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Java / MapStruct Java Interview questions part 2

What is the difference between returning a new object and using @MappingTarget?

A regular mapping method (TargetDto toDto(Source s)) always constructs a brand-new target instance and populates it from the source. Any existing state on a target you might have had beforehand is irrelevant - you get a fresh object every time.

A method that accepts a parameter annotated @MappingTarget instead updates an already-existing instance in place, leaving properties with no matching source value untouched rather than resetting them to defaults.

void updateEntity(Dto dto, @MappingTarget Entity entity);

This distinction matters most for partial updates - for example, a PATCH endpoint should only overwrite the fields present in the request, which is exactly what an @MappingTarget update method does, whereas a plain toEntity method would replace everything with a new object.

A method without @MappingTarget will:
@MappingTarget methods are especially useful for:

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