quarkusio/quarkus · error · ReflectiveOperationException

Could not find the namingStrategy property on JsonBuilder

Error message

Could not find the namingStrategy property on JsonBuilder

What it means

JsonProducer configures kotlinx.serialization Json naming strategies reflectively by setting the `namingStrategy` property on JsonBuilder. If the kotlinx.serialization version in use no longer exposes that mutable property (API change), the lookup returns null and a ReflectiveOperationException is thrown. This is a library-version compatibility failure, typically surfaced indirectly as IllegalArgumentException wrapping.

Source

Thrown at extensions/resteasy-reactive/rest-kotlin-serialization-common/runtime/src/main/kotlin/io/quarkus/resteasy/reactive/kotlin/serialization/common/runtime/JsonProducer.kt:60

        configuration.json().namingStrategy().ifPresent { strategy ->
            loadStrategy(this, strategy, this@JsonProducer)
        }
        val sortedCustomizers = sortCustomizersInDescendingPriorityOrder(customizers)
        for (customizer in sortedCustomizers) {
            customizer.customize(this)
        }
    }

    @ExperimentalSerializationApi
    private fun loadStrategy(
        jsonBuilder: JsonBuilder,
        strategy: String,
        jsonProducer: JsonProducer,
    ) {
        val strategyProperty: KMutableProperty1<JsonBuilder, JsonNamingStrategy> =
            (JsonBuilder::class.memberProperties.find { member -> member.name == "namingStrategy" }
                ?: throw ReflectiveOperationException(
                    "Could not find the namingStrategy property on JsonBuilder"
                ))
                as KMutableProperty1<JsonBuilder, JsonNamingStrategy>
        strategyProperty.isAccessible = true

        strategyProperty.set(
            jsonBuilder,
            if (strategy.startsWith("JsonNamingStrategy")) {
                jsonProducer.extractBuiltIn(strategy)
            } else {
                jsonProducer.loadStrategyClass(strategy)
            },
        )
    }

    @ExperimentalSerializationApi
    private fun loadStrategyClass(strategy: String): JsonNamingStrategy {
        try {

View on GitHub (pinned to e1c734241f)

Solutions

1. Align kotlinx-serialization-json version with the one supported by your Quarkus version (check the Quarkus BOM)
2. Construct the Json instance manually with Json { namingStrategy = ... } instead of relying on the CDI producer if versions cannot be aligned
3. Upgrade Quarkus to a version compatible with your kotlinx-serialization version
4. Remove the naming-strategy config to fall back to the default Json instance

Example fix

// before (config-driven, version-dependent)
quarkus.kotlin-serialization.json.naming-strategy=io.ktor...JsonNamingStrategy.SnakeCase
// after (explicit)
val json = Json { namingStrategy = JsonNamingStrategy.SnakeCase }

// Verify kotlinx-serialization exposes the property before configuring
val hasNamingStrategy = JsonBuilder::class.memberProperties.any { it.name == "namingStrategy" }
require(hasNamingStrategy) { "kotlinx-serialization version incompatible with naming-strategy config" }

fun JsonBuilder.hasNamingStrategy(): Boolean =
    JsonBuilder::class.memberProperties.any { it.name == "namingStrategy" }

try {
    json = produceJson()
} catch (e: ReflectiveOperationException) {
    json = Json.Default // fall back to default Json
}

When it happens

Trigger: Setting quarkus.kotlin-serialization.json.naming-strategy at build/runtime time with a kotlinx-serialization version whose JsonBuilder lacks the namingStrategy property, or where it is not a KMutableProperty1.

Common situations: Upgrading or downgrading kotlinx-serialization-json so its internal JsonBuilder API differs from what the Quarkus rest-kotlin-serialization-common extension expects; Quarkus/kotlinx version skew in a project BOM.

Related errors

• No no-arg constructor found on $strategy
• Unexpected index $index
• Unexpected index $index
• Error loading naming strategy: ${strategy.substringAfter('.
• Expected : after attribute

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/dbd6366db78d0427. Report an issue: GitHub.