Custom Json Deserializer in Jackson for Hashmap only

Question

I'm writing json serialization (using Jackson) for a hierarchy of Java classes, ie the classes are composed of other classes. Since, I'm not serializing all properties, I've used JsonViews and have annotated only those properties that I want to serialize. The class at the top of this hierarchy contains a Map which also needs to be serialized/deserialized. Is it possible to write a serializer/deserializer only for the Map ? I want the default serializer to take care of serializing the rest of the objects

Why this requirement ? If I define a serializer for the topmost class, then I would need to do the serialization for all the objects. The JsonGenerator object seems to ignore the JsonView annotations and serializes all properties.

Answer 1

Sure its possible. You define your custom Serializer with the Map class generic type, then bind it using Jackson module subsystem.

Here is an example: (it produces silly custom serialization, but the principal is valid)

public class Test
{
    // the "topmost" class
    public static class DTO {
        public String name = "name";
        public boolean b = false; 
        public int i = 100;

        @JsonView(MyView.class)
        public Map<String, String> map; {
            map = new HashMap<>();
            map.put("key1", "value1");
            map.put("key2", "value2");
            map.put("key3", "value3");
        }
    }

    // just to prove it works with views...
    public static class MyView {}

    // custom serializer for Map 
    public static class MapSerializer extends JsonSerializer<Map> {
        @Override
        public void serialize(Map map, JsonGenerator gen, SerializerProvider serializers) throws IOException, JsonProcessingException {
            // your custom serialization goes here ....
            gen.writeStartObject();
            gen.writeFieldName("map-keys");
            gen.writeStartArray();
            gen.writeString(map.keySet().toString());
            gen.writeEndArray();
            gen.writeFieldName("map-valuess");
            gen.writeStartArray();
            gen.writeString(map.values().toString());
            gen.writeEndArray();
            gen.writeEndObject();
        }
    }

    public static void main(String[] args) {
        SimpleModule module = new SimpleModule();
        module.addSerializer(Map.class, new MapSerializer());
        ObjectMapper mapper = new ObjectMapper();
        mapper.disable(MapperFeature.DEFAULT_VIEW_INCLUSION);
        mapper.registerModule(module);
        try {
            mapper.writerWithView(MyView.class).writeValue(System.out, new DTO());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}