SpringBoot: Request body with inheritance

Question

I have following classes

public class Car {
     private String make; 
     private String model; 
     
     private TransmissionType transmissionType; // Manual/Automatic
     private Transmission transmission; 
}

public class Transmission {
}

public class AutomaticTransmission {
     public Technology technology; // DCT/CVT/AMT
}

public class ManualTransmission {
     public int numGears; 
}

In this example, when the payload passed could be

{
    "make": "Toyota", 
    "model": "Iris", 
    "transmissionType": "Automatic", 
    "transmission" {
         "technology": "DCT"
    }
}

or it could be

{
    "make": "Merc", 
    "model": "C", 
    "transmissionType": "Manual", 
    "transmission" {
         "numGears": 5
    }
}

When this is passed to controller as body, it should be able to create respective class. How do I achieve this.

I tried using JsonTypeInfo and JsonSubTypes as shown in this answer but it necessitates me to move transmissionType within Transmission class, which is not the case for me.

Answer 1

You're looking for @JsonTypeInfo(include = As.EXTERNAL_PROPERTY) Transmission transmission . This tells Jackson to look for the property that defines the type in the containing object .

That said, if you have any opportunity of changing your JSON shape, you should avoid "externally tagged types" like this and put the type property into the object.

Answer 2

With help of @chrylis's pointers.

I wrote the code as below

class Car { 
    ... 
    private TransmissionType transmissionType;
    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXTERNAL_PROPERTY, property = "transmissionType")
    private Transmission transmission; 
}

@JsonSubTypes({
        @JsonSubTypes.Type(value = AutomaticTransmission.class, name = "AUTOMATIC"),
        @JsonSubTypes.Type(value = ManualTransmission.class, name = "MANUAL")
})
public abstract class Transmission {
}

Few gotchas I found, mentioning FYI.

If you put JsonSubTypes on the field in Car class, Swagger documentation won't work. If you put, @JsonTypeInfo on the class Transmission then SpringBoot won't recognise the field "transmissionType".