Raising a different exception in "after" or "after_raising" advice

Dean Wampler — Sat, 26 Apr 2008 16:13:04 -0500

An Aquarium user asked recently if you can use after_raising advice to raise a different exception, e.g., to wrap the original exception. Now you can, with the commits I did today. (This change will appear in the forthcoming V0.4.2 release.) Actually, this feature was partially implemented already, but never finished (OOPS!)

The following made-up example illustrates what you can do. Imagine you are one of the handful of Ruby programmers who aren’t using ActiveRecord ;) and you have an OracleDriver class that handles Oracle database transactions.


class OracleDriver
  class OracleDriverException < StandardException; ...; end
  class ConnectionError < OracleDriverException; ...; end
  def connect
    if try_to_connect == false
      raise ConnectionError 
    end
    ...
  end
  def find query_parameters
    ...
  end
end

In the application code that uses the OracleDriver you ignore any exceptions raised (because we will implement an exception handling strategy elsewhere…).


  ...
  def load_object oracle_driver, query_parameters
    oracle_driver.connect
    oracle_driver.find query_parameters
  end
  ...

Finally, you implement an application-wide exception handling strategy for any exceptions raised by the driver.


Aspect.new :after_raising => OracleDriverException, 
    :in_types => ... do |jp, object, *args|
  # Ruby needs a standard way to wrap one exception in another.
  original = jp.context.raised_exception
  app_exception = ApplicationException.new(original.message)
  app_exception.set_backtrace(original.backtrace)
  jp.context.raised_exception = app_exception
end

In other words, any exceptions raised by calls to OracleDriver are wrapped in an ApplicationException, which will be thrown by Aquarium when the advice block finishes.

This technique can also be used when handling exceptions in after advice.

By the way, in a similar way, you can also change the return value in after and after_returning advice. In this case, you assign a new value to jp.context.returned_value.

Dean Wampler's Mind: Raising a different exception in "after" or "after_raising" advice

Raising a different exception in "after" or "after_raising" advice