Weaving Advice at Runtime

Understanding how Spring weaves advice into beans at runtime is essential for mastering AOP in real-world applications. When your application context is initialized, Spring begins by scanning all bean definitions. As it encounters each bean, it checks whether any defined aspects, such as those annotated with @Aspect, have pointcuts matching the bean's methods. This matching process is crucial: only beans with methods that match an advice's pointcut expression are considered for proxying.

Spring uses a special post-processor, typically the AnnotationAwareAspectJAutoProxyCreator, to analyze beans and aspects. This component inspects each bean before instantiation, determining if any advice should be applied. If a bean matches one or more pointcuts, Spring decides to create a proxy for that bean. This proxy will intercept method calls and weave in the relevant advice at the correct join points.

The lifecycle of proxy creation involves several steps. First, Spring identifies all advisors (wrappers for advice and pointcut) that apply to the bean. Then, it decides the proxying mechanism based on the bean's type: JDK dynamic proxies for interfaces, or CGLIB proxies for concrete classes. The proxy object is then registered in the application context in place of the original bean. As a result, whenever you retrieve the bean or inject it elsewhere, you interact with the proxy, which manages the invocation of advice alongside your business logic.