Ruby中的异常处理代码编写示例

单个异常使用 fail 关键字仅仅当捕获一个异常并且反复抛出这个异常(因为这里你不是失败，而是准确的并且故意抛出一个异常)。

  begin
   fail 'Oops'
  rescue => error
   raise if error.message != 'Oops'
  end

不要为 fail/raise 指定准确的 RuntimeError。

 # bad
  fail RuntimeError, 'message'

  # good - signals a RuntimeError by default
  fail 'message'

宁愿提供一个异常类和一条消息作为 fail/raise 的两个参数，而不是一个异常实例。

 # bad
  fail SomeException.new('message')
  # Note that there is no way to do `fail SomeException.new('message'), backtrace`.

  # good
  fail SomeException, 'message'
  # Consistent with `fail SomeException, 'message', backtrace`.

不要在 ensure 块中返回。如果你明确的从 ensure 块中的某个方法中返回，返回将会优于任何抛出的异常，并且尽管没有异常抛出也会返回。实际上异常将会静静的溜走。

 def foo
   begin
    fail
   ensure
    return 'very bad idea'
   end
  end

Use implicit begin blocks when possible.如果可能使用隐式 begin 代码块。

 # bad
  def foo
   begin
    # main logic goes here
   rescue
    # failure handling goes here
   end
  end

  # good
  def foo
   # main logic goes here
  rescue
   # failure handling goes here
  end

通过 contingency methods 偶然性方法。 (一个由 Avdi Grimm 创造的词) 来减少 begin 区块的使用。

  # bad
  begin
   something_that_might_fail
  rescue IOError
   # handle IOError
  end

  begin
   something_else_that_might_fail
  rescue IOError
   # handle IOError
  end

  # good
  def with_io_error_handling
    yield
  rescue IOError
   # handle IOError
  end

  with_io_error_handling { something_that_might_fail }

  with_io_error_handling { something_else_that_might_fail }

不要抑制异常输出。

  # bad
  begin
   # an exception occurs here
  rescue SomeError
   # the rescue clause does absolutely nothing
  end

  # bad
  do_something rescue nil

避免使用 rescue 的修饰符形式。

 # bad - this catches exceptions of StandardError class and its descendant classes
  read_file rescue handle_error($!)

  # good - this catches only the exceptions of Errno::ENOENT class and its descendant classes
  def foo
   read_file
  rescue Errno::ENOENT => ex
   handle_error(ex)
  end

不要用异常来控制流。

 # bad
  begin
   n / d
  rescue ZeroDivisionError
   puts "Cannot divide by 0!"
  end

  # good
  if d.zero?
   puts "Cannot divide by 0!"
  else
   n / d
  end

应该总是避免拦截(最顶级的) Exception 异常类。这里(ruby自身)将会捕获信号并且调用 exit，需要你使用 kill -9 杀掉进程。

  # bad
  begin
   # calls to exit and kill signals will be caught (except kill -9)
   exit
  rescue Exception
   puts "you didn't really want to exit, right?"
   # exception handling
  end

  # good
  begin
   # a blind rescue rescues from StandardError, not Exception as many
   # programmers assume.
  rescue => e
   # exception handling
  end

  # also good
  begin
   # an exception occurs here

  rescue StandardError => e
   # exception handling
  end

将更具体的异常放在救援（rescue）链的上方，否则他们将不会被救援。

  # bad
  begin
   # some code
  rescue Exception => e
   # some handling
  rescue StandardError => e
   # some handling
  end

  # good
  begin
   # some code
  rescue StandardError => e
   # some handling
  rescue Exception => e
   # some handling
  end

在 ensure 区块中释放你程式获得的外部资源。

 f = File.open('testfile')
  begin
   # .. process
  rescue
   # .. handle error
  ensure
   f.close unless f.nil?
  end

除非必要, 尽可能使用 Ruby 标准库中异常类，而不是引入一个新的异常类。(而不是派生自己的异常类)