Applying Spring BeanPostProcessor for A/B Testing and Dynamic Routing

When building a Java project with Spring, the BeanPostProcessor extension point offers a simple way to address many seemingly hard problems. The article first introduces the BeanPostProcessor interface, which defines two callbacks— postProcessBeforeInitialization and postProcessAfterInitialization —that are invoked before and after a bean's initialization methods.

By registering a BeanPostProcessor, developers can modify bean properties, generate dynamic proxy instances, or implement low‑level AOP logic. The article then presents a practical case from an online ticketing system where A/B testing creates two implementations of the same service (V1 and V2) that must coexist at runtime.

Before using BeanPostProcessor, the calling code contains numerous conditional branches to select the appropriate implementation, leading to duplicated and hard‑to‑maintain code. The article shows the original, cumbersome code (illustrated with images) and explains the maintenance challenges it creates.

To solve this, the author defines two custom annotations: @RoutingInjected (similar to @Autowired) to mark fields that should receive a routing proxy, and @RoutingSwitch to control routing behavior per method or class. A RoutingBeanPostProcessor scans beans for these annotations after initialization and injects a dynamically generated proxy.

The proxy is created by RoutingBeanProxyFactory, which determines the target implementation based on the presence of @RoutingSwitch on methods (method‑level routing takes precedence) or on the class otherwise. This approach eliminates the repetitive conditional logic and centralizes routing decisions.

After refactoring, the calling code becomes much cleaner, as the routing logic is encapsulated within the post‑processor and proxy factory. The article concludes by encouraging readers to experiment with BeanPostProcessor for similar problems and invites feedback.