Spring Boot: Wrapping JSON response in dynamic parent objects

Question

I have a REST API specification that talks with back-end microservices, which return the following values:

On "collections" responses (e.g. GET /users) :

{
    users: [
        {
            ... // single user object data
        }
    ],
    links: [
        {
            ... // single HATEOAS link object
        }
    ]
}

On "single object" responses (e.g. GET /users/{userUuid}) :

{
    user: {
        ... // {userUuid} user object}
    }
}

This approach was chosen so that single responses would be extensible (for example, maybe if GET /users/{userUuid} gets an additional query parameter down the line such at ?detailedView=true we would have additional request information).

Fundamentally, I think it is an OK approach for minimizing breaking changes between API updates. However, translating this model to code is proving very arduous.

Let's say that for single responses, I have the following API model object for a single user:

public class SingleUserResource {
    private MicroserviceUserModel user;

    public SingleUserResource(MicroserviceUserModel user) {
        this.user = user;
    }

    public String getName() {
        return user.getName();
    }

    // other getters for fields we wish to expose
}

The advantage of this method is that we can expose only the fields from the internally used models for which we have public getters, but not others. Then, for collections responses I would have the following wrapper class:

public class UsersResource extends ResourceSupport {

    @JsonProperty("users")
    public final List<SingleUserResource> users;

    public UsersResource(List<MicroserviceUserModel> users) {
        // add each user as a SingleUserResource
    }
}

For single object responses, we would have the following:

public class UserResource {

    @JsonProperty("user")
    public final SingleUserResource user;

    public UserResource(SingleUserResource user) {
        this.user = user;
    }
}

This yields JSON responses which are formatted as per the API specification at the top of this post. The upside of this approach is that we only expose those fields that we want to expose. The heavy downside is that I have a ton of wrapper classes flying around that perform no discernible logical task aside from being read by Jackson to yield a correctly formatted response.

My questions are the following:

• How can I possibly generalize this approach? Ideally, I would like to have a single BaseSingularResponse class (and maybe a BaseCollectionsResponse extends ResourceSupport class) that all my models can extend, but seeing how Jackson seems to derive the JSON keys from the object definitions, I would have to user something like Javaassist to add fields to the base response classes at Runtime - a dirty hack that I would like to stay as far away from as humanly possible.

• Is there an easier way to accomplish this? Unfortunately, I may have a variable number of top-level JSON objects in the response a year from now, so I cannot use something like Jackson's SerializationConfig.Feature.WRAP_ROOT_VALUE because that wraps everything into a single root-level object (as far as I am aware).

• Is there perhaps something like @JsonProperty for class-level (as opposed to just method and field level)?

Answer 1

There are several possibilities.

You can use a java.util.Map:

List<UserResource> userResources = new ArrayList<>();
userResources.add(new UserResource("John"));
userResources.add(new UserResource("Jane"));
userResources.add(new UserResource("Martin"));
Map<String, List<UserResource>> usersMap = new HashMap<String, List<UserResource>>();
usersMap.put("users", userResources);
ObjectMapper mapper = new ObjectMapper();
System.out.println(mapper.writeValueAsString(usersMap));

---

You can use ObjectWriter to wrap the response that you can use like below:

ObjectMapper mapper = new ObjectMapper();
ObjectWriter writer = mapper.writer().withRootName(root);
result = writer.writeValueAsString(object);

Here is a proposition for generalizing this serialization.

A class to handle simple object:

public abstract class BaseSingularResponse {

    private String root;

    protected BaseSingularResponse(String rootName) {
        this.root = rootName;
    }

    public String serialize() {
        ObjectMapper mapper = new ObjectMapper();
        ObjectWriter writer = mapper.writer().withRootName(root);
        String result = null;
        try {
            result = writer.writeValueAsString(this);
        } catch (JsonProcessingException e) {
            result = e.getMessage();
        }
        return result;
    }
}

