How is AOP used in Java?

AOP (Aspect-Oriented Programming) is a programming paradigm that separates the concerns of a system from the business logic code in order to better achieve cross-cutting concerns reuse. In Java, AOP can be implemented in several ways.

1. Agent-based AOP: AOP is achieved by creating proxy objects for target objects. Proxy objects can add additional logic before and after the execution of target object methods. The dynamic proxy mechanism in Java and the CGLIB library can be used to create proxy objects.
2. Annotation-based AOP: This involves using annotations to identify the methods where logic needs to be applied, and then using aspect classes to implement the logic. During runtime, the AOP framework automatically weaves the aspect logic into the target object’s methods.
3. XML-based AOP: by defining pointcuts and aspects in an XML configuration file, the aspect logic is combined with the target object. During runtime, the AOP framework automatically weaves the aspect logic into the methods of the target object based on the definitions in the configuration file.

No matter which method is used, AOP can achieve common cross-cutting concerns, such as logging, transaction management, exception handling, etc. By separating these concerns from business logic code, code maintainability and reusability can be improved.