MCS-287 Notes for 2006-05-12

Comments on the Laddad article, Part 1:

Beware that the way Laddad uses the phrase "mix-in classes" is not the only way that phrase is used. (Also, the first word is more commonly spelled as "mixin".)

The section entitled "Weaving example" includes some manually woven code, namely the CreditCardProcessorWithLogging class. The logging messages in that example are messed up. The messages in the debit method say that the credit operation is starting or completing. Conversely, the first message in the credit method says that the debit method is starting.

Moreover, the code in that example fails to include requirement number 3 from the list immediately above it, "Log any exception thrown by each public operation." To address this, each of the two methods (debit and credit) would need an appropriate try/catch> construct. For example, here is a possible version of debit:

   public void debit(CreditCard card, Money amount)
        throws InvalidCardException, NotEnoughAmountException,
               CardExpiredException {
        _logger.log("Starting CreditCardProcessor.debit(CreditCard, Money) "
                    + "Card: " + card + " Amount: " + amount);
        try{
            // Debiting logic
            _logger.log("Completing CreditCardProcessor.debit(CreditCard, Money) "
                        + "Card: " + card + " Amount: " + amount);
        }catch(Exceptiion e){
            _logger.log("Exception " + e
                        + " CreditCardProcessor.debit(CreditCard, Money) "
                        + "Card: " + card + " Amount: " + amount);
            throw e;
        }
    }

Later, in the section "AspectJ: An AOP implementation for Java", the same example is reprised as the aspect LogCreditCardProcessorOperations. This example could be enhanced a bit to log also any public operations that don't have the card and amount as arguments. The code also got a bit bashed somewhere on its way to publication. Each place where "thisjoin point" appears, it should really be "thisJoinPoint".