StdDeserializer for abstract class and it's subtypes

Question

Let there be an abstract class A with a property a and three non-abstract subclasses B, C and D. B has no additional property, C contains the property c and D contains both properties c and d.

I would like to subclass StdDeserializer for the abstract class A to be able to decide based on the existence of properties to be deserialized which subclass to choose.

I did that with some Jackson release from Codehaus before and it was working fine using the following implementation:

class AbstractADeserializer extends StdDeserializer<A> {
    AbstractADeserializer () {
        super(A.class);
    }

    @Override
    public A deserialize(JsonParser jp, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        ObjectMapper mapper = (ObjectMapper) jp.getCodec();
        ObjectNode root = (ObjectNode) mapper.readTree(jp);
        Class<? extends A> requestClass = null;

        // root.getFieldNames() is an iterator over all field names
        JsonNode cValue = root.findValue("c");
        JsonNode dValue = root.findValue("d");

        /*
         * Check for existence of required fields and choose the
         * corresponding request class.
         */
        logger.debug(Boolean.toString(c != null));
        logger.debug(Boolean.toString(d != null));

        if(c != null && d == null) {
            logger.debug("Found C");
            requestClass = C.class;
        } else if(c != null && d != null) {
            logger.debug("Found D");
            requestClass = D.class;
        } else {
            logger.debug("Found B");
            requestClass = B.class;
        }

        return mapper.readValue(root, requestClass);
    }
}

This worked fine but after migration to Jackson 2.4 from FasterXML ObjectMapper does not allow ObjectNode as a parameter for it's readValue method.

When I modified the code to use return mapper.readValue(jp, requestClass); I always get

    com.fasterxml.jackson.databind.JsonMappingException: No content to map due to end-of-input
     at [Source: org.apache.catalina.connector.CoyoteInputStream@1286ec89; line: 1, column: 559]
        at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)
        at com.fasterxml.jackson.databind.ObjectMapper._initForReading(ObjectMapper.java:3095)
        at com.fasterxml.jackson.databind.ObjectMapper._readValue(ObjectMapper.java:3009)
        at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1637)

What alternatives do you see the flexible determine the class of a given input without needing to deserialize the object manually? To me it looks complicated to clone the JsonParser to avoid the exhaustion of it's input source.

Answer 1

Just ran into similar issues, and for me it was all fixed by replacing mapper.readValue(jp, requestClass) with mapper.treeToValue(root, requestClass)