Golang學習筆記（6）資源管理和錯誤處理

第六章 資源管理和錯誤處理

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defer調用

• 確保調用在函數結束時發生
• 參數在defer語句時計算
• defer列表爲先進後出

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

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

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

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

何時使用defer調用

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

錯誤處理

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)
	}

}

錯誤處理二

• 如何實現統一的錯誤處理邏輯

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) {

		err := handler(writer, request)

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

			// 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)
		}
	}
}

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

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

panic

• 停止當前函數執行
• 一直向上返回，執行每一層的defer
• 如果沒有遇見recover，程序退出

recover

• 僅在defer調用中使用
• 獲取panic的值
• 如果無法處理，可以重新panic

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"))		// Output : Error occurred: this is an error

	// Division by zero
	b := 0
	a := 5 / b
	fmt.Println(a)								// Output : Error occurred: runtime error: 
												//			integer divide by zero

	// Causes re-panic
	panic(123)									// Output :
												// 			panic: 123 [recovered]
												// 			panic: I don't know what to do: 123
}

func main() {
	tryRecover()
}

error vs panic

• 意料之中的 : 使用error. 如 : 文件打不開
• 意料之外的: 使用panic. 如 : 數組越界

錯誤處理綜合實例

• defer + panic + recover
• Type Assertion
• 函數式編程的應用