How to register a custom list serializer with Spring Boot / Jackson

Recently, I ran into a situation where I wanted to customize the Json serialization of a list in an API response.

TL/DR: you can find the full source code on my github: https://github.com/alexthered/spring-jackson-custom-list-serializer

Concretely, I have a list of language strings:

public class LanguageString {

    Locale locale;
    String value;
}

and it should be serialized to: (a map-like format instead of a Json list)

{
  "de": "hallo",
  "en": "hello",
  "fr": "bonjour"
}

To change the default json serialization with Jackson, you can implement a custom serializer and then you can either:

• Annotate every field that you want to use that serializer with @JsonSerializer(use = YourCustomSerializerClass.class). This is not a choice for us as we simply have the List<LanguageString> everywhere and we want to set a default behaviour for it.
• Register your custom serializer with Jackson config and the data type. This normally works fine without collection type. After quite a bit of struggling to make it work and still failed, I came across the comment in Jackson’s SimpleModule.java:

/**
 * Method for adding serializer to handle values of specific type.
 *<p>
 * WARNING! Type matching only uses type-erased {@code Class} and should NOT
 * be used when registering serializers for generic types like
 * {@link java.util.Collection} and {@link java.util.Map}.
 */

So, it turns out that you can not add a custom serializer with generic type. I tried to google quite a bit but people just recommend to add annotation in the filed. Time to dig in the source code!!

Finally, after a bit more struggling, I found out that there is a class name SimpleSerializers where Jackson uses to define which serializer it should use for a collection type. So easy job now, you just need to override the method to point to your own custom serializer.

public class CollectionTypeJsonSerializer extends SimpleSerializers {

  @Override
  public JsonSerializer<?> findCollectionSerializer(SerializationConfig config,
                                                    CollectionType type,
                                                    BeanDescription beanDesc,
                                                    TypeSerializer elementTypeSerializer,
                                                    JsonSerializer<Object> elementValueSerializer) {
    //if the collection is of type LanguageString, then use custom collection serializer
    if (isLanguageStringListType(type)) {
      return new LanguageStringListSerializer();
    }

    return findSerializer(config, type, beanDesc);
  }


  private boolean isLanguageStringListType(CollectionType type) {
    CollectionType languageStringArrayListType = TypeFactory.defaultInstance()
        .constructCollectionType(ArrayList.class, LanguageString.class);

    CollectionType languageStringListType = TypeFactory.defaultInstance()
        .constructCollectionType(List.class, LanguageString.class);

    return (type.equals(languageStringListType) || type.equals(languageStringArrayListType));
  }
}

And then, you can register your new SimpleSerializers by setting it as the serializer for a module in Jackson config:

@Configuration
public class JacksonConfig {
    @Bean
    public ObjectMapper jsonObjectMapper() {
        ArrayList<Module> modules = new ArrayList<>();

        //CollectionType Serialization
        SimpleModule collectionTypeSerializerModule = new SimpleModule();
        collectionTypeSerializerModule.setSerializers(new CollectionTypeJsonSerializer());
        modules.add(collectionTypeSerializerModule);

        return Jackson2ObjectMapperBuilder.json()
                .modules(modules)
                .build();
    }
}

Then that’s it.

I guess the take-away lesson here is that as a developer, sometimes it’s worthwhile (and fruitful) to dig into the source code, instead of just googling around.