What is the implementation principle of Spring AOP?

The mechanism behind Spring AOP implementation relies on dynamic proxies. When a proxied object is invoked, Spring AOP will create a proxy object that can intercept method calls and execute appropriate enhancement logic before, after, or when an exception is thrown during the method call.

There are primarily two ways of proxying in Spring AOP: JDK dynamic proxy and CGLIB dynamic proxy.

1. Spring AOP uses JDK dynamic proxy when the target object implements an interface. At runtime, a proxy object that implements the target object’s interface is dynamically created using Java’s reflection mechanism. The proxy object delegates method calls to the target object and can perform additional logic before and after method calls.
2. CGLIB dynamic proxy: When the target object does not implement an interface, Spring AOP will use CGLIB dynamic proxy. CGLIB is a powerful high-performance bytecode generation library that creates a proxy subclass by inheriting the target object. The proxy subclass overrides the methods of the target object and can execute enhanced logic before and after method calls.

The implementation principle of Spring AOP can be summarized in a few simple steps:

1. Determine if the target object needs to be proxied, and continue if so.
2. Choose between using JDK dynamic proxy or CGLIB dynamic proxy based on whether the target object has implemented the interface.
3. Create a proxy object dynamically based on aspect definitions and proxy methods.
4. Execute corresponding enhancement logic before and after method invocation or when an exception is thrown based on the rules defined in the aspect in the proxy object.

Spring AOP achieves cross-cutting concerns through dynamic proxy, which decouples business logic from cross-cutting concerns such as logging and transactions, thus increasing code maintainability and reusability.