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"}]
Related
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"}
so I am currently stuck with a problem using Jackson. I would like to de-serialize a JSON which can contain within ClassB one of multiple POJOs (I marked them in ClassB with the #JsonProperty). Since this ClassB is more like a generic container of multiple options, I don't know how to let Jackson know about these multiple sub-POJOs. I tried to do it with the #JacksonTypeInfo annotation; however, this is not possible in my opinion since the property value decides to which sub-class it goes. For example I would like to parse:
{
"classA": {
"classB": {
"ClassC": {...}
}
}
}
but also:
{
"classA": {
"classB": {
"ClassD": {...}
}
}
}
and many other POJOs. This was solved in the past with JAXB using the #XmlAnyElement (lax = true) annotation together with object factories using #XmlRegistry and #XmlElementDecl, but now also JSON input needs be supported and Jackson looks like the most promising library. These are my classes:
#JsonRootName(value = "classA")
#JsonIgnoreProperties
public class ClassA {
private ClassB classB;
#JsonProperty("classB")
public void setClassB(ClassB value) {
this.classB = value;
}
#JsonRootName(value = "classB")
#JsonIgnoreProperties
public class ClassB {
private Object object;
#JsonProperty("classC")
#JsonProperty("classD")
#JsonProperty("classE")
public void setObject(Object value) {
this.object = value;
}
Here is my reader:
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE);
JSONObject jsonObject = new JSONObject((LinkedHashMap<?, ?>) element);
String json = objectMapper.writeValueAsString(jsonObject);
Object object = objectMapper.readValue(json, ClassA.class);
How can I conditionally deserialize a JSON string to a POJO field?
I receive a JSON string like so:
{
"status": "we stuck",
"data" : "someData"
}
but "someData" can be just a string "under the bridge" or can be something like "['bridge 5', 'Mandela bridge']" or "[{'incident 1' : '['bridge 1', 'bridge 2]'},{'incident 2' : ['bridge 99', 'what ever else']}]"
I want to return the json string AS IS if "data" is an array then I will map to a different Type that deals with the array
I have a java class:
class Response {
String status;
String data;
}
the other Type will have data as
ArrayList<SomeOtherType> data;
This is what i have so far
ObjectMapper mapper = new ObjectMapper();
Response rspns = mapper.readValue(<theJSONStrHere>, Response.class);
this fails when data is an array, giving me the message
can not deserialize instance of java.lang.String out of START_ARRAY token
I don't know where to go from here.
You can either use a custom deserializer as aussie said or you can just modify your working solution.
class Response {
String status;
String data;
}
class Other {
String status;
ArrayList<SomeOtherType> data;
}
ObjectMapper mapper = new ObjectMapper();
Other rspns = mapper.readValue(<theJSONStrHere>, Other.class);
This will parse the JSON String to the Other class with the ArrayList.
Now it's your turn to implement the decision of then to use
Other rspns = mapper.readValue(<theJSONStrHere>, Other.class);
or when to use
Responserspns = mapper.readValue(<theJSONStrHere>, Response.class);
Note: The above is a quick and dirty solution. It works like that but I would highly recommend to use a custom deserializer, which handles the logic of what it is and what to return.
Also keep in mind that for this to work the best you might consider building the POJO structure to multiple classes which extend a base class and then work generic.
example:
class response {
String status;
}
class simpleResponse extends response {
String data;
}
class listResponse extends response {
ArrayList<Type> data;
}
class MyDeserializer extends JSONDeserializer<E extends response> {
public E deserialize...) {
}
}
To get an actual working example read about Jackson
Dont make it complicated think simple..
There are two ways
First taking List os string/(or any other type)
private List<String> tags;
Second taking List of class (if you need more than one parameters)
List<PageLink> pagelinks;
See below case example......
