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How can I have different names for the same field in different APIs?—Jackson

I have an API whose response is as follows:
{
ruleId:”123”,
ruleName:”Rule1”
}
Now I am introducing a new Api which exactly has these fields but the response should not have name as ruleId ,ruleName but as id,name:
{
id:”123”,
name:”Rule1”
}
I should change in such a way so that the previous Api response should not be impacted.
Thought to use JsonProperty /JsonGetter but it will change the previous Api response as well.
Is there any way that I can have 2 getters for the same field and then use one getter for previous Apis and other one for my purpose? (My concern is only when converting Pojo to JSON)
Can anyone help?

Since you want serialize the object differently in different cases, using jackson mix-in is preferred.
Here is example how to do that.
If your pojo looks something like this:
public class CustomPojo {
private String ruleId;
private String ruleName;
public String getRuleId() {
return ruleId;
}
public void setRuleId(String ruleId) {
this.ruleId = ruleId;
}
public String getRuleName() {
return ruleName;
}
public void setRuleName(String ruleName) {
this.ruleName = ruleName;
}
}
First, you need to create one interface (or class) like this:
import com.fasterxml.jackson.annotation.JsonProperty;
public interface CostomPojoMixin {
#JsonProperty("Id")
String getRuleId();
#JsonProperty("name")
String getRuleName();
}
This interface will be used to rename fields ruleId and ruleName during serilization.
Then when you have all this setup you can write controller method and customize ObjectMapper:
#GetMapping(value = "/test/mixin")
public String testMixin() throwsJsonProcessingException {
CostomPojo cp = new CostomPojo();
cp.setRuleId("rule");
cp.setRuleName("name");
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.addMixIn(CustomPojo.class, CostomPojoMixin.class);
String json = objectMapper.writeValueAsString(cp);
return json;
}
This endpoint should return response like this:
{"Id":"rule","name":"name"}

Custom JSON Serialization Wrapper for Any Object

I use external application which expects an Object that Serializable from me like his function:
externalFunction(Object input);
So I should give that function an input that will be correctly serialized into JSON when the method is invoked (not controlled by me).
But I don't know how data is structured since I receive input from another external application dynamically. So case like this:
1. Get data from 3rd party
2. MyApp should annotate data for Json Serialization
3. Send data to 3rd party as input
4. Response will be produced as JSON
How can I achieve this? How can I give input to the function that is correctly serialized when the function is invoked?
What I tried so far:
So first thing I try is wrap data with some Wrapper like:
public class JsonWrapper<T> implements Serializable
{
public T attributes;
public JsonWrapper( T attributes )
{
this.attributes = attributes;
}
#JsonValue
public T getAttributes( )
{
return attributes;
}
}
So I wrap data like ->
data = getFromThirdParty();
wrapped = new JsonWrapper<>(data);
externalFunction(wrapped);
But it produces a response with "attributes" field which I don't want. Also I tried to use #JsonUnwrapped public T attributes; but the result is same.
I don't want this:
{
"attributes": {
... some fields/values that I don't know, get from 3rd party
}
}
I want like this:
{
... some fields/values that I don't know, get from 3rd party
}

The #JsonUnwrapped annotation doesn't work when T is a Collection (see this answer from the Jackson's creator). But the #JsonValue annotation actually does the trick:
public class JsonWrapper<T> {
#JsonValue
private T value;
public JsonWrapper(T value) {
this.value = value;
}
public T getValue() {
return value;
}
}
If you use Lombok, you can have:
#Getter
#AllArgsConstructor
public class JsonWrapper<T> {
#JsonValue
private T value;
}
Example
Consider the following class:
#Data
#AllArgsConstructor
public class Person {
private String firstName;
private String lastName;
}
When serializing an Person instance, the following result JSON is produced:
ObjectMapper mapper = new ObjectMapper();
JsonWrapper<?> wrapper = new JsonWrapper<>(new Person("John", "Doe"));
String json = mapper.writeValueAsString(wrapper);
{"firstName":"John","lastName":"Doe"}
When serializing a list of Person instances, the following result JSON is produced:
ObjectMapper mapper = new ObjectMapper();
JsonWrapper<?> wrapper = new JsonWrapper<>(
Arrays.asList(
new Person("John", "Doe"),
new Person("Jane", "Poe")
));
String json = mapper.writeValueAsString(wrapper);
[{"firstName":"John","lastName":"Doe"},{"firstName":"Jane","lastName":"Poe"}]

