Go资源与出错处理

Posted 2020-12-25 ycx95

 

2018-07-25

1. 资源管理：defer

 

确保调用在函数结束时发生

参数在defer语句时计算

defer列表为先进后出

error vs panic : 意料之中用error比如文件打不开，意料之外用panic比如数组越界

注意：示例用到defer+panic+recover   用到Type Assertion   函数式编程很重要

例子1

创建目录：errorhandling下面再创建目录defer

//defer里面是相当于有一个栈 先进后出  defer好处：不怕中间有return

func tryDefer() {  
　　defer fmt.Println(1)
　　defer fmt.Println(2)
　　fmt.Println(3)
    //第一种：return
    panic("eror occurred") //第二种
    fmt.Println(4)
}//输出是3 2 1

　　

例子2

    详情参加functional  https://www.cnblogs.com/ycx95/p/9362175.html

package main

import (
	"fmt"
	"os"

	"bufio"

	"imooc.com/ccmouse/learngo/functional/fib"
)

func tryDefer() {
	for i := 0; i < 100; i++ {
		defer fmt.Println(i)
		if i == 30 {  //由于现进后出 输出就是：30 29 ... 1
			// 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) //err只是一个值 可以针对值进行处理

	if err != nil { //错误处理  细致的错误处理  如果文件存在：输出会提示文件存在
		if pathError, ok := err.(*os.PathError); !ok {
			panic(err)
		} else {
			fmt.Printf("%s, %s, %s
",
				pathError.Op,
				pathError.Path,
				pathError.Err)
		}
		return
	}
	defer file.Close() //file写完了要关闭   

	writer := bufio.NewWriter(file)  // 直接file写文件很慢，因此用bufio包装一下
	defer writer.Flush()  //导进新创建的fib.txt文件里面

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

func main() {
	tryDefer()
	writeFile("fib.txt")  //运行程序这里会创建一个fib.txt文件
}　　

2. 错误处理

错误处理一：人为处理

制造一个错误（程序会挂掉）

将原来的：
file,err := os.Create(filename)  //Create是openfile加了一些状态
直接修改为：
file, err := os.OpenFile(filename,
		os.O_EXCL|os.O_CREATE|os.O_WRONLY, 0666) //0666是一个权限　

处理挂掉的错误：如何对err进行细化处理（详见上面代码writefile的error处理部分

自己写error：err = errors.New("this is a custom error“) 其实error就是一个interface

错误处理：
file,err := os.Open("abc.txt")
if err!= nil {
      if pathError,ok := err.(*os.PathError);ok {
           fmt.Println(pathError.Err)
     }else{
             fmt.Println("unknown error",err)  
}

　

错误处理二:统一错误处理

新建目录：errorhanding目录下创建 filelistingserver 目录 

handler.go ( 位置 errorhandling/filelistingserver/filelisting)

package filelisting

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

const prefix = "/list/"

type userError string

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 {
		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)
	if err != nil {
		return err  //有错就return出去外面有处理
	}
	defer file.Close()

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

	writer.Write(all)
	return nil
}

 

web.go(位置 errorhandling/filelistingserver)

package main

import (
	"log"   //这里视频中所写是 github.com/gpmgo/gopm/modules/log 
	"net/http"
	_ "net/http/pprof"
	"os"

	"imooc.com/ccmouse/learngo/errhandling/filelistingserver/filelisting"
)
//返回一个err
type appHandler func(writer http.ResponseWriter,
	request *http.Request) error
//包装errHandler返回的err，然后返回一个函数  特点：输入是一个函数 输出也是一个函数 
func errWrapper(  
	handler appHandler) func(
	http.ResponseWriter, *http.Request) {
	return func(writer http.ResponseWriter,
		request *http.Request) {
		// panic
		defer func() {
			if r := recover(); r != nil {
				log.Printf("Panic: %v", r)
				http.Error(writer,
					http.StatusText(http.StatusInternalServerError),
					http.StatusInternalServerError)
			}
		}()

		err := handler(writer, request)
                //处理err进行处理
		if err != nil {
			log.Printf("Error occurred "+
				"handling request: %s",
				err.Error())

