JsonTypeInfo does not work instance based in collections

Using:

@JsonTypeInfo(
    use = Id.NAME,
    include = As.PROPERTY,
    property = "_class"
)

On a Hibernate entity class (this could make a difference, as Hibernate tends to proxy objects), when serializing a single object, I correctly get the _class field:

new ObjectMapper().writeValueAsString(base.getSubScopes().get(0));
// Returns: {"_class":"Institute","id":36}

But for the entire collection it returns the JSON without _class data:

new ObjectMapper().writeValueAsString(base.getSubScopes());
// Returns: [{"id":36}]

The collection is of the Hibernate type PersistentBag. If I wrap the samething in an Arraylist though, I get the same result:

new ObjectMapper().writeValueAsString(Lists.newArrayList(base.getSubScopes()));
// Returns: [{"id":36}]

I see this behaviour on both 2.7.x and 2.8.0 versions.

[wetneb]

Here is a workaround to force Jackson to serialize the class name at all times:

@JsonTypeInfo(
        use=JsonTypeInfo.Id.CLASS,
        include=JsonTypeInfo.As.EXISTING_PROPERTY,
        property="class")
public interface MyInterface {
	
	@JsonProperty("class")
	public default String getClassName() {
		return this.getClass().getName();
	}
}

Note the use of EXISTING_PROPERTY to avoid adding duplicate "class" attributes if serializing the instance as a root object.

[cowtowncoder]

@JWGmeligMeyling super-class annotations are considered, that is not the problem. Problem is that type information for root value (List) has element type of ?, that is, roughly Object. So information is simply not available when considering serializer for List value; unless explicitly passed. This is how 2 work-arounds work: they make type information available for root values too, either by sub-classing (in which case generic types are available from class definition), or by telling ObjectMapper/ObjectWriter generic type.

And yes, third work around would indeed be to have non-generic POJO as root value; in that case full type is available. That is probably the cleanest approach.