The issue, really, is in Jackson's API here. Here's how the readValue method is declared:

public <T> T readValue(String content, TypeReference valueTypeRef)

They are using the raw type of TypeReference for some reason, even though they could easily take a TypeReference<T> as their parameter. If they did, you code would work as is, as Kotlin could then infer the T generic type parameter of the function, and therefore know its return type.

You can work around this issue a couple different ways, however, by being explicit with your types.

• Either by providing the generic type parameter for the function, and having the type of the msg variable inferred:

    val typeRef: TypeReference<Map<K, V>> = object : TypeReference<Map<K, V>>() {} 
    val msg = objectMapper.readValue<HashMap<String, String>>(message.payload, typeRef)
  

• Or alternatively, by explicitly typing your variable, and having the function's type parameter inferred:

    val msg: HashMap<String, String> = objectMapper.readValue(message.payload, typeRef)
  

With Jackson Kotlin Module current versions, if you import the full module package or the specific extension function you'll have all extension methods available. Such as:

import com.fasterxml.jackson.module.kotlin.*  
val JSON = jacksonObjectMapper()  // keep around and re-use
val myList: List<String> = JSON.readValue("""["a","b","c"]""")

Therefore the Jackson Module for Kotlin will infer the the correct type and you do not need a TypeReference instance.

so your case (slightly renamed and fixed the data class, and JSON):

import com.fasterxml.jackson.module.kotlin.readValue

data class MyData(val a: String, val b: Int)
val JSON = jacksonObjectMapper()  

val jsonStr = """[{"a": "value1", "b": 1}, {"a": "value2", "b": 2}]"""
val myList: List<MyData> = JSON.readValue(jsonStr)

You can also use the form:

val myList = JSON.readValue<List<MyData>>(jsonStr)

Without the import you will have an error because the extension function is not found.