			// user error
			if userErr, ok := err.(userError); ok {
				http.Error(writer,
					userErr.Message(),
					http.StatusBadRequest)
				return
			}

			// system error
			code := http.StatusOK
			switch {
			case os.IsNotExist(err):
				code = http.StatusNotFound
			case os.IsPermission(err):
				code = http.StatusForbidden
			default:
				code = http.StatusInternalServerError
			}
			http.Error(writer,
				http.StatusText(code), code)
		}
	}
}

type userError interface {
	error
	Message() string
}

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

	err := http.ListenAndServe(":8888", nil)  //开服务器  打开服务器输入：localhost:8888/list/fib.txt
	if err != nil {
		panic(err)
	}
}

//测试时候：管理员目录时候cp fib.txt fib2.txt  浏览器可以访问　让别人没有权限：chmod 500 fib2.txt  浏览器输出：Forbidden

 

errwrapper_test.go(位置 errorhandling/filelistingserver)　　

package main

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

func errPanic(_ http.ResponseWriter,
	_ *http.Request) error {
	panic(123)
}

type testingUserError string

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

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

func errUserError(_ http.ResponseWriter,
	_ *http.Request) error {
	return testingUserError("user error")
}

func errNotFound(_ http.ResponseWriter,
	_ *http.Request) error {
	return os.ErrNotExist
}

func errNoPermission(_ http.ResponseWriter,
	_ *http.Request) error {
	return os.ErrPermission
}

func errUnknown(_ http.ResponseWriter,
	_ *http.Request) error {
	return errors.New("unknown error")
}

func noError(writer http.ResponseWriter,
	_ *http.Request) error {
	fmt.Fprintln(writer, "no error")
	return nil
}

var tests = []struct {
	h       appHandler
	code    int
	message string
}{
	{errPanic, 500, "Internal Server Error"},
	{errUserError, 400, "user error"},
	{errNotFound, 404, "Not Found"},
	{errNoPermission, 403, "Forbidden"},
	{errUnknown, 500, "Internal Server Error"},
	{noError, 200, "no error"},
}

func TestErrWrapper(t *testing.T) {
	for _, tt := range tests {
		f := errWrapper(tt.h)
		response := httptest.NewRecorder()
		request := httptest.NewRequest(
			http.MethodGet,
			"http://www.imooc.com", nil)
		f(response, request)

		verifyResponse(response.Result(),
			tt.code, tt.message, t)
	}
}

func TestErrWrapperInServer(t *testing.T) {
	for _, tt := range tests {
		f := errWrapper(tt.h)
		server := httptest.NewServer(
			http.HandlerFunc(f))
		resp, _ := http.Get(server.URL)

		verifyResponse(
			resp, tt.code, tt.message, t)
	}
}

func verifyResponse(resp *http.Response,
	expectedCode int, expectedMsg string,
	t *testing.T) {
	b, _ := ioutil.ReadAll(resp.Body)
	body := strings.Trim(string(b), "
")
	if resp.StatusCode != expectedCode ||
		body != expectedMsg {
		t.Errorf("expect (%d, %s); "+
			"got (%d, %s)",
			expectedCode, expectedMsg,
			resp.StatusCode, body)
	}
}

　

3. panic

注意：不能经常用

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则程序退出

 

4.recover

与panic对应

仅在defer调用中使用，获取panic的值，如果无法处理可重新panic

例子：

package main

import (
	"fmt"
)

func tryRecover() {
	defer func() { //这里写一个匿名函数 注意最后加的()
		r := recover()  //哈哈，这里是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) //的确是err
		} else {
			panic(fmt.Sprintf(
				"I don‘t know what to do: %v", r))
		}
	}()

	// 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()
}