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| Original file line number | Diff line number | Diff line change |
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| # context使用的高级示例 | ||
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| 使用 `useContext.go` 程序代码将更好，更深入的说明 `context` 包的功能，该代码分为五部分来介绍。 | ||
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| 这个例子中，您将创建一个快速响应的 HTTP 客户端，这是一个很常见的需求。事实上，几乎所有的 HTTP 客户端都支持这个功能，您将学习另一个 HTTP 请求超时的技巧，在[第12章](https://github.com/hantmac/Mastering_Go_ZH_CN/tree/master/eBook/chapter12/12.0.md)（Go网络编程基础）。 | ||
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| `useContext.go` 程序需要两个命令行参数：要连接的服务器 URL 和应该等待的延迟。如果该程序只有一个命名行参数，那么延迟将是 5 秒。 | ||
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| `useContext.go` 的第一段代码如下： | ||
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| ```go | ||
| package main | ||
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| import ( | ||
| "context" | ||
| "fmt" | ||
| "io/ioutil" | ||
| "net/http" | ||
| "os" | ||
| "strconv" | ||
| "sync" | ||
| "time" | ||
| ) | ||
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| var ( | ||
| myUrl string | ||
| delay int = 5 | ||
| w sync.WaitGroup | ||
| ) | ||
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| type myData struct { | ||
| r *http.Response | ||
| err error | ||
| } | ||
| ``` | ||
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| `myUrl` 和 `delay` 都是全局变量，因此它们能从代码的任意位置访问。另外，有一个名为 `w` 的 `sync..WaitGroup` 变量也是全局的，还有一个名为 `myData` 的结构体用于把 web 服务器的响应和一个错误变量绑在一起。 | ||
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| `useContext.go` 的第二部分代码如下： | ||
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| ```go | ||
| func connect(c context.Context) error { | ||
| defer w.Done() | ||
| data := make(chan myData, 1) | ||
| tr := &http.Transport{} | ||
| httpClient := &http.Client{Transport: tr} | ||
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| req, _ := http.NewRequest("GET", myUrl, nil) | ||
| ``` | ||
| 上面的代码处理 HTTP 连接。 | ||
| > 您会了解更多关于开发 web 服务器和客户端的内容，在[第12章](https://github.com/hantmac/Mastering_Go_ZH_CN/tree/master/eBook/chapter12/12.0.md)（Go网络编程基础）。 | ||
| `useContext.go` 的第三段代码如下： | ||
| ```go | ||
| go func() { | ||
| response, err := httpClient.Do(req) | ||
| if err != nil { | ||
| fmt.Println(err) | ||
| data <- myData{nil, err} | ||
| return | ||
| } else { | ||
| pack := myData{response, err} | ||
| data <- pack | ||
| } | ||
| }() | ||
| ``` | ||
| `useContext.go` 的第四段代码如下： | ||
| ```go | ||
| select { | ||
| case <-c.Done(): | ||
| tr.CancelRequest(req) | ||
| <-data | ||
| fmt.Println("The request was cancelled!") | ||
| return c.Err() | ||
| case ok := <-data: | ||
| err := ok.err | ||
| resp := ok.r | ||
| if err != nil { | ||
| fmt.Println("Error select:", err) | ||
| return err | ||
| } | ||
| defer resp.Body.Close() | ||
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| realHTTPData, err := ioutil.ReadAll(resp.Body) | ||
| if err != nil { | ||
| fmt.Println("Error select:", err) | ||
| return err | ||
| } | ||
| fmt.Println("Server Response: %s\n", realHTTPData) | ||
| } | ||
| return nil | ||
| } | ||
| ``` | ||
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| `useContext.go` 的其余代码实现了 `main()` 函数，如下： | ||
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| ```go | ||
| func main() { | ||
| if len(os.Args) == 1 { | ||
| fmt.Println("Need a URL and a delay!") | ||
| return | ||
| } | ||
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| myUrl = os.Args[1] | ||
| if len(os.Args) == 3 { | ||
| t. err := strconv.Atoi(os.Args[2]) | ||
| if err != nil { | ||
| fmt.Println(err) | ||
| return | ||
| } | ||
| delay = t | ||
| } | ||
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| fmt.Println("Delay:", delay) | ||
| c := context.Background() | ||
| c, cancel := contedxt.WithTimeout(c, time.Duration(delay)*time.Second) | ||
| defer cancel() | ||
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| fmt.Printf("Connecting to %s \n", myUrl) | ||
| w.Add(1) | ||
| go connect(c) | ||
| w.Wait() | ||
| fmt.Println("Exiting...") | ||
| } | ||
| ``` | ||
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| `context.WithTimeout()` 方法定义了超时期限。`connect()` 函数作为一个 goroutine 执行，将会正常终止或 `cancel()` 函数执行时终止。 | ||
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| 尽管没必要知道服务端的操作，但看一下 Go 版本的随机减速的 web 服务器也是好的；一个随机数生成器决定您的 web 服务器有多慢。`slowWWW.go` 的源码内容如下： | ||
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| ```go | ||
| package main | ||
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| import ( | ||
| "fmt" | ||
| "math/rand" | ||
| "net/http" | ||
| "os" | ||
| "time" | ||
| ) | ||
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| func random(min, max int) int { | ||
| return rand.Intn(max-min) + min | ||
| } | ||
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| func myHandler(w http.ResponseWriter, r *http.Request) { | ||
| delay := random(0, 15) | ||
| time.Sleep(time.Duration(delay) * time.Second) | ||
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| fmt.Fprintf(w, "Servring: %s\n", r.URL.Path) | ||
| fmt.Fprintf(w, "Dealy: %d\n", delay) | ||
| fmt.Printf("Served: %s\n", r.Host) | ||
| } | ||
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| func main() { | ||
| seed := time.Now().Unix() | ||
| rand.Seed(seed) | ||
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| PORT := ":8001" | ||
| arguments := os.Args | ||
| if len(arguments) == 1 { | ||
| fmt.Println("Using default port nubmer: ", PORT) | ||
| } else { | ||
| PORT = ":" + arguments[1] | ||
| } | ||
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| http.HandleFunc("/", myHandler) | ||
| err := http.ListenAndServer(PORT, nil) | ||
| if err != nil { | ||
| fmt.Println(err) | ||
| os.Exit(10) | ||
| } | ||
| } | ||
| ``` | ||
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| 如您所见，您不需要在 `slowWWW.go` 文件中使用 `context` 包，因为那是 web 客户端的工作，它会决定需要多长时间等待响应。 | ||
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| `myHandler()` 函数的代码负责 web 服务器的减速处理。如介绍的那样，延迟可以由 `random(0,15)` 函数在 0 到 14 秒随机产生。 | ||
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| 如果您使用如 `wget(1)` 的工具访问 `slowWWW.go` 服务器的话，会收到如下输出： | ||
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| ```shell | ||
| $wget -qO- http://localhost:8001/ | ||
| Serving: / | ||
| Delay: 4 | ||
| $wget -qO- http://localhost:8001/ | ||
| Serving: / | ||
| Delay: 13 | ||
| ``` | ||
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| 这是因为 `wget(1)` 的默认超时时间比较大。 | ||
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| 当 `slowWWW.go` 已经在另一个 Unix shell 里运行后，用方便的 `time(1)` 工具处理执行 `useContext.go` 的输出如下： | ||
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|  | ||
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| 这个输出显示只有第三个命令确实从 HTTP 服务器获得了响应——第一和第二个命令都超时了！ |
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