38 — Metaprogramming

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▶️ java pkg17metaprogramming/metaprogramming1DynamicProxy.java · metaprogramming5AnnotationProcessor.java

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What is metaprogramming?

Code that inspects or generates other code at compile time or runtime — the foundation of frameworks like Spring, Hibernate, and Mockito.

Technique	When	Use case
Reflection	Runtime	Inspect classes, invoke methods by name
Dynamic proxy	Runtime	Wrap interfaces (logging, transactions)
MethodHandles	Runtime	Faster than raw reflection (Java 7+)
VarHandles	Runtime	Atomic field access (Java 9+)
Annotation processing	Compile time	Generate boilerplate (Lombok-style)

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Dynamic proxies

java.lang.reflect.Proxy creates an object implementing your interface; an InvocationHandler receives every method call — used by Spring AOP and mock frameworks.

Run: java pkg17metaprogramming/metaprogramming1DynamicProxy.java

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MethodHandles & VarHandles

Prefer MethodHandles over classic reflection for better performance and type safety. VarHandles replace some sun.misc.Unsafe patterns for atomic operations on fields.

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Annotation processing

javax.annotation.processing runs at compile time to generate source files from annotations — how @Entity, @Autowired, and Lombok @Getter work under the hood.

Full guide → Reflection & Annotations

Interview drill → 12 Reflection & Annotations

Next → 39 Resilience Patterns