golang 错误处理

一、defer

package main

import (
	"fmt"
	"os"

	"bufio"
)

func tryDefer() {
	for i := 0; i < 100; i++ {
		defer fmt.Println(i)
		if i == 30 {
			// Uncomment panic to see
			// how it works with defer
			panic("printed too many")
		}
	}
}

func writeFile(filename string) {
	file, err := os.OpenFile(filename,
		os.O_EXCL|os.O_CREATE|os.O_WRONLY, 0666)

	if err != nil {
		if pathError, ok := err.(*os.PathError); !ok {//错误处理，向下转型
			panic(err)
		} else {
			fmt.Printf("%s, %s, %s\n",
				pathError.Op,
				pathError.Path,
				pathError.Err)
		}
		return
	}
	defer file.Close()//无论return、panic最后都会被执行

	writer := bufio.NewWriter(file)
	defer writer.Flush()//先进后执行，后进先执行，退出时先执行Flush，再执行Close

	f := Fibonacci()
	for i := 0; i < 20; i++ {
		fmt.Fprintln(writer, f())
	}
}

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

输出：

30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
panic: printed too many

goroutine 1 [running]:
main.tryDefer()
        /Users/jltxgcy/go/src/learn/errhandling/defer/defer.go:24 +0x111
main.main()
        /Users/jltxgcy/go/src/learn/errhandling/defer/defer.go:56 +0x20
defer执行的顺序是先进后执行，无论是提前return还是panic，defer都会执行，用于关闭资源等，通常是成对出现。

二、recovery

package main

import (
	"fmt"
)

func tryRecover() {
	defer func() {
		r := recover()
		if r == nil {
			fmt.Println("Nothing to recover. " +
				"Please try uncomment errors " +
				"below.")
			return
		}
		if err, ok := r.(error); ok {
			fmt.Println("Error occurred:", err)
		} else {
			panic(fmt.Sprintf(
				"I don't know what to do: %v", r))//panic(123)
		}
	}()

	// Uncomment each block to see different panic
	// scenarios.
	// Normal error
	panic(errors.New("this is an error"))

	// Division by zero
	//b := 0
	//a := 5 / b
	//fmt.Println(a)

	// Causes re-panic
	//panic(123)
}

func main() {
	tryRecover()
}

输出：

Error occurred: this is an error
打开：

b := 0
a := 5 / b
fmt.Println(a)

输出：

Error occurred: runtime error: integer divide by zero

打开：

panic(123)

输出：

panic: 123 [recovered]
        panic: I don't know what to do: 123

goroutine 1 [running]:
main.tryRecover.func1()
        /Users/jltxgcy/go/src/learn/errhandling/recover/recover.go:19 +0x1f6
panic(0x10ac4a0, 0x10eb4d0)
        /usr/local/go/src/runtime/panic.go:679 +0x1b2
main.tryRecover()
        /Users/jltxgcy/go/src/learn/errhandling/recover/recover.go:35 +0x5f
main.main()
        /Users/jltxgcy/go/src/learn/errhandling/recover/recover.go:39 +0x20
 

总结：

recover是用来接收panic的错误的，类似于try catch，用于捕捉panic的信息，不至于让panic输出信息那么难看。

如上panic(123)，123并不是error类型，所以打印panic: I don't know what to do: 123。

三、错误处理实例

1、目录结构

filelistingserver(目录)

   filelisting(目录)     

     --handler.go

--web.go

handle.go

package filelisting

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

const prefix = "/list/"

type userError string//string也能实现接口

func (e userError) Error() string {
	return e.Message()
}

func (e userError) Message() string {
	return string(e)
}

func HandleFileList(writer http.ResponseWriter,
	request *http.Request) error {
	fmt.Println()
	if strings.Index(
		request.URL.Path, prefix) != 0 {//localhost:8888/abc.txt，不以list开始
		return userError(
			fmt.Sprintf("path %s must start "+
				"with %s",
				request.URL.Path, prefix))
	}
	path := request.URL.Path[len(prefix):]
	file, err := os.Open(path)//本地打开文件，可能不存在，可能没有权限，都向上返回，让上一层来处理
	if err != nil {
		return err
	}
	defer file.Close()

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

	writer.Write(all)
	return nil
}

web.go

package main

import (
	"log"
	"net/http"
	_ "net/http/pprof"
	"os"

	"learn/errhandling/filelistingserver/filelisting"
)

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

func errWrapper(
	handler appHandler) func(
	http.ResponseWriter, *http.Request) {//参数和返回值都是函数
	return func(writer http.ResponseWriter,
		request *http.Request) {
		// panic
		defer func() {
			if r := recover(); r != nil {//统一处理panic
				log.Printf("Panic: %v", r)
				http.Error(writer,
					http.StatusText(http.StatusInternalServerError),
					http.StatusInternalServerError)
			}
		}()

		err := handler(writer, request)//HandleFileList

		if err != nil {
			log.Printf("Error occurred "+
				"handling request: %s",
				err.Error())

			// user error
			if userErr, ok := err.(userError); ok {//localhost:8888/abc.txt，不以/list开始
				http.Error(writer,
					userErr.Message(),
					http.StatusBadRequest)
				return
			}

			// system error
			code := http.StatusOK
			switch {
			case os.IsNotExist(err)://本地没有abc.txt
				code = http.StatusNotFound
			case os.IsPermission(err)://本地abc.txt没有权限访问
				code = http.StatusForbidden
			default:
				code = http.StatusInternalServerError
			}
			http.Error(writer,
				http.StatusText(code), code)
		}
	}
}

type userError interface {//定义接口
	error
	Message() string
}

func main() {
	http.HandleFunc("/",
		errWrapper(filelisting.HandleFileList))

	err := http.ListenAndServe(":8888", nil)
	if err != nil {
		panic(err)
	}
}

启动server，在浏览器输入localserver:8888/list/abc.txt，就可以显示本地目录下的abc.txt。