public class PagesJson {
private String ln;
private int pageno;
private List<String> tags;
private List<PageLink> pagelinks;
private String error;
}
public class PageLink {
private String title= null;
private String url;
}
Now json of PagesJson class as below
{"ln":en,"count":100,"viewcount":23,"pageno":17,"tags":["Ensuring safe motherhood","pregnancy health in women","Abortion"],"pagelinks":[{"title":"Abortion","url":"http://vikaspedia.in/health/women-health"},{"title":"Acts and Rules","url":"http://vikaspedia.in/social-welfare/scheduled-tribes-welfare/acts-and-rules"},{"title":"Acts and Rules ","url":"http://vikaspedia.in/social-welfare/unorganised-sector-1/acts-and-rules"}],"error":"Parameter Validation Error"}
{"ln":en,"count":100,"viewcount":23,"pageno":17,"tags":["Ensuring safe motherhood","pregnancy health in women","Abortion"],"pagelinks":[{"title":"Abortion","url":"http://vikaspedia.in/health/women-health"},{"title":"Acts and Rules","url":"http://vikaspedia.in/social-welfare/scheduled-tribes-welfare/acts-and-rules"},{"title":"Acts and Rules ","url":"http://vikaspedia.in/social-welfare/unorganised-sector-1/acts-and-rules"}],"error":"Parameter Validation Error"}
For Mapping json to class use jackson library as below.....
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
....
.....
ObjectMapper mapper = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
PagesJson pj = mapper.readValue(response.toString(), PagesJson.class);
You can use Custom Deserializer on a Method OR a Class using below :
extends JsonDeserializer
then
#Override
public ReturnObject deserialize(JsonParser parser, DeserializationContext ctx)
throws IOException, JsonProcessingException {
JsonToken token = parser.getCurrentToken();
if(JsonToken.START_ARRAY.equals(token)){
//TODO your JSON Array code handling
}else{
//TO DO you JSON Object Handling
}
}
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/
I am building JSON from Java object tree using Jackson ObjectMapper. Some of my Java objects are collections and sometimes they might be empty. So if they are empty that ObjectMapper generates me: "attributes": [], and I want to exclude those kind of empty JSON arrays from my result. My current ObjectMapper config:
SerializationConfig config = objectMapper.getSerializationConfig();
config.setSerializationInclusion(JsonSerialize.Inclusion.NON_NULL);
config.set(SerializationConfig.Feature.WRAP_ROOT_VALUE, true);
From this post I've read that I can use:
config.setSerializationInclusion(JsonSerialize.Inclusion.NON_DEFAULT);
But that is generating me an error:
Caused by: java.lang.IllegalArgumentException: Class com.mycomp.assessments.evaluation.EvaluationImpl$1 has no default constructor; can not instantiate default bean value to support 'properties=JsonSerialize.Inclusion.NON_DEFAULT' annotation.
So how should I prevent those empty arrays to appear in my result?
You should use:
config.setSerializationInclusion(JsonSerialize.Inclusion.NON_EMPTY);
for Jackson 1 or
config.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
for Jackson 2
A very good example describing :
JsonInclude.Include.NON_NULL
JsonInclude.Include.ABSENT
JsonInclude.Include.NON_EMPTY
In : https://www.logicbig.com/tutorials/misc/jackson/json-include-non-empty.html
#JsonInclude(JsonInclude.Include.NON_EMPTY)
public class Employee {
private String name;
private String dept;
private String address;
private List<String> phones;
private AtomicReference<BigDecimal> salary;
.............
}
public class ExampleMain {
public static void main(String[] args) throws IOException {
Employee employee = new Employee();
employee.setName("Trish");
employee.setDept("");
employee.setAddress(null);
employee.setPhones(new ArrayList<>());
employee.setSalary(new AtomicReference<>());
ObjectMapper om = new ObjectMapper();
String jsonString = om.writeValueAsString(employee);
System.out.println(jsonString);
}
}
=====> Result :
If we don't use #JsonInclude annotation at all then output of the above example will be:
{"name":"Trish","dept":"","address":null,"phones":[],"salary":null}
If we use #JsonInclude(JsonInclude.Include.NON_NULL) on Employee class then output will be:
{"name":"Trish","dept":"","phones":[],"salary":null}
If we use #JsonInclude(JsonInclude.Include.NON_ABSENT) then output will be:
{"name":"Trish","dept":"","phones":[]}
If we use #JsonInclude(JsonInclude.Include.NON_EMPTY) :
{"name":"Trish"}
If you can modify the object to be serialized, you can also place an annotation directly on the field, for example (Jackson 2.11.2):
#JsonProperty
#JsonInclude(JsonInclude.Include.NON_EMPTY)
private Set<String> mySet = new HashSet<>();
In this way, no further configuration of the ObjectMapper is required.