I'm encountering an inconsistency in how @JsonTypeInfo is applied when serializing collections.

When I serialize an array of polymorphic objects, Jackson includes the @type metadata as expected. However, when I serialize a List of the same objects, the @type metadata is not included, resulting in different behavior between arrays and lists.

Jackson should include @type metadata for both arrays and lists, as long as the element type is correctly annotated with @JsonTypeInfo

Example code

import com.fasterxml.jackson.annotation.*;
import com.fasterxml.jackson.databind.*;

import java.util.Arrays;
import java.util.List;

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "@type")
@JsonSubTypes({
    @JsonSubTypes.Type(value = Dog.class, name = "dog")
})
abstract class Animal {
    public String name;
}

@JsonTypeName("dog")
class Dog extends Animal {
    public String breed;

    public Dog() {
        this.name = "Fido";
        this.breed = "Labrador";
    }
}

``` java ObjectMapper mapper = new ObjectMapper(); mapper.registerSubtypes(Dog.class);

// Case 1: Array Animal[] array = new Animal[] { new Dog() }; System.out.println(mapper.writeValueAsString(array));

// Output: // [{"@type":"dog","name":"Fido","breed":"Labrador"}] ✅

// Case 2: List List list = Arrays.asList(new Dog()); System.out.println(mapper.writeValueAsString(list));

// Output: // [{"name":"Fido","breed":"Labrador"}] ❌ Missing @type

Questions

  • Is this behavior intentional? Why does Jackson serialize type info correctly in arrays but not in lists ?
  • Is there a way to globally or locally configure Jackson to include @type when serializing List ?
  • While using TypeReference> with ObjectMapper.writerFor(...) does force the inclusion of @type, this approach isn't feasible when using higher-level libraries like Spring's RestTemplate, which handles serialization internally. => Is there a recommended way to ensure @type metadata is included when sending a List through such frameworks — without manually serializing to JSON strings or switching to arrays?

Environment Jackson version: 2.14.2 (also reproduced in earlier versions) Java: 21+ Module: jackson-databind

Comment From: pjfanning

Jackson 2.19.2 is the latest release. Could you try that?

Comment From: JooHyukKim

Also, try declaring a wrapper class to contain the polymorphic list and serialize that container? considering type erasure

Comment From: cowtowncoder

Definitely infamous Type Erasure problem: when directly serializing ("root value") List<T> values, all runtime sees (and can see) is List<?>, i.e. List<Object>. And since java.lang.Object has no @JsonTypeInfo, no type info is included.

There are couple of ways to work-around this:

  1. Use helper type like public class AnimalList extends ArrayList<Animal> { }
  2. Use wrapper (like @JooHyukKim suggested) -- non-generic Java class with List<Animal> animals -- type information is retained in class declarations

  3. Construct ObjectWriter with specified type

    ObjectWriter w = mapper.writerFor(new TypeReference>() { }); String json = w.writeValueAsString(animalList);

I hope this helps.