Google资深工程师深度讲解Go语言错误处理和资源管理(七)

一.defer调用:实现资源管理

• 确保调用在函数结束时发生
• 参数在defer语句时计算
• defer列表为后进先出

何时使用defer调用

• Open/Close
• Lock/Unlock
• PrintHeader/PrintFooter

package main

import "fmt"

func tryDefer(){
	defer fmt.Println(1)
	defer fmt.Println(2)//defer 相当于栈:先进后出
	fmt.Println(3)
	//结果:3 2 1
}
func main() {
	tryDefer()
}

package main

import (
	"../../functional/fib"
	"bufio"
	"fmt"
	"os"
)

func tryDefer() {
	defer fmt.Println(1)
	defer fmt.Println(2) //defer 相当于栈:先进后出
	fmt.Println(3)
	//结果:3 2 1
	//return
	panic("error occurred")
	fmt.Println(4)
	//3
	//2
	//1
	//panic: error occurred
}
//参数在defer语句时计算
func tryDefer2() {
	for i := 0; i < 100; i++ {
		defer fmt.Println(i) //
		if i == 10 {
			panic("print too many")
		}
	}
}

func writeFile(filename string) {
	file, err := os.Create(filename)
	if err != nil {
		panic(err)
	}
	defer file.Close()

	writer := bufio.NewWriter(file)
	defer writer.Flush()

	f := fib.Fibonacci()
	for i := 0; i < 10; i++ {
		fmt.Fprintln(writer, f())
	}
}

func main() {
	//tryDefer()
	tryDefer2()
	writeFile("fib.txt")
}

二.错误处理理念

func writeFile(filename string) {
	//file, err := os.Create(filename)
	file, err := os.OpenFile(filename, os.O_EXCL|os.O_CREATE, 0666) //O_EXCL 如果文件存在的话打开不了
	// panic: open fib.txt: file exists
	err = errors.New("this is a custum error")
	if err != nil {
		//fmt.Println("Error:",err.Error())
		if pathError, ok := err.(*os.PathError); !ok {
			panic(err)
		} else {
			fmt.Printf("%s ,%s ,%s\n ", pathError.Op, pathError.Path, pathError.Err)
		}
		return
	}
	defer file.Close()

	writer := bufio.NewWriter(file)
	defer writer.Flush()

	f := fib.Fibonacci()
	for i := 0; i < 10; i++ {
		fmt.Fprintln(writer, f())
	}
}

三.服务器统一出错处理

web.go

package main

import (
	"./filelisting"
	"log"
	"net/http"
	"os"
)

type appHandler func(writer http.ResponseWriter, request *http.Request) error

//错误包装
func errWrapper(handler appHandler) func(http.ResponseWriter, *http.Request) {
	return func(writer http.ResponseWriter, request *http.Request) {
		err := handler(writer, request)
		if err != nil {
			log.Printf("Error handing request:%s",err.Error()) //2020/08/29 17:17:36 Error handing request:open fib2.txt: permission denied
			code := http.StatusOK
			switch {
			case os.IsNotExist(err):
				//http.Error(writer, http.StatusText(http.StatusNotFound), http.StatusNotFound)
				code = http.StatusNotFound
			case os.IsPermission(err):
				code = http.StatusForbidden
			default:
				code = http.StatusInternalServerError
			}
			http.Error(writer, http.StatusText(code), code)
		}
	}
}
func main() {
	http.HandleFunc("/list/", errWrapper(filelisting.HandleFileList))

	err := http.ListenAndServe(":8081", nil)
	if err != nil {
		panic(err)
	}
}

handler.go

package filelisting

import (
	"io/ioutil"
	"net/http"
	"os"
)

func HandleFileList(writer http.ResponseWriter, request *http.Request) error {
	path := request.URL.Path[len("/list/"):]
	file, err := os.Open(path)
	if err != nil {
		//panic(err)
		/*http.Error(writer,
			err.Error(),
			http.StatusInternalServerError)
		return*/
		return err
	}
	defer file.Close()

	all, err := ioutil.ReadAll(file)
	if err != nil {
		//panic(err)
		return err
	}
	writer.Write(all)
	return nil
}

四.panic和recover

panin功能作用

• 停止当前函数执行
• 一直向上返回,执行每一层的defer
• 如果没有遇见recover,程序退出

recover功能作用

• 仅在defer调用中使用
• 获取panic的值
• 如果无法处理,可重新panic

package main

import (
	"fmt"
)

func tryRecover() {
	defer func() {
		r := recover()
		if err, ok := r.(error); ok {
			fmt.Println("Error occurred:", err)
		} else {
			fmt.Println(r)
		}
	}()

	//panic(errors.New("this is an error")) //Error occurred: this is an error,下面不执行,panic停止当前函数执行

	b := 0
	a := 5 / b
	fmt.Println(a) //Error occurred: runtime error: integer divide by zero
}
func main() {
	tryRecover()
}

五.服务器统一出错处理

• 意料之中的:使用error,如文件打不开
• 意料之外的:使用panic,如数组越界