ClassCastException when deserializing with ObjectMapper onto parametrized class, even using TypeReference and TypeFactory

Question

I'm getting the following exception when trying to deserialize with ObjectMapper onto a parameterized class (works fine for non-parametrized classes):

java.lang.ClassCastException: class java.util.LinkedHashMap cannot be cast to class com.xyz.A (java.util.LinkedHashMap is in module java.base of loader 'bootstrap'; com.xyz.A is in unnamed module of loader 'app')

Here's the original code:

Foo<A> request = OBJECT_MAPPER.readValue(payload, Foo.class);

I tried:

Foo<A> request = OBJECT_MAPPER.readValue(payload, new TypeReference<Foo<A>>() {});

As well as:

JavaType myType = OBJECT_MAPPER.getTypeFactory()
    .constructParametricType(Foo.class, A.class);
Foo<A> request = OBJECT_MAPPER.readValue(payload, myType);

But I still get the same exception.

Could there be something special about my scenario that's not covered in these questions?

• Jackson is not deserialising a generic list that it has serialised
• Jackson and generic type reference
• Jackson - Deserialize using generic class

One thing I can think of is that my Foo is actually an @AutoMatter-annotated interface that generates the class:

@AutoMatter
public interface Foo<T> {
  Optional<T> parent;
  Optional<List<T>> children;
}

Normally we have no issues mapping onto AutoMatter-generated classes though. It's just adding the parametrization <T> that seems to be causing issues.

Does anyone have an idea?

---

Edit to answer @MichalZiober's questions:

In my test code I'm actually just serializing what I know is a valid object, i.e. then deserializing that to get back the object I started with:

Foo<A> myExampleObject;
ByteString.encodeUtf8(OBJECT_MAPPER.writeValueAsString(myExampleObject));

---

Edit 2

Okay, so it looks like we are already importing that module:

  @VisibleForTesting
  public static final ObjectMapper OBJECT_MAPPER = new ObjectMapper()
      .registerModule(new JodaModule())
      .registerModule(new GuavaModule())
      .registerModule(new AutoMatterModule())
      .registerModule(new Jdk8Module())
      .registerModule(new ProtobufModule())
      .setSerializationInclusion(JsonInclude.Include.NON_NULL)
      .configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false)
      .configure(DeserializationFeature.FAIL_ON_IGNORED_PROPERTIES, false)
      .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
      .configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);

Answer 1

When you use Optional in POJO structure you need to enable Jdk8Module from jackson-modules-java8. Below example shows that with this module registered we can serialise and deserialise data:

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.datatype.jdk8.Jdk8Module;
import io.norberg.automatter.AutoMatter;
import io.norberg.automatter.jackson.AutoMatterModule;

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

public class JsonApp {

    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);
        mapper.registerModule(new AutoMatterModule());
        mapper.registerModule(new Jdk8Module());

        String json = "{\"parent\":\"Aaaa\", \"children\":[\"a\"]}";
        System.out.println(mapper.readValue(json, Foo.class));


        Foo<StringWrapper> foo = new FooBuilder<StringWrapper>()
                .parent(new StringWrapperBuilder().value("PARENT").build())
                .children(Arrays.asList(new StringWrapperBuilder().value("CHILD1").build()))
                .build();
        json = mapper.writeValueAsString(foo);
        System.out.println(json);
        System.out.println(mapper.readValue(json, Foo.class));
    }
}
@AutoMatter
interface Foo<T> {
    Optional<T> parent();
    Optional<List<T>> children();
}

@AutoMatter
interface StringWrapper {
    String value();
}

Above code prints:

Foo{parent=Optional[Aaaa], children=Optional[[a]]}
{
  "parent" : {
    "value" : "PARENT"
  },
  "children" : [ {
    "value" : "CHILD1"
  } ]
}
Foo{parent=Optional[{value=PARENT}], children=Optional[[{value=CHILD1}]]}