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15.5 确保网页应用健壮

当网页应用的处理函数发生 panic，服务器会简单地终止运行。这可不妙：网页服务器必须是足够健壮的程序，能够承受任何可能的突发问题。

首先能想到的是在每个处理函数中使用 defer/recover()，不过这样会产生太多的重复代码。13.5 节使用闭包的错误处理模式是更优雅的方案。我们把这种机制应用到前一章的简单网页服务器上。实际上，它可以被简单地应用到任何网页服务器程序中。

为增强代码可读性，我们为页面处理函数创建一个类型：

type HandleFnc func(http.ResponseWriter, *http.Request)

我们的错误处理函数应用了 13.5 节的模式，变成了以下的 logPanics() 函数：

func logPanics(function HandleFnc) HandleFnc {
	return func(writer http.ResponseWriter, request *http.Request) {
		defer func() {
			if x := recover(); x != nil {
				log.Printf("[%v] caught panic: %v", request.RemoteAddr, x)
			}
		}()
		function(writer, request)
	}
}

然后我们用 logPanics() 来包装对处理函数的调用：

http.HandleFunc("/test1", logPanics(SimpleServer))
http.HandleFunc("/test2", logPanics(FormServer))

处理函数现在可以恢复 panic 调用，类似 13.5 节中的错误检测函数。完整代码如下：

示例 15.11 robust_webserver.go

package main

import (
	"io"
	"log"
	"net/http"
)

const form = `<html><body><form action="#" method="post" name="bar">
		<input type="text" name="in"/>
		<input type="submit" value="Submit"/>
	</form></html></body>`

type HandleFnc func(http.ResponseWriter, *http.Request)

/* handle a simple get request */
func SimpleServer(w http.ResponseWriter, request *http.Request) {
	io.WriteString(w, "<h1>hello, world</h1>")
}

/* handle a form, both the GET which displays the form
   and the POST which processes it.*/
func FormServer(w http.ResponseWriter, request *http.Request) {
	w.Header().Set("Content-Type", "text/html")
	switch request.Method {
	case "GET":
		/* display the form to the user */
		io.WriteString(w, form)
	case "POST":
		/* handle the form data, note that ParseForm must
		   be called before we can extract form data*/
		//request.ParseForm();
		//io.WriteString(w, request.Form["in"][0])
		io.WriteString(w, request.FormValue("in"))
	}
}

func main() {
	http.HandleFunc("/test1", logPanics(SimpleServer))
	http.HandleFunc("/test2", logPanics(FormServer))
	if err := http.ListenAndServe(":8088", nil); err != nil {
		panic(err)
	}
}

func logPanics(function HandleFnc) HandleFnc {
	return func(writer http.ResponseWriter, request *http.Request) {
		defer func() {
			if x := recover(); x != nil {
				log.Printf("[%v] caught panic: %v", request.RemoteAddr, x)
			}
		}()
		function(writer, request)
	}
}

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