资源管理与错误处理

Go学习笔记

   |  2 minutes  |  271 words

Defer

调用特点

  • 确保调用在函数结束时发生

  • defer列表为后进先出，参数在defer语句时才计算

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

常见使用defer调用场景

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

错误处理

  • 常用错误处理

    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()

  • 服务器统一错误处理

    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
}

Panic

  • 停止当前函数执行
  • 一直向上返回，执行每一层的defer
  • 如果没有遇见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"))
  
  // Division by zero
  //b := 0
  //a := 5 / b
  //fmt.Println(a)
  
  // Causes re-panic
  //panic(123)
}

Error vs Panic

  • 意料之中的使用error，如：文件打不开等
  • 意料之外的使用panic，如：数组越界
Go 

See also