Mapstruct can't map properties while using ObjectFactory

I need some community help to point me out where I'm wrong in my code... :)
I try to use mapstruct to map fields between 2 entities with the help of an #ObjectFactory.
Entity 1:
public class ScimUser {
#JsonProperty("addresses")
#Valid
private List<UserAddress> addresses = null;
}
Entity 2:
public class User {
#JsonProperty("postalAddress")
private PostalAddress postalAddress = null;
}
Mapper:
#Mapper(componentModel = "spring", uses = { AddressFactory.class })
public interface ScimUserMapper {
#Mapping(target = "postalAddress", source = "scimUser.addresses")
User toUser(ScimUser scimUser);
#Mapping(target = "addresses", source = "user.postalAddress")
ScimUser toScimUser(User user);
}
ObjectFactory:
#Component
public class AddressFactory {
#Autowired
private CountryMapper countryMapper;
#Autowired
private CountryRepository countryRepository;
#ObjectFactory
public PostalAddress toPostalAddress(List<UserAddress> addresses, #TargetType Class<PostalAddress> type) {
PostalAddress postalAddress = new PostalAddress();
if (addresses != null && !addresses.isEmpty()) {
UserAddress userAddress = addresses.stream().filter(UserAddress::isPrimary).findFirst().orElse(null);
if (userAddress == null) {
userAddress = addresses.get(0);
}
postalAddress.setAddressLine1(userAddress.getStreetAddress());
postalAddress.setPostCode(userAddress.getPostalCode());
postalAddress.setState(userAddress.getRegion());
postalAddress.setCity(userAddress.getLocality());
CountryJpa countryJpa = countryRepository.getCountryByIso2Code(userAddress.getCountry());
if (countryJpa != null) {
Country country = countryMapper.fromJPA(countryJpa);
postalAddress.setCountry(country);
}
}
return postalAddress;
}
#ObjectFactory
public List<UserAddress> toUserAddressList(PostalAddress address, #TargetType Class<List<UserAddress>> type) {
UserAddress userAddress = new UserAddress();
userAddress.setCountry(address.getCountry().getIso2());
userAddress.setFormatted("?");
userAddress.setLocality(address.getCity());
userAddress.setPostalCode(address.getPostCode());
userAddress.setPrimary(true);
userAddress.setRegion(address.getState());
userAddress.setStreetAddress(address.getAddressLine1());
userAddress.setType("?");
return Collections.singletonList(userAddress);
}
}
The code above gets me this error during source code generation:
Can't map property "java.util.List addresses" to "PostalAddress postalAddress". Consider to declare/implement a mapping method: "PostalAddress map(java.util.List value)".
Can't map property "PostalAddress postalAddress" to "java.util.List addresses". Consider to declare/implement a mapping method: "java.util.List map(PostalAddress value)".
It's not the first time that I struggle with using these object factories and I really don't get what I am doing wrong.
So if someone has an idea, I'd be glad to read it. :)

You are using the #ObjectFactory wrong. What you want to achieve is a custom mapping method.
#ObjectFactory needs to be used to create the target instance object. In your case if you just remove #ObjectFactory and #TargetType from your method then it should work correctly.
I have to stress out that you are doing quite some manual mapping there. You can easily provide methods for mapping between a single UserAddress and PostalAddress and just add wrappers for the collections.

Spring Boot: Wrapping JSON response in dynamic parent objects

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)?

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)

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.

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.

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/

How to (De)serialize field from object based on annotation using Jackson?

I need to configure Jackson in a specific way which I'll describe below.
Requirements
Annotated fields are serialized with only their id:
If the field is a normal object, serialize its id
If the field is a collection of objects, serialize an array of id
Annotated fields get their property names serialized differently:
If the field is a normal object, add "_id" suffix to property name
If the field is a collection of objects, add "_ids" suffix to property name
For the annotation I was thinking something like a custom #JsonId, ideally with an optional value to override the name just like #JsonProperty does
The id property should be defined by the user, either using:
The already existing Jackson's #JsonIdentityInfo
Or by creating another class or field annotation
Or by deciding which annotation to inspect for id property discoverability (useful for JPA scenarios, for example)
Objects should be serialized with a wrapped root value
Camel case naming should be converted to lower case with underscores
All of this should be deserializable (by constructing an instance with just the id setted)
An example
Considering these POJO's:
//Inform Jackson which property is the id
#JsonIdentityInfo(
generator = ObjectIdGenerators.PropertyGenerator.class,
property = "id"
)
public abstract class BaseResource{
protected Long id;
//getters and setters
}
public class Resource extends BaseResource{
private String name;
#JsonId
private SubResource subResource;
#JsonId
private List<SubResource> subResources;
//getters and setters
}
public class SubResource extends BaseResource{
private String value;
//getters and setters
}
A possible serialization of a Resource instance could be:
{
"resource":{
"id": 1,
"name": "bla",
"sub_resource_id": 2,
"sub_resource_ids": [
1,
2,
3
]
}
}
So far...
Requirement #5 can be accomplished by configuring ObjectMapper in the following way:
objectMapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
objectMapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
And then using #JsonRootName("example_root_name_here") in my POJO's.
Requirement #6 can be accomplished by configuring ObjectMapper in the following way:
objectMapper.setPropertyNamingStrategy(
PropertyNamingStrategy.CAMEL_CASE_TO_LOWER_CASE_WITH_UNDERSCORES);
As you can see there are still lots of requirements to fulfill. For those wondering why I need such a configuration, it's because I'm developing a REST webservice for ember.js (more specifically Ember Data).
You would appreciate very much if you could help with any of the requirements.
Thanks!

