Deserialize JSON array containing instance of sealed class

Hello,

I am getting unusual behavior when I deserialize a JSON array that contains an instance of a sealed class. I expect an exception to be thrown since a sealed class cannot be instantiated, but instead it is being deserialized as a LinkedHashMap.

I have the following class definitions:

val json = ObjectMapper().registerKotlinModule()

@JsonTypeInfo(
        use = JsonTypeInfo.Id.NAME,
        include = JsonTypeInfo.As.EXISTING_PROPERTY,
        property = "schema",
        visible = true
)
@JsonSubTypes(value = *arrayOf(
        JsonSubTypes.Type(value = Child::class, name = "Child")
))
private sealed class AbstractParent {
    abstract val schema: String
    abstract val a: String
}

@JsonTypeName("Child")
private data class Child(
        override val schema: String,
        override val a: String,
        val b: String
): AbstractParent()

I have posted a Gist that contains all tests I am running related to this here. All are successful, except for one. The specific test that exposes the behavior in question is copied below.

@Test(expected = JsonMappingException::class)
fun testSerializeDeserializeParentList() {
    val source = json.readTree("""
    [{
        "schema": "AbstractParent",
        "a": "foo"
    }]
    """.trimIndent())

    val deserialized: List<AbstractParent> = json.treeToValue(source)
    val serialized = json.readTree(json.writeValueAsString(deserialized))

    assertEquals(source, serialized)
}

As you can see, the JSON object within the array follows the model of the AbstractParent class, which should not be able to be deserialized since AbstractParent is a sealed class. In a test where I try to deserialize a standalone instance of AbstractParent, a JsonMappingException is thrown. I would expect to get that same exception when it is contained in an array, but instead no exception is thrown. When I inspect deserialized in a debugger, I see that the object it contains is a LinkedHashMap which is very unusual. When I try to access deserialized.first() I get a ClassCastException because LinkedHashMap is not a sub-type of AbstractParent. It seems like a fix is needed to make sure that there is an immediate exception when deserializing the JSON, instead of the late exception we see here.

Thanks

[cowtowncoder]

@alexrwasserman Ok good. This makes sense. Glad you have a solution that works.

Fwtw, use of generic types as root values has been (… and, will be) a big source of problems, so I recommend users to consider avoiding it when possible. As you have found it can be made to work, but there’s bit extra code, and can be non-obvious to tackle. Work-around include use of helper class that binds type (class AbstractParentList extends List<AbstractParent>), to achieve the end result by passing non-generic type.

[alexrwasserman]

Just to clarify, we are using this function declared in com.fasterxml.jackson.module.kotlin.Extensions:

inline fun <reified T: Any> ObjectMapper.treeToValue(n: TreeNode): T = treeToValue(n, T::class.java)

Your response helped us pinpoint the issue and resolve it. We thought that the value being passed for T was List<AbstractParent>, but it actually was just the generic List interface, which would explain the results we are seeing.

The solution we went with is this:

val deserialized = json.readValue<List<AbstractParent>>(json.writeValueAsString(source), object : TypeReference<List<AbstractParent>>(){})

Not the most elegant, but we didn’t see a better way