What is the implementation principle of AOP in Java?

In Java, the implementation principle of AOP (Aspect-Oriented Programming) is achieved through dynamic proxy.

AOP is a programming concept that separates cross-cutting concerns (such as logging, performance statistics, etc.) from business logic, modularizing them to improve code reusability and maintainability.

In Java, AOP is mainly implemented in two ways: static proxy and dynamic proxy. In static proxy, a proxy class needs to be manually written to insert additional logic into the business methods. In dynamic proxy, the Proxy class and InvocationHandler interface provided by Java are used to dynamically generate proxy classes at runtime using reflection mechanism.

The implementation principle of dynamic proxy is: when a class is being proxied, a proxy class object is created through the static method newProxyInstance() of the Proxy class. This proxy class object will implement the interfaces implemented by the target class, and also implement the InvocationHandler interface. It will forward the method call to the invoke() method of InvocationHandler when calling methods of the proxy object. In the invoke() method, additional logic can be added before and after the execution of the target method.

The steps to implement AOP using dynamic proxies are as follows:

1. Define an advice class that contains additional logic to be added before and after the execution of the target method.
2. Create a proxy object, specifying the target object and the aspect class.
3. When a method of the proxy object is called, the invoke() method of the proxy class will forward the method invocation to the corresponding method of the aspect class.
4. In aspect-oriented programming, additional logic can be added before and after the execution of the target method in the corresponding methods.

Implementing AOP through dynamic proxies allows for the reuse of cross-cutting concerns without needing to modify the existing business logic code.