objectMapper.treeToValue is error-prone, should be either deprecated or fixed (or removed?)

[Fuud]

Use case
There is old issue: #45

Main problem: ObjectMapper.treeToValue looks very similar to ObjectMapper.readValue but suffers from type erasure.
It is very simple to missuse it.
Current behaviour is because ObjectMapper.treeToValue(TreeNode, TypeReference) does not exist.
As suggested in FasterXML/jackson-databind#1294 we can use readValue or convertValue.

Describe the solution you'd like
Let's either:

1. reimplement ObjectMapper.treeToValue in type-safe way
2. deprecate it with descriptive message

[dinomite]

Huh, I've never used treeToValue, I think the best fix would be to implement it using Tatu's suggestion in that ticket: FasterXML/jackson-databind#1294 (comment)

[cowtowncoder]

@Fuud why is this in Kotlin module? If it's about ObjectMapper, this belong under jackson-databind.

I am also not quite certain I understand why you claim treeToValue() is error-prone, compared to, say, readValue().
And I certainly would not want to deprecate the existing method, as that is convenient for vast majority of use cases where target type is not generic.

However: I'd be +1 for adding overloads where type can be passed as JavaType or TypeReference<T>: these would be rather easy to add one(s) for 2.13 at this point. I'd be happy to do that (or help merge a PR) if this is something something wants. All we need is an issue in jackson-databind asking for 1 or 2 overloads (might as well do both: one with JavaType is mandatory, TypeReference optional).

Note, also, that ObjectReader has 2 variants: JavaType taking one added in 2.13.

[Fuud]

@cowtowncoder

There is no such problem in core jackson library because it is clean signature there: treeToValue

public <T> T treeToValue(TreeNode n, Class<T> valueType)

It is clear that type erasure will work in this case.

But jackson-module-kotlin brings extension functions
And without diving in implementations you cannot say which line is wrong:

import com.fasterxml.jackson.module.kotlin.convertValue
import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
import com.fasterxml.jackson.module.kotlin.readValue
import com.fasterxml.jackson.module.kotlin.treeToValue

data class Person(val name: String)
fun main() {
    val mapper = jacksonObjectMapper()
    val node = mapper.readTree("""[ {"name":"Alice"}, {"name" : "Bob"} ]""")


    val readValueResult: List<Person> = mapper.readValue(node.toString())
    val convertValueResult: List<Person> = mapper.convertValue(node)
    val treeToValueResult: List<Person> = mapper.treeToValue(node)!!


    println("readValueResult Classes: ")
    println(readValueResult.map { it::class.simpleName })

    println("convertValueResult Classes: ")
    println(convertValueResult.map { it::class.simpleName })

    println("treeToValueResult Classes: ")
    println(treeToValueResult.map { it::class.simpleName })

}

Click to see output

output:

readValueResult Classes: 
[Person, Person]
convertValueResult Classes: 
[Person, Person]
treeToValueResult Classes: 
Exception in thread "main" java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to Person
  at MainKt.main(main.kt:42)
  at MainKt.main(main.kt)

[cowtowncoder]

Ahhhh. Thank you @Fuud that makes sense. Thank you for clarifying!