quarkusio/quarkus · error · IllegalArgumentException

Error loading naming strategy: ${strategy.substringAfter('.

Error message

Error loading naming strategy:  ${strategy.substringAfter('.')}

What it means

This is thrown by JsonProducer.loadStrategyClass when Quarkus tries to instantiate a user-configured JSON naming strategy class (quarkus.rest.kotlin-serialization.naming-strategy) but loading or constructing it via reflection fails. The underlying ReflectiveOperationException (ClassNotFound, no no-arg constructor, etc.) is wrapped into an IllegalArgumentException naming the strategy's short name.

Source

Thrown at extensions/resteasy-reactive/rest-kotlin-serialization-common/runtime/src/main/kotlin/io/quarkus/resteasy/reactive/kotlin/serialization/common/runtime/JsonProducer.kt:89

                jsonProducer.loadStrategyClass(strategy)
            },
        )
    }

    @ExperimentalSerializationApi
    private fun loadStrategyClass(strategy: String): JsonNamingStrategy {
        try {
            val strategyClass: Class<JsonNamingStrategy> =
                Thread.currentThread().contextClassLoader.loadClass(strategy)
                    as Class<JsonNamingStrategy>
            val constructor =
                strategyClass.constructors.find { it.parameterCount == 0 }
                    ?: throw ReflectiveOperationException(
                        "No no-arg constructor found on $strategy"
                    )
            return constructor.newInstance() as JsonNamingStrategy
        } catch (e: ReflectiveOperationException) {
            throw IllegalArgumentException(
                "Error loading naming strategy:  ${strategy.substringAfter('.')}",
                e,
            )
        }
    }

    @ExperimentalSerializationApi
    private fun extractBuiltIn(strategy: String): JsonNamingStrategy {
        val kClass = Builtins::class
        val property =
            kClass.memberProperties.find { property ->
                property.name == strategy.substringAfter('.')
            }
                ?: throw IllegalArgumentException(
                    "Unknown naming strategy provided:  ${strategy.substringAfter('.')}"
                )

        return property.get(JsonNamingStrategy) as JsonNamingStrategy

View on GitHub (pinned to e1c734241f)

Solutions

Verify the configured class exists and is on the runtime classpath (check package/name spelling).
Give the class a public no-arg constructor and make it implement kotlinx.serialization.json.JsonNamingStrategy.
If using a built-in strategy, use the Builtins form (e.g. JsonNamingStrategy.Builtins.snake_case) instead of a custom class name.
For native builds, register the class for reflection (@RegisterForReflection).

Example fix

// before
@ConfigProperty(name = "quarkus.rest.kotlin-serialization.naming-strategy")
// com.example.MyStrategy (has only a constructor taking a String)
class MyStrategy(private val cfg: String) : JsonNamingStrategy { ... }
// after
class MyStrategy : JsonNamingStrategy { ... } // public no-arg ctor

Defensive patterns

Strategy: validation

Validate before calling

// before configuring
val klass = runCatching { Class.forName("com.example.MyStrategy") }.getOrNull()
check(klass != null) { "naming strategy class not on classpath" }
check(klass.constructors.any { it.parameterCount == 0 }) { "needs a public no-arg constructor" }
check(JsonNamingStrategy::class.java.isAssignableFrom(klass))

Type guard

fun Class<*>.isValidNamingStrategy() =
    JsonNamingStrategy::class.java.isAssignableFrom(this) && constructors.any { it.parameterCount == 0 }

Prevention

Prefer built-in strategy names over custom classes
Keep strategy classes with public no-arg constructors
Add reflection registration for native builds

When it happens

Trigger: Setting quarkus.rest.kotlin-serialization.naming-strategy to a fully-qualified custom class name that does not exist on the classpath, is not public, lacks a no-arg constructor, or is not a JsonNamingStrategy.

Common situations: Typos in the config value; pointing at a class from a dependency not present at runtime; writing a constructor with args (e.g. to receive config); refactor renamed the strategy class but the property was not updated; native-image builds missing reflection registration.

Related errors

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/2e601c7bdbc5dabe. Report an issue: GitHub.