I need to conditionally serialize a Map to a String. I.e. when Map meets some condition, I want to replace the whole resulting JSON object with the single custom value, e.g. String. I am trying to implement it like this:

public class MapParseTest
{
    private final ObjectMapper mapper;

    public MapParseTest()
    {
        mapper = new ObjectMapper();
        SimpleModule module = new SimpleModule();
        module.setSerializerModifier(new MyMapSerializerModifier());
        mapper.registerModule(module);
    }

    public void doTest() throws JsonProcessingException
    {
        MyMap map1 = new MyMap();
        MyMap map2 = new MyMap();

        map1.setName("shtame");
        map2.setName("magic name");

        map1.put("First", 1);
        map1.put("Second", "Two");
        map1.put("Third", map2);

        map2.put("Eleventh", 11);
        map2.put("Twelfth", "12");

        String result = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(map1);
        System.out.println(result);
    }

    public static void main(String[] args) throws JsonProcessingException
    {
        MapParseTest test = new MapParseTest();
        test.doTest();
    }

    @Setter
    @Getter
    public static class MyMap extends HashMap<String, Object>
    {
        private String name;
    }

    public static class MyMapSerializer extends JsonSerializer<MyMap>
    {
        private final JsonSerializer<Object> defaultSerializer;

        public MyMapSerializer(JsonSerializer<Object> defaultSerializer)
        {
            this.defaultSerializer = defaultSerializer;
        }

        @Override
        public void serialize(MyMap value,
                              JsonGenerator gen,
                              SerializerProvider provider) throws IOException
        {
            String className = value.getName();
            if (className.equalsIgnoreCase("magic name"))
            {
                gen.writeString("magic string");
            } else
            {
                defaultSerializer.serialize(value, gen, provider);
            }
        }
    }

    public static class MyMapSerializerModifier extends BeanSerializerModifier
    {
        @Override
        public JsonSerializer<?> modifyMapSerializer(SerializationConfig config,
                                                     MapType mapType,
                                                     BeanDescription beanDesc,
                                                     JsonSerializer<?> serializer)
        {
            Class<?> type = beanDesc.getBeanClass();
            if (type.equals(MyMap.class))
            {
                return new MyMapSerializer((JsonSerializer<Object>) serializer);
            }
            return serializer;
        }
    }
}

I expect it will produce

{
  "Second" : "Two",
  "Third" : "magic string",
  "First" : 1
}

instead of

{
  "Second" : "Two",
  "Third" : {
    "Eleventh" : 11,
    "Twelfth" : "12"
  },
  "First" : 1
}

But I rather have a

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Cannot invoke "com.fasterxml.jackson.databind.JsonSerializer.serialize(Object, com.fasterxml.jackson.core.JsonGenerator, com.fasterxml.jackson.databind.SerializerProvider)" because "keySerializer" is null (through reference chain: st.notexi.test.MapParseTest$MyMap["Second"])

I probably have to implement modifyKeySerializer as well, but I don’t understand how, and cannot find any documentation for it. I realize that Map itself should have some key, for example in case it is a root map. But I don’t understand how to deal with it.

Please help – how could it be implemented?