Json schema with anyOf fields to POJO

Question

I wonder what's the recommended way to generate POJOs for Json schemas with "anyOf" fields?

For example, given the following json schemas:

hobby.json

{
    "anyOf": [
        { "type": {"$ref": "./exercise.json" } },
        { "type": {"$ref": "./music.json" } }
    ]    
}

exercise.json

{
    "type": "object"
    "properties" {
        "hobbyType": {"type": "string"}
        "exerciseName": { "type": "string" },
        "timeSpent": { "type": "number" },
        "place": { "type": "string" }
    }
}

music.json

{
    "type": "object"
    "properties" {
        "hobbyType": {"type": "string"}
        "instrument": { "type": "string" },
        "timeSpent": { "type": "number" }
    }
}

How could I generate a POJO for Hobby.java with Jackson?

Answer 1

The approach I ended up taking is using polymorphic marshaling/unmarshaling functionality provided by Jackson.

Specifically -

1. Make hobby to be an interface and annotate it with @JsonTypeInfo and @JsonSubTypes

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NAME,
    property = "hobbyType",
    include = JsonTypeInfo.As.EXISTING_PROPERTY,
    visible = true
)
@JsonSubTypes({
        @Type(value = Exercise.class, name = "exercise"),
        @Type(value = Music.class, name = "music")
})
public interface Hobby {
}

2. Create Exercise.java and Music.java that implements this interface

@Builder
@Data
@AllArgsConstructor
public class Exercise implements Hobby {
    @JsonProperty("hobbyType")
    @Builder.Default
    @NonNull
    private final String hobbyType = "exercise";    

    @JsonProperty("exerciseName")
    private String exerciseName;

    @JsonProperty("place")
    private String place;
    //... (other fields)
}

3. Use Hobby for serialization and deserialization.

// create a Hobby object
Hobby exercise = Exercise.builder().exerciseName("swimming").place("swimmingPool").build();

// serialization
String serializedHobby = new ObjectMapper.writeValueAsString(exercise)
/**
serializedHobby looks like this ->
{
    "hobbyType": "exercise",
    "exerciseName": "swimming",
    "place": "swimmingPool"
}
*/

// deserialization
Hobby deserializedObject = new ObjectMapper.readValue(jsonString, Hobby.class)
// deserializedObject.getClass() would return Exercise.java or Music.java based on the hobbyType

Ref: https://www.baeldung.com/jackson-inheritance