Weaving Multiple Aspects in Sequence Diagrams - Transactions on Aspect-Oriented Software Development - page 174

Fig. 5. Result of the weaving

In Fig. 1, the cSD log in represents a customer log in on a server. The customer
tries to log in and either he succeeds, or he fails. In this last case, the customer
can try again to log in, and either he succeeds, or the server answers “no”. The
expected weaving of the three aspects depicted in Fig. 4 into the cSD

log in is

represented by the cSD in Fig. 5.

3 Various Definitions of Join Points

As mentioned in the introduction, weaving multiple aspects at the same join
point can be dicult if a join point is simply defined as a strict sequence of
messages, because aspects previously woven might have inserted messages in
between. In this case, the only way to support multiple static weaving is to define
each aspect in function of the other aspects, which is clearly not acceptable.
The weaving of the three aspects depicted in Fig. 4 allows us to better explain
the problem. If the join points are defined as the strict sequence of messages cor-
responding to those specified in the pointcut, the weaving of these three aspects is
impossible. Indeed, when the aspect security is woven, a message save bad attempt

is added between the two messages . Since the pointcut detects

only a strict sequence of messages, after the weaving of the aspect

log in and try again

security ,theas-

pect display cannot be woven anymore. We obtain the same problem if we weave

the aspect afterwards.

To solve this problem of multiple weaving, we introduce new formal defini-
tions of join points which make possible the detection of join points where some
events can occur between the events specified in the pointcut. In this way, when
the aspect

display first and the aspect security

security is woven, the pointcut of the aspect display will allow the

detection of the join point formed by the messages log in and try again ,evenif

the message save bad attempt has been added.