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Java / MapStruct Java Interview questions part 2

Explain the execution flow of a generated MapStruct mapper method?

Once compiled, calling a MapStruct mapper method behaves exactly like calling any ordinary Java method - there is no framework machinery involved at runtime.

sequenceDiagram participant Caller participant MapperImpl participant NestedMapper Caller->>MapperImpl: toDto(entity) MapperImpl->>MapperImpl: new TargetDto() MapperImpl->>MapperImpl: target.setName(entity.getName()) MapperImpl->>NestedMapper: toAddressDto(entity.getAddress()) NestedMapper-->>MapperImpl: AddressDto MapperImpl->>MapperImpl: target.setAddress(addressDto) MapperImpl-->>Caller: TargetDto

Each field assignment is a direct method call baked in at compile time; nested object properties are delegated to whichever mapping method was resolved for that type during code generation. There is no lookup, reflection, or dynamic dispatch beyond normal Java virtual method calls.

This is exactly why stepping through a mapper call in a debugger feels the same as stepping through hand-written code: the stack frames belong to the generated class itself, with named local variables and straightforward branches for any null checks, rather than opaque proxy or reflection frames.

At runtime, a generated mapper method's field assignments are:
Nested object conversion inside a generated method is handled by:

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