Spring AOP: JDK Dynamic Proxy vs CGLIB

Spring AOP generates a proxy object that wraps your bean; external callers hold the proxy, and every method call first runs through advice chains (before, after, around) before delegating to the real method. Two proxy mechanisms. JDK dynamic proxy uses java.lang.reflect.Proxy to generate a class at runtime that implements all the target's interfaces. Method calls route through an InvocationHandler whose invoke() inspects the method and applies advice. Only works when the target implements interfaces. CGLIB uses bytecode manipulation (via ASM) to generate a subclass of the target at runtime. Methods are overridden; calls invoke the subclass, which applies advice before calling super. Works on plain classes but can't subclass final classes or override final/private methods. Spring's selection: default was "interface → JDK proxy, no interface → CGLIB". Spring Boot 2.x flipped the default to always-CGLIB (proxyTargetClass=true) because it avoids surprises when callers cast to the concrete type. The production-critical gotcha is self-invocation. If class Foo has two methods a() and b(), and b() is @Transactional, then this.b() from within a() calls the raw Foo instance method directly — not through the proxy — so the transaction advice never fires. Fixes: inject the bean into itself, or use AopContext.currentProxy() to get the proxy manually, or restructure so the @Transactional method is called from a different bean.