Mapping one custom Java field to many JSON fields using Jackson @JsonDeserializer

I have a java class representing a JSON using Jackson. All of the fields, with one exception, can be translated using no annotations at all. 1-to-1, simple translations (although some of them are nested POJOs).

@Data
@AllArgsConstructor
@NoArgsConstructor
public class MyPojo {
    private String someString;
    private AnotherPojo someOtherPojo;
    //The problem child:
    private Object value;
}

The field value which is an exception to this rule, can represent any JSON field matching value* where * is a wildcard of indefinite length. That means valueString or valueReference in JSON will be assigned to this field with the assertion that only one may be present.

{
  "someString": "asdasdadasdsa",
  "someOtherPojo": {
    "someOtherProperty": "whatever"
  },
  "valueCodeableConcept": {
    "text": "text value",
    "coding": [
      {
        "code": "my-code"
      }
    ]
  }
}

Using a custom deserializer on the top-level class, I can scrape all of the fields from the root node (baseNode in the following example) that start with value and set the value field appropriately. That works great! However, in doing so, I now have to set every other field in this MyPojo class manually in my deserializer, and I have to put a custom copy of this deserializer on each POJO that uses a field like value*.

private Object parseValueX(JsonNode baseNode, DeserializationContext context) throws IOException {
    //Find the concrete implementation referred to by the value[x] field
    Set<String> concreteNames = new HashSet<>();
    baseNode.fieldNames().forEachRemaining(name -> {
        if (name.startsWith("value")) {
            concreteNames.add(name);
        }});

    if (concreteNames.isEmpty()) {
        return null;
    }

    if (concreteNames.size() > 1) {
        throw JsonMappingException.from(context, "The field value[x] must have no more than one concrete " +
                "implementation, ex: valueCode, valueCodeableConcept, valueReference");
    }

    String concreteName = concreteNames.stream().findFirst().orElseThrow(() -> new RuntimeException(""));
    JsonNode jsonSource = baseNode.get(concreteName);

    //...deserialize from jsonSource, solved, but not relevant to question...
}

To make this apply to any value* property on any POJO, I tried to move the deserializer to the value attribute in the POJO (whereas it's on the top-level resource now). The first flaw is that the deserializer isn't even invoked unless the JSON property exactly matches value. What I actually need is for the entire parent JSON resource to be passed to that field-specific deserializer, so that I may find the matching field and assign it -- OR -- I need to be able to have the deserializer on MyPojo only assign the one field value and allow the automatic deserialization to take care of the others. How do I do either of these?

For those curious about my motivation, I am implementing the HL7 FHIR Specification, which specifies generic attributes called value[x] (here's one example: https://www.hl7.org/fhir/extensibility.html#Extension) where [x] becomes the type of the resource.

Solution

I think a good fit for you problem is @JsonAnySetter. This method annotation tells Jackson to route unknown properties to it. the arg (in your case) is a Map containing the json tree of the unknown property. if I understand your code properly, the name of the value property contains the class name of the target Pojo. so once you have a class name, you can tell Jackson how to "deserialize" the map into an instance of the target class.

Here is an example based on the code from the question

public class MyPojo {
    public String someString;  // made properties into public for this example...
    public AnotherPojo someOtherPojo;
    public Object value;

    @JsonAnySetter
    public void setValue(String name, Object value) {
        System.out.println(name + " " + value.getClass());
        System.out.println(value);

        // basic validation
        if (name.startsWith("value") && value instanceof Map) {
            String className = "com.company." + name.substring("value".length());
            System.out.println(name + " " + value.getClass() + " " + className);
            System.out.println(value);
            try {
                // nice of Jackson to be able to deserialize Map into Pojo :)
                ObjectMapper mapper = new ObjectMapper();
                this.value = mapper.convertValue(value, Class.forName(className));
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println(this.value + " " + this.value.getClass());
        }

    }
}

public class AnotherPojo {
    public String someOtherProperty;
}

public class CodeableConcept {
    public String text;
    public Code[] coding;
}

public class Code {
    public String code;
}