Jackson Bean Validation

Jackson Bean Validation module performs JSR 380 validation mapping during deserialization. This process ensures validation checks occur before the actual type is deserialized. It is particularly useful for validating non-nullable fields in Kotlin, ensuring they are not erroneously mapped to null in the incoming JSON.

Any detected constraint violations are communicated through a DataValidationException, which is thrown during BeanDeserializer::deserialize.

Features

• Automatic @NotNull Validation: All non-nullable fields in Kotlin are implicitly treated as if they are annotated with @NotNull. This applies unless a field is already constrained by another annotation (e.g., @NotBlank).

• Handling Existing Constraint Violations: If a field with an existing constraint does not emit violations on null values, it is still treated as a @NotNull field.

• Extended Validation Scope: Implicit @NotNull behavior also applies to the values inside maps, collections, and arrays, wherever applicable.

• Support for Inline Value Classes: Value classes are boxed before validation to preserve and recognize any constraint annotations.

• Null Handling for Primitive Fields: When enabled, DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES triggers a @NotNull violation for null values in primitive fields.

As an example:

Old structure to ensure @NotNull constraints in kotlin:

// @NotBlank likely won't be evaluated, because value classes are 
// typically inlined as their underlying type
@JvmInline
value class Inlined(@get:NotBlank val value: String?)

data class FooBar(
    @get:NotNull val foo: List<@NotNull String>?,
    @get:NotNull val bar: Inlined?,
    @get:NotBlank val string: String?,
)

With jackson bean validation, and the same constraint mapping and behavior, this becomes:

// constraint annotations now work on value classes
@JvmInline
value class Inlined(@get:NotBlank val value: String)

// @NotNull inferred from kotlin's type system
data class FooBar(
    val foo: List<String>,
    val bar: Inlined,
    @get:NotBlank val string: String, // no @NotNull due to @NotBlank
)

Usage

Declare the dependency:

<dependency>
    <groupId>com.assaabloyglobalsolutions.jacksonbeanvalidation</groupId>
    <artifactId>jackson-jakarta-validation</artifactId>
    <version>${jackson-bean-validation.version}</version>
</dependency>

Alternatively, for javax:

<dependency>
    <groupId>com.assaabloyglobalsolutions.jacksonbeanvalidation</groupId>
    <artifactId>jackson-javax-validation</artifactId>
    <version>${jackson-bean-validation.version}</version>
</dependency>

And then register the module:

objectMapper
    .registerModule(kotlinBeanValidationModule(validator))
    .configure(FAIL_ON_NULL_FOR_PRIMITIVES, true) // optional: for @NotNull on primitive fields

A DataValidationException is thrown during deserialization if validation fails, containing the constraint violations. It likely requires an exception mapper similar to that of a mapper for ConstrainViolationException.

@Provider
class JacksonConstraintViolationExceptionMapper : ExceptionMapper<DataValidationException> {
    override fun toResponse(exception: DataValidationException): Response {
        return ErrorResponseFactory.error(null, BAD_REQUEST, ErrorCodeV2.CONSTRAINT_VIOLATION, exception
            .violations
            .map { violation -> "${violation.message} (${violation.propertyPath})" }
            .sorted()
            .joinToString(", ")
        )
    }
}