This paper by Kotrappa Sirbi and Prakash Jayanth Kulkarni (link goes to HTML abstract, full text PDF is free) discusses implementation of an application’s security requirements in Java using Aspect-Oriented Programming (AOP).
We have AOP for Objective-C (of sorts), but as hardly anyone has used it I think it’s worth taking a paragraph or two out to explain. If you’ve ever written a non-trivial Cocoa[ Touch] application, you’ll have found that even when you have a good object-oriented design, you have code that addresses different concerns mixed together. A method that performs some useful task (deleting a document, for example) also calls some logging code, checks for authorisation, reports errors, and maybe some other things. Let’s define the main business concerns of the application as, well, business concerns, and all of these other things like logging and access control as cross-cutting concerns.
AOP is an attempt to reduce the coupling between business and cross-cutting code by introducing aspects. An aspect is a particular cross-cutting concern that must be implemented in an application, such as logging. Rather than calling the logging code from the business code, we’ll define join points, which are locations in the business code where it’s useful to insert cross-cutting code. These join points are usually method calls, but could also be exception throw points or anywhere else that program control changes. We don’t necessarily need logging at every join point, so we also define predicates that say which join points are useful for logging. Whenever one of the matching join points is met, the application will run the logging code.
This isn’t just useful for adding code. Your aspect code can also define whether the business code actually gets run at all, and can even inspect and modify the return value of the business code. That’s where it gets useful for security purposes. You don’t need to put your access control (for instance) checks inside the business code, you can define them as modifications to join points in the business code. If you need to change the way access control works (say going from a single-user to directory service scheme, or from password checks to access tokens) you can just change the implementation of the aspect rather than diving through all of the app code.
Of course, that doesn’t mean you can just implement the business logic then add security stuff later, like icing a cake or sprinkling fairy dust. You still need to design the business objects such that the security control decisions and the join points occur at the same places. However, AOP is useful for separating concerns, and for improving maintainability of non-core app behaviour.