JsonSubTypes annotated class not serialized properly when no property using Micronaut Serdes

[loicgreffier]

Expected Behavior

Using Micronaut serdes, a class with no property but annotated with @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "validation-type") and @JsonSubTypes should contain the JsonType property validation-type when serialized.

Actual Behaviour

When serializing a class that has no property but a property defined by @JsonTypeInfo jackson annotation:

E.g.:

@JsonTypeInfo(
        use = JsonTypeInfo.Id.NAME,
        property = "validation-type")
@JsonSubTypes({
        @JsonSubTypes.Type(value = NonEmptyString.class, name = "NonEmptyString"),
})
public interface Validator { }

@Data
@Serdeable
@NoArgsConstructor
public static class NonEmptyString implements ResourceValidator.Validator { }

I was expecting the serialization output to contain the property validation-type, but it does contain any field at all, using the Micronaut serdes.

The same implementation is working with the jackson-databind feature.

Using micronaut-serde-jackson

13:46:55.987 [Test worker] INFO  com.example.DemoTest - Result: Optional[{validator={}}]

The output does not contain the property validation-type.

Using micronaut-jackson-databind

13:46:55.987 [Test worker] INFO  com.example.DemoTest - Result: Optional[{validator={validation-type=NonEmptyString}}]

The output contains the property validation-type as expected.

Additional Info

When adding any field to the class, it is serialized as expected using Micronaut serdes:

13:51:04.925 [Test worker] INFO  com.example.DemoTest - Result: Optional[{validator={validation-type=NonEmptyStringWithField, anyField=true}}]

The output contains the property validation-type as expected.

Steps To Reproduce

1. Clone https://github.com/loicgreffier/issue-micronaut-serdes
2. Checkout the branch subtypes-nofield and run the unit test. It runs with implementation("io.micronaut.serde:micronaut-serde-jackson").
3. See the output logs
4. Checkout the branch subtypes-nofield-jackson and run the unit test. It runes with implementation("io.micronaut:micronaut-jackson-databind").
5. See the output logs

Environment Information

• JDK Eclipse Temurin v17.0.2

Example Application

https://github.com/loicgreffier/issue-micronaut-serdes

Version

4.1.0

[loicgreffier]

Investigated a little bit more...

Based on the fact that it works for a class with attributes (e.g., Range in my example), but does not for a class without attributes (e.g., NonEmptyString, I could find, from my example, that:

• Range child class is serialized by a CustomObjectSerializer, which is able to get all the attribute, including @JsonTypeInfo
• NonEmptyString child class is serialized by a SimpleObjectSerializer, which is not able to

I could see this in the CustomizedObjectSerializer class itself: https://github.com/micronaut-projects/micronaut-serialization/blob/master/serde-support/src/main/java/io/micronaut/serde/support/serializers/CustomizedObjectSerializer.java#L81.

Going deeper, I could see that when selecting the appropriate serializer for child attributes, it drives me to this condition:

micronaut-serialization/serde-support/src/main/java/io/micronaut/serde/support/serializers/SerBean.java

Line 184 in 8f0e808

if (!properties.isEmpty() || !jsonGetters.isEmpty()) {

which precisely compute the attribute of @JsonTypeInfo. Unfortunately, it requires properties not to be empty to be honored.. which is not the case of my NonEmptyString child class that has no properties.