One of the guiding principles of the AspectJ design
was that we were trying to enable modular implementation of crosscutting
concerns -- aka aspects. That is what led us to put pointcuts, advice and
other member declarations together into aspects. For example, being able to
see the entire ObserverProtocol aspect in a single screenful of code was
important to us. It gave us the nice modularity we were after. I
personally believe that is critical, that's what separation of concerns is
about, getting the concerns into separate modular pieces.
From that perspective, putting pointcuts and advice
bodies in separate files isn't what we want.
But what I don't understand is to what degree tool
support will be able to bring the separate things back together. One of the
claims about XML and meta-data is that of course you'll never edit the raw
meta-data or XML, there will be tool support. If there is tool support, then
that support might be able to make things look more modular. But then again,
everytime I see someone talking about XML they insist on showing me the raw
XML, so I'm not sure how present the tool support is.
3) What role, if any, should metadata play in AOP?
I know Gregor has commented on the use of
metadata a couple of times, but the JBoss group is charging ahead with their
"AOP" product which encourages developers to use class metadata to load
library aspects. This strikes me as wrong in many ways, but the message that
is reaching the development community is that interceptors and metadata are
all you need to do AOP. Is this just a function of what's easy to implement,
or are people really having trouble grasping the concepts?
My opinion on this has softened. I
now believe that meta-data support (ala JSR-175) is useful, and that when it
appears in Java, tools like AspectJ should support it instantly. I think
programmers should use attributes (aka tags or metadata) in pointcuts in
certain circumstances when property-based or enumeration based pointcuts are
not appropriate. Without going into detail, I think the 3 cases
are:
- if the pointcut reaches a large number of
join points, use a property based pointcut
- if the pointcut reaches a small number of
join points, and a property-based pointcut is hard to write, use
tags
- if the pointcut reaches a small number of
join points, and a property-based pointcut is hard to write, and you don't
have access to the target source, then use an enumeration-based
pointcut.