Jackson JSON: Unmarshalling a Map<String,Object> of attributes back into its original class

Question

I'm using a third-party library ( Elasticsearch 2.3.3 , to be exact) which already parses a given JSON structure into a Map<String,Object> instance for me, and I want to create an instance of the class of the object which was originally marshalled into JSON:

1. Marshalling: MarshallableObject → marshalled_object.json
2. Unmarshalling: marshalled_object.json → Map<String,Object> → MarshallableObject )

Additionally, here is some example code illustrating the logic:

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.Arrays;
import java.util.Map;
import java.util.stream.Stream;

import org.elasticsearch.action.search.SearchResponse;
import org.elasticsearch.client.transport.TransportClient;
import org.elasticsearch.common.transport.InetSocketTransportAddress;
import org.elasticsearch.index.query.QueryBuilders;

import com.fasterxml.jackson.databind.ObjectMapper;

public final class AttrMapUnmarshallingTest {

    /**
     * This class was directly used in the process for creating JSON documents indexed by Elasticsearch
     */
    public static final class MarshallableObject {

        public String bar;

        public String foo;

    }

    private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();

    public static void main(final String[] args) throws UnknownHostException {
        try (final TransportClient client = TransportClient.builder().build().addTransportAddress(new InetSocketTransportAddress(InetAddress.getLoopbackAddress(), 9300))) {

            final SearchResponse response = client.prepareSearch("news").setTypes("article")
                .setQuery(QueryBuilders.matchQuery("text", "advertisement")).execute().actionGet();
        final Stream<Map<String, Object>> attrMaps = Arrays.stream(response.getHits().getHits())
                .map(hit -> hit.sourceAsMap());
        final Stream<MarshallableObject> objs = attrMaps.map(attrMap -> {
            // TODO: Find way of converting attr map into original class
            // which was marshalled into JSON
            // final MarshallableObject obj = OBJECT_MAPPER.read(attrMap);
            });
        }
    }

}

Answer 1

I did some reading on the SearchHit class, and this is what I think may work:

Stream<MarshallableObject> objs = Arrays.stream(response.getHits().getHits())
    .filter(hit->hit.hasSource()) // For some reason source could be null.
    .map(hit->
         OBJECT_MAPPER.readValue(hit.sourceAsString(), 
                                 MarshallableObject.class));

Note: You need to declare the MarshallableObject class static to make the reading work. Apparently, the hidden ParentClass$this reference cannot be reconstructed from the json string.

Good luck.