How does Jackson JSON binding work in Spring MVC, and what can go wrong?

Explains message-converter selection, deserialization, Java records and constructors, unknown fields, dates, enums, and stable JSON configuration.

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For an application/json request body, Spring MVC selects a Jackson-backed HttpMessageConverter when it supports the target type. Jackson parses the JSON and constructs the request DTO using its visible properties, constructors, records, and configured modules. If JSON is malformed or a value cannot become the declared Java type, conversion fails before the controller and before Bean Validation. Successful deserialization can still produce an object that later fails constraints.

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The JSON conversion boundary, mapper configuration, compatibility policy, and separation from Bean Validation.

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Spring selects a JSON message converter; Jackson then constructs and populates the declared body type before validation runs.
For an application/json request body, Spring MVC selects a Jackson-backed HttpMessageConverter when it supports the target type. Jackson parses the JSON and constructs the request DTO using its visible properties, constructors, records, and configured modules. If JSON is malformed or a value cannot become the declared Java type, conversion fails before the controller and before Bean Validation. Successful deserialization can still produce an object that later fails constraints.
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Common failures include an invalid enum, an unsupported date format, no usable creator, unexpected polymorphic data, or a property name mismatch. I define one application-wide mapper policy for dates, naming, unknown properties, and modules, then use type-specific annotations or serializers only when the contract requires them. Records work well for immutable request shapes when component names and nullability are explicit. I avoid returning arbitrary domain graphs and hoping Jackson produces a stable API.

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Calling a conversion failure a validation failure.
Customizing independent ObjectMappers in many controllers.
Enabling unrestricted polymorphic type handling.

How does Jackson JSON binding work in Spring MVC, and what can go wrong?

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