A class to handle collection:

public abstract class BaseCollectionsResponse<T extends Collection<?>> {
    private String root;
    private T collection;

    protected BaseCollectionsResponse(String rootName, T aCollection) {
        this.root = rootName;
        this.collection = aCollection;
    }

    public T getCollection() {
        return collection;
    }

    public String serialize() {
        ObjectMapper mapper = new ObjectMapper();
        ObjectWriter writer = mapper.writer().withRootName(root);
        String result = null;
        try {
            result = writer.writeValueAsString(collection);
        } catch (JsonProcessingException e) {
            result = e.getMessage();
        }
        return result;
    }
}

And a sample application:

public class Main {

    private static class UsersResource extends BaseCollectionsResponse<ArrayList<UserResource>> {
        public UsersResource() {
            super("users", new ArrayList<UserResource>());
        }
    }

    private static class UserResource extends BaseSingularResponse {

        private String name;
        private String id = UUID.randomUUID().toString();

        public UserResource(String userName) {
            super("user");
            this.name = userName;
        }

        public String getUserName() {
            return this.name;
        }

        public String getUserId() {
            return this.id;
        }
    }

    public static void main(String[] args) throws JsonProcessingException {
        UsersResource userCollection = new UsersResource();
        UserResource user1 = new UserResource("John");
        UserResource user2 = new UserResource("Jane");
        UserResource user3 = new UserResource("Martin");

        System.out.println(user1.serialize());

        userCollection.getCollection().add(user1);
        userCollection.getCollection().add(user2);
        userCollection.getCollection().add(user3);

        System.out.println(userCollection.serialize());
    }
}

---

You can also use the Jackson annotation @JsonTypeInfo in a class level

@JsonTypeInfo(include=As.WRAPPER_OBJECT, use=JsonTypeInfo.Id.NAME)

Answer 2

Personally I don't mind the additional Dto classes, you only need to create them once, and there is little to no maintenance cost. And If you need to do MockMVC tests, you will most likely need the classes to deserialize your JSON responses to verify the results.

As you probably know the Spring framework handles the serialization/deserialization of objects in the HttpMessageConverter Layer, so that is the correct place to change how objects are serialized.

If you don't need to deserialize the responses, it is possible to create a generic wrapper, and a custom HttpMessageConverter (and place it before MappingJackson2HttpMessageConverter in the message converter list). Like this:

public class JSONWrapper {

    public final String name;
    public final Object object;

    public JSONWrapper(String name, Object object) {
        this.name = name;
        this.object = object;
    }
}


public class JSONWrapperHttpMessageConverter extends MappingJackson2HttpMessageConverter {

    @Override
    protected void writeInternal(Object object, Type type, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        // cast is safe because this is only called when supports return true.
        JSONWrapper wrapper = (JSONWrapper) object;
        Map<String, Object> map = new HashMap<>();
        map.put(wrapper.name, wrapper.object);
        super.writeInternal(map, type, outputMessage);
    }

    @Override
    protected boolean supports(Class<?> clazz) {
        return clazz.equals(JSONWrapper.class);
    }
}

You then need to register the custom HttpMessageConverter in the spring configuration which extends WebMvcConfigurerAdapter by overriding configureMessageConverters(). Be aware that doing this disables the default auto detection of converters, so you will probably have to add the default yourself (check the Spring source code for WebMvcConfigurationSupport#addDefaultHttpMessageConverters() to see defaults. if you extend WebMvcConfigurationSupport instead WebMvcConfigurerAdapter you can call addDefaultHttpMessageConverters directly (Personally I prefere using WebMvcConfigurationSupport over WebMvcConfigurerAdapter if I need to customize anything, but there are some minor implications to doing this, which you can probably read about in other articles.

Answer 3

Jackson doesn't have a lot of support for dynamic/variable JSON structures, so any solution that accomplishes something like this is going to be pretty hacky as you mentioned. As far as I know and from what I've seen, the standard and most common method is using wrapper classes like you are currently. The wrapper classes do add up, but if you get creative with your inheretence you may be able to find some commonalities between classes and thus reduce the amount of wrapper classes. Otherwise you might be looking at writing a custom framework.

Answer 4

I guess you are looking for Custom Jackson Serializer. With simple code implementation same object can be serialized in different structures

some example: https://stackoverflow.com/a/10835504/814304 http://www.davismol.net/2015/05/18/jackson-create-and-register-a-custom-json-serializer-with-stdserializer-and-simplemodule-classes/