted --> gopl.io/ch8/pipeline3<!-- raw HTML omitted --> <!-- raw HTML omitted --> < / p >
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< 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 >
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< h3 > 8.4.4. 带缓存的Channels< / h3 >
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< 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 >
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< p > < img src = "../images/ch8-02.png" alt = "" > < / p >
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< 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 >
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< p > < img src = "../images/ch8-03.png" alt = "" > < / p >
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< 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 >
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< p > < img src = "../images/ch8-04.png" alt = "" > < / p >
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< 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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