Why Kotlin data classes can have nulls in non-nullable fields with Gson?

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This happens because Gson uses an unsafe (as in java.misc.Unsafe) instance construction mechanism to create instances of classes, bypassing their constructors, and then sets their fields directly.

See this Q&A for some research: Gson Deserialization with Kotlin, Initializer block not called.

As a consequence, Gson ignores both the construction logic and the class state invariants, so it is not recommended to use it for complex classes which may be affected by this. It ignores the value checks in the setters as well.

Consider a Kotlin-aware serialization solution, such as Jackson (mentioned in the Q&A linked above) or kotlinx.serialization.

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