JsonTypeInfo and Optional

[matstatman]

I think i have found a bug, while upgrading my project from 2.8.7 -> 2.8.8. The classname is no longer part of the serialized json string. The test testWithOptional below fails in 2.8.8:

import static org.junit.Assert.assertTrue;

import java.io.IOException;
import java.util.Optional;

import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.datatype.jdk8.Jdk8Module;

public class Test {
    ObjectMapper mapper = new ObjectMapper().registerModule(new Jdk8Module());

    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
    @JsonSubTypes({@JsonSubTypes.Type(name = "Container", value = ChildClass.class)})
    public static class Base {
    }

    public static class ChildClass extends Base {
    }

    @org.junit.Test
    public void testWithOptional() throws IOException {
        String json = mapper.writeValueAsString(Optional.of(new ChildClass()));
        // breaks in com.fasterxml.jackson.datatype:jackson-datatype-jdk8:2.8.8 and above
        assertTrue(json, json.contains("type"));
    }

    @org.junit.Test
    public void test() throws IOException {
        String json = mapper.writeValueAsString(new ChildClass());
        assertTrue(json, json.contains("type"));
    }
}

[cowtowncoder]

I don't know how it could have worked differently before, but I don't think type information should be included in this case. Problem is Java Type Erasure, and the type Jackson sees is simply Optional<?>, which does not allow determination of base type as anything other than Object.
This is different from case of directly serializing ChildClass since it is non-generic class and runtime type is used as is.

This is different from having a POJO with such Optional: if you had

public class Wrapper {
   public Optional<Base> value;
}

type information would be included.

So, this is really FAQ, although more commonly encountered with Lists and Collections.

Unfortunate part is if earlier versions did make this appear to work: it should not have behaved that way. I think change is probably due to fix for #15.

As to work-around: two common methods are:

1. Sub-classing, something like

   public class BaseOptional extends Optional { }
   // is above possible wrt factory methods?

2. Declaring of full generic type

   mapper.writerFor(new TypeReference<Optional>() { })
   .writeValueAsString(value);

I hope this helps.

[matstatman]

Ok, thanks a lot!

[alexism]

FYI This is the root cause of the deserialization issue described at #59