Raising a different exception in "after" or "after_raising" advice
Posted by Dean Wampler Sat, 26 Apr 2008 21:13:04 GMT
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.
