There are many ways to modify the behavior of existing code with actually changing the core logic. Some patterns you might be familiar with are the decorator pattern or the observer pattern. Both allow you to take another object and modify the behavior by wrapping your modifications around the original code. One pattern you might not be familiar with though, is the interceptor pattern.
The interceptor pattern is a core concept of what is called aspect oriented programming (AOP). AOP aims to improve the modularity of your code by allowing you to separate cross-cutting concerns from the rest of your code.
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