What is Cross Cutting Concern? Why AOP? What is Aspect?

There are certain aspects of the application that has to be addressed at every layer of the application and cannot be categorized to be a responsibility of any one single layer. Examples are logging, transaction management, security, Caching, Validation etc…

These aspects that span across the several layers of the application are cross cutting concerns. AOP helps decouple cross cutting concerns from the application logic that they impact.

If there was no AOP, one would have to implement common functionality in a base class and have all classes make use it. But this will make the object hierarchy brittle. Other common approach is to delegate common functionality is to utility class. But this will require calls to the utility method spread all across application logic code.

In AOP, the common functionality is implemented in a single class. But this class is invoked by declaratively defining how and where the functionality needs to be applied without having to modify the class to which the functionality is applied to.

These classes that hold common functionality are called Aspects.

Now, the application logic code or service modules contain code pertaining only to their primary responsibility or primary concern and secondary concerns move to aspects. The second advantage is that all common functionality is nicely held in a single place.