go 基础学习 异常

概述

• panic

  • 停止当前函数执行

  • 一直向上返回，执行每一层的defer

  • 如果没有遇到recover,程序退出

• recover

  • 仅在defer调用中使用

  • 获取panic的值

  • 如果无法处理，可重新panic

代码

package main
 
import (
	"fmt"
	"os"
 
	"bufio"
 
	"golearn/functional/fib"
)
 
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 {
			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 < 20; i++ {
		fmt.Fprintln(writer, f())
	}
}
 
func main() {
	tryDefer()
	writeFile("fib.txt")
}

分装错误处理

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

package main
 
import (
	"log"
	"net/http"
	_ "net/http/pprof"
	"os"
 
	"golearn/errhandling/filelistingserver/filelisting"
)
 
type appHandler func(writer http.ResponseWriter,
	request *http.Request) error
 
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)
 
		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)
	if err != nil {
		panic(err)
	}
}

recover

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))
		}
	}()
 
	// 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()
}