Java / MapStruct Java Interview questions part 2

What is the difference between MapStruct's compile-time approach and CGLIB proxy-based mapping?

CGLIB-style approaches generate subclasses or proxies at runtime (via bytecode generation libraries) to intercept method calls and perform mapping or enhancement dynamically as the application runs.

MapStruct	CGLIB-style proxying
Mapping class generated once, at compile time.	Proxy/subclass generated at runtime, usually on first use.
No extra classloading cost during startup.	Adds bytecode generation and classloading overhead at startup or first call.
Generated code is visible, ordinary `.java`/`.class` files.	Proxy bytecode is generated dynamically, harder to inspect or debug.
Errors caught by the compiler.	Errors typically only surface when the proxy method is actually invoked.

The trade-off is flexibility versus predictability: runtime proxying can adapt to types it has never seen before without recompilation, while MapStruct requires a build step but gives you certainty about what a mapping will do before the application ever starts.

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CGLIB-style proxies for mapping are typically generated: At compile time alongside the source code

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At runtime, often on first use

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Only inside unit tests

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A downside of runtime proxy-based mapping compared to MapStruct is: It cannot map any nested objects

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It requires no dependency at all

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Errors often surface only when the proxy method runs

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Java / MapStruct Java Interview questions part 2

What is the difference between MapStruct's compile-time approach and CGLIB proxy-based mapping?

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