Java / MapStruct Java Interview questions part 2
Explain the lifecycle of a MapStruct mapper from source code to runtime call?
A MapStruct mapper passes through distinct build-time and run-time phases, and understanding the split explains why most mapping problems show up as compiler diagnostics rather than production bugs.
flowchart TD
A[Developer writes @Mapper interface] --> B[Build triggers javac]
B --> C[MapStruct annotation processor runs]
C --> D[MapperImpl.java generated in build folder]
D --> E[javac compiles MapperImpl to bytecode]
E --> F[Application starts, mapper instance obtained]
F --> G[Caller invokes mapping method]
G --> H[Generated plain Java code executes]
Steps A through E happen once per build, entirely before the application ever runs. Steps F through H happen repeatedly at runtime but involve nothing more than an ordinary object instantiation and plain method calls - MapStruct itself is not present on the runtime classpath's active code path at that point, only its small annotation/runtime support library.
Which phase happens only once per build, not per application run?
At runtime, invoking a mapping method executes:
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