资源管理与错误处理

  func tryDefer() {
  	defer fmt.Println(1)
  	defer fmt.Println(2)
  	fmt.Println(3)
  	//return
  	panic("error occurred")
  	fmt.Println(4)
  }
  ---------
  3
  2
  1

  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, %sn",
          pathError.Op,
          pathError.Path,
          pathError.Err)
    }
    return
  }
  defer file.Close()

  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 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)
  }