How to Serialize a Map as List using Jackson

Question

How can I serialize a property which is a Map as a List of the Map's values? I've been able to do other simple conversions using the @JsonSerialize(using=...) annotation on the getter. However, I am not sure if one exists for what I want to do.

Answer 1

We needed something similar, in our case we used a customized @JsonSerialize as you commented, and it was stupid simple:

public class MyCustomSerializer extends JsonSerializer<Map<?, ?>> {
    @Override
    public void serialize(final Map<?, ?> value, final JsonGenerator jgen, final SerializerProvider provider)
            throws IOException, JsonProcessingException {
        jgen.writeObject(value.values());
    }
}

Code using it:

import java.io.IOException;
import java.util.Collections;
import java.util.Map;

import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.JsonProcessingException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.JsonSerializer;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.map.SerializerProvider;
import org.codehaus.jackson.map.annotate.JsonSerialize;

public class JacksonTest {

    public static class ModelClass {
        private final Map<String, String> map;

        public ModelClass(final Map<String, String> map) {
            super();
            this.map = map;
        }

        @JsonSerialize(using = MyCustomSerializer.class)
        public Map<String, String> getMap() {
            return map;
        }

    }

    public static void main(final String[] args) throws JsonGenerationException, JsonMappingException, IOException {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.writeValue(System.out, new ModelClass(Collections.singletonMap("test", "test")));
    }

}

Answer 2

I implemented using default Serializer to handle values that are not just String :

@Override
public void serialize(final Map<Long, ?> value, final JsonGenerator jgen, final SerializerProvider provider) throws IOException,
            JsonProcessingException {
   provider.defaultSerializeValue(value.values(), jgen);
}

EDIT : As mentioned by Radu Simionescu this solution only works for Maps of Pojos.