How to use my custom generator while deserializing data classes

[aiguofer]

I wrote a custom generator to truncate text fields past a certain length but it doesn't seem to work on data class fields.

I'm using Kotlin with Klogger and have a function like this that allows me to pass a lambda that builds a map and allows me to set key, value pairs as Markers:

fun KLogger.debug(msg: () -> Any?, extra: (MutableMap<String, Any?>) -> Unit) {
    if (isDebugEnabled) {
        val entries = mutableMapOf<String, Any?>().apply { extra(this) }
        debug(appendEntries(entries), msg)
    }
}

To test this, I wrote a custom serializer for a NestedObject data class, a custom JsonFactory to add that serializer, and a custom JsonGenerator to do the actual truncation:

class CustomJsonGeneratorDecorator : JsonGeneratorDecorator {
    override fun decorate(generator: JsonGenerator) = CappedFieldJsonGenerator(generator)
}

class CappedFieldJsonGenerator(delegate: JsonGenerator) : JsonGeneratorDelegate(delegate) {
    companion object {
        private const val MAX_FIELD_LENGTH = 3
    }

    override fun writeRawUTF8String(text: ByteArray?, offset: Int, length: Int) {
        super.writeRawUTF8String(text, offset, min(MAX_FIELD_LENGTH, length))
    }

    override fun writeUTF8String(text: ByteArray, offset: Int, length: Int) {
        super.writeUTF8String(text, offset, min(MAX_FIELD_LENGTH, length))
    }

    override fun writeString(reader: Reader, len: Int) {
        super.writeString(reader, min(MAX_FIELD_LENGTH, len))
    }

    override fun writeString(text: CharArray, offset: Int, len: Int) {
        super.writeString(text, offset, min(MAX_FIELD_LENGTH, len))
    }

    override fun writeString(text: SerializableString) {
        this.writeString(text.value)
    }

    override fun writeString(text: String) {
        super.writeString(text.substring(0, min(MAX_FIELD_LENGTH, text.length)))
    }
}

data class NestedObject(val name: String = "nested")

data class TestObject(
    val name: String = "hello",
    val nested: NestedObject = NestedObject(),
)

class TestObjectSerializer @JvmOverloads constructor(t: Class<NestedObject>? = null) :
    StdSerializer<NestedObject>(t) {

    override fun serialize(value: NestedObject, gen: JsonGenerator, provider: SerializerProvider) {
        gen.writeStartObject()
        gen.writeStringField("name", "serialized")
        gen.writeStringField("class", gen::class.java.name)
        gen.writeEndObject()
    }
}

class CustomJsonFactoryDecorator : JsonFactoryDecorator {
    override fun decorate(factory: JsonFactory): JsonFactory {
        val mapper = factory.codec as ObjectMapper
        val module = SimpleModule()
        module.addSerializer(NestedObject::class.java, TestObjectSerializer())
        mapper.registerModule(module)
        return factory
    }
}

When I call the following log:

        logger.info({ "debug" }) {
            it["test_object"] = TestObject()
            it["plain_string"] = "string"
        }

I get the following result:

{"@timestamp":"202","@version":"1","message":"deb","logger_name":"com","thread_name":"mai","level":"INF","level_value":20000,"dd.service":"sem","dd.env":"aws","dd.version":"0.0","test_object":{"name":"hello","nested":{"name":"serialized","class":"com.fasterxml.jackson.core.json.UTF8JsonGenerator"}},"plain_string":"str","pod_name":"sem","pod_namespace":"dev","node_name":"ip-","node_ip":"10."}

As you can see, the truncate logic works on all fields (including plain text markers) except for those on the TestObject and those on the NestedObject with a custom serializer. It seems the custom serializer is getting UTF8JsonGenerator instead of my CappedFieldJsonGenerator.

Any ideas on how to fix this?