Most (all?) of your requirements can be accomplished through the use of a contextual serializer. Taking one answer from ContextualDeserializer for mapping JSON to different types of maps with Jackson and Jackson's wiki (http://wiki.fasterxml.com/JacksonFeatureContextualHandlers) I was able to come up with the following.
You need to start with the #JsonId annotation, which is the key indicating a property needs to only use the Id property.
import com.fasterxml.jackson.annotation.*;
import java.lang.annotation.*;
#Target({ElementType.FIELD, ElementType.METHOD, ElementType.TYPE})
#Retention(RetentionPolicy.RUNTIME)
#JacksonAnnotation // important so that it will get included!
public #interface JsonId {
}
Next is the actual ContextualSerializer, which does the heavy lifting.
import com.fasterxml.jackson.databind.ser.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.core.*;
import java.io.*;
public class ContextualJsonIdSerializer
extends JsonSerializer<BaseResource>
implements ContextualSerializer/*<BaseResource>*/
{
private ObjectMapper mapper;
private boolean useJsonId;
public ContextualJsonIdSerializer(ObjectMapper mapper) { this(mapper, false); }
public ContextualJsonIdSerializer(ObjectMapper mapper, boolean useJsonId) {
this.mapper = mapper;
this.useJsonId = useJsonId;
}
#Override
public void serialize(BaseResource br, JsonGenerator jgen, SerializerProvider provider) throws IOException
{
if ( useJsonId ) {
jgen.writeString(br.getId().toString());
} else {
mapper.writeValue(jgen, br);
}
}
#Override
public JsonSerializer<BaseResource> createContextual(SerializerProvider config, BeanProperty property)
throws JsonMappingException
{
// First find annotation used for getter or field:
System.out.println("Finding annotations for "+property);
if ( null == property ) {
return new ContextualJsonIdSerializer(mapper, false);
}
JsonId ann = property.getAnnotation(JsonId.class);
if (ann == null) { // but if missing, default one from class
ann = property.getContextAnnotation(JsonId.class);
}
if (ann == null ) {//|| ann.length() == 0) {
return this;//new ContextualJsonIdSerializer(false);
}
return new ContextualJsonIdSerializer(mapper, true);
}
}
This class looks at BaseResource properties and inspects them to see if the #JsonId annotation is present. If it is then only the Id property is used, otherwise a passed in ObjectMapper is used to serialize the value. This is important because if you try to use the mapper that is (basically) in the context of the ContextualSerializer then you will get a stack overflow since it will eventually call these methods over and over.
You're resource should look something like the following. I used the #JsonProperty annotation instead of wrapping the functionality in the ContextualSerializer because it seemed silly to reinvent the wheel.
import java.util.*;
import com.fasterxml.jackson.annotation.*;
public class Resource extends BaseResource{
private String name;
#JsonProperty("sub_resource_id")
#JsonId
private SubResource subResource;
#JsonProperty("sub_resource_ids")
#JsonId
private List<SubResource> subResources;
//getters and setters
public String getName() {return name;}
public void setName(String name) {this.name = name;}
public SubResource getSubResource() {return subResource;}
public void setSubResource(SubResource subResource) {this.subResource = subResource;}
public List<SubResource> getSubResources() {return subResources;}
public void setSubResources(List<SubResource> subResources) {this.subResources = subResources;}
}
Finally the method that performs the serialization just creates an additional ObjectMapper and registers a module in the original ObjectMapper.
// Create the original ObjectMapper
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
objectMapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
objectMapper.setPropertyNamingStrategy(PropertyNamingStrategy.CAMEL_CASE_TO_LOWER_CASE_WITH_UNDERSCORES);
// Create a clone of the original ObjectMapper
ObjectMapper objectMapper2 = new ObjectMapper();
objectMapper2.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
objectMapper2.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
objectMapper2.setPropertyNamingStrategy(PropertyNamingStrategy.CAMEL_CASE_TO_LOWER_CASE_WITH_UNDERSCORES);
// Create a module that references the Contextual Serializer
SimpleModule module = new SimpleModule("JsonId", new Version(1, 0, 0, null));
// All references to SubResource should be run through this serializer
module.addSerializer(SubResource.class, new ContextualJsonIdSerializer(objectMapper2));
objectMapper.registerModule(module);
// Now just use the original objectMapper to serialize