Jackson - Generics- T as reified parameter

Issue

I'm trying to implement an abstract message-consumer with kotlin and jackson for RabbitMQ. The setup for the consumers works fine, so I omit that in the code-snippets. The Problem is only in regard to parsing the String-payload to a generic-type object.

My Goal is to have an abstract class like :

abstract class AbstractRetryConsumer<T>(val queueName:String) : MessageListener {
// Autowiring and seting up the Beans
...
    override fun onMessage(msg: Message) {
       val msgPayload = String(msg.body, Charsets.UTF_8)
  // the following line doesn't compile
       val readValue = objectMapper.readValue<T>(msgPayload)
       processEvent(readValue)
    }

    abstract fun processEvent(payload: T)
...
}

Then I'd be able to implement a Consumer like:

@Service
class NotificationConsumer(
) : AbstractRetryConsumer<Notification>("myQueue") {

// gets the payload as a parsed object
    override fun processEvent(payload: Notification) {
        println("payload ist: $payload")
    }
}

The compiler won't accept that. The error is: Cannot use 'T' as reified type parameter. Use a class instead.

I know there are many similar questions, but I didn't find an answer to my problem.

Thanks for your inquires and answers

Solution

When using the Jackson Kotlin module, the object mapper has an inline extension readValue with reified generic parameter. Since the paramter in your call objectMapper.readValue<T>(msgPayload) is reified, your class AbstractRetryConsumer<T> also needs to define a reified parameter T. However, that means your class has to be inline, and it looks too big to be inline. Therefore, you should add a JavaClass argument to your constructor and call the non-extension version of readValue:

val readValue = objectMapper.readValue(msgPayload, payloadClass)

Answered By - k314159