Deserialize JsonManagedReference on many to many relations
Jackson Version: 2.9.7
Objects with a ManyToMany relationship which is managed by #JsonManagedReference/#JsonBackReference cannot be deserialized. This is easiest to show with an example.
public class JsonReferenceTest {
class Customer {
#JsonManagedReference("users")
public Collection<User> users = new ArrayList<>();
public String name = "company";
}
class User {
#JsonBackReference("users")
public Collection<Customer> customers = new ArrayList<>();
public String name = "user";
}
ObjectMapper objectMapper = new ObjectMapper();
#Test
public void testDeserialize() throws IOException {
String customer = "{\"name\":\"asdf\"}";
objectMapper.readValue(customer, Customer.class);
}
}
Running the following test results in:
com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot handle managed/back reference 'users': back reference type (java.util.Collection) not compatible with managed type (com.widewail.JsonReferenceTest$Customer)
at [Source: (String)"{"name":"asdf"}"; line: 1, column: 1]
Digging into BeanDeserializerBase it looks like when checking that the back reference type matches the reference type it is not checking the contained type of the collection on the back reference side.
As the documentation from JsonBackReference says you can't use it with a Collection.
Annotation used to indicate that associated property is part of
two-way linkage between fields; and that its role is "child" (or
"back") link. Value type of the property must be a bean: it can not
be a Collection, Map, Array or enumeration.
You can use JsonIdentityInfo as decribed here.
Related
I'm using spring data mongo. I have a collection within a document that when I add an item to it I would like to assign a new automatically generated unique identifier to it e.g. (someGeneratedId)
#Document(collection = "questionnaire")
public class Questionnaire {
#Id
private String id;
#Field("answers")
private List<Answer> answers;
}
public class Answer {
private String someGeneratedId;
private String text;
}
I am aware I could use UUID.randomUUID() (wrapped in some kind of service) and set the value, I was just wondering if there was anything out of the box that can handle this? From here #Id seems to be specific to _id field in mongo:
The #Id annotation tells the mapper which property you want to use for
the MongoDB _id property
TIA
No there is no out of the box solution for generating ids for properties on embedded documents.
If you want to keep this away from your business-logic you could implement a BeforeConvertCallback which generates the id's for your embedded objects.
#Component
class BeforeConvertQuestionnaireCallback implements BeforeConvertCallback<Questionnaire> {
#Override
public Questionnaire onBeforeConvert(#NonNull Questionnaire entity, #NonNull String collection) {
for (var answer : entity.getAnswers()) {
if (answer.getId() == null) {
answer.setId(new ObjectId().toString());
}
}
return entity;
}
}
You could also implement this in a more generic manner:
Create a new annotation: #AutogeneratedId.
Then listen to all BeforeConvertCallback's of all entities and iterate through the properties with reflection. Each property annotated with the new annotation gets a unique id if null.
I have two entities: Book and Category and a repository for both. In the controller, I have set up the methods correctly as such:
#RequestMapping(value="/books", method = RequestMethod.GET)
#CrossOrigin
public #ResponseBody List<Book> bookListRest() {
return (List<Book>) bookRepository.findAll();
}
This obviously shows all books and every field in the entity that isn't #JsonIgnore'd. The problem is, I need to have:
One page with Book data (book name, author name, isbn..) without category
One page with Category data (Category name) without books
One page with Everything (book data along with categories where they belong in)
How can one accomplish this?
I somehow need to in a way ignore #jsonignore on some occasions. Should I make a new entity that extends say, Question and also make a repository for that? Surely that can't be the correct way to do this.
As khalid Ahmed Said you can use costum dtos or you can add Filters to ignore specific fields in Jackson. First, we need to define the filter on the java object:
#JsonFilter("myFilterBook")
public class Book{
...
}
#JsonFilter("myFilterCategory")
public class Category{
...
}
Before you return your ResponseBody you try to use ObjectMapper (Jackson):
The case of one page with Book data (book name, author name, isbn..) without category:
ObjectMapper mapper = new ObjectMapper();
SimpleBeanPropertyFilter theFilter = SimpleBeanPropertyFilter
.serializeAllExcept("category");
FilterProvider filters = new SimpleFilterProvider()
.addFilter("myFilterBook", theFilter);
String dtoAsString = mapper.writer(filters).writeValueAsString(book);
You can do the same think by putting what you want o ignore for the other example.
And for more details to ignore field during marshalling with jackson you can check here
What about using DTOs data transfer objects
you can create multiple DTOs to use them in the response of your API
DTO is a pojo class that customize the returning data from your entity
public class BookWithoutCategoryDTO {
private String name;
private String authorName;
.....
/// and make setters and getters for them
}
public class BookWithCategoryDTO {
private String name;
private String authorName;
private String category;
.....
/// and make setters and getters for them
}
and create your custom mapper to convert from Book to BookDTO
I got the following classes:
#JsonIdentityInfo(
generator = ObjectIdGenerators.IntSequenceGenerator.class,
property = "oid"
)
#JsonTypeInfo(
use = JsonTypeInfo.Id.NAME,
include = JsonTypeInfo.As.PROPERTY,
property = "clazz")
#JsonSubTypes({
#JsonSubTypes.Type(value = MySubEntity.class, name = "MySubEntity"),
})
public abstract class Entity {
...
}
public class MySubEntity extends Entity {
...
}
Now when I serialize that MySubEntity wrapped in an Optional then JSON does not contain the clazz attribute containing the type ID. Bug? When I serialize to List<MySubEntity> or just to MySubEntity it works fine.
Setup: jackson-databind 2.9.4, jackson-datatype-jdk8 2.9.4, serialization is done in Spring Boot application providing a RESTful web service.
EDIT: Here is the Spring REST method that returns the Optional:
#RequestMapping(method = RequestMethod.GET, value = "/{uuid}", produces = "application/json")
public Optional<MySubEntity> findByUuid(#PathVariable("uuid") String uuid) {
...
}
EDIT:
I made a SSCCE with a simple Spring REST controller and two tests. The first test is using ObjectMapper directly which is successful in deserialization although the clazz is missing. The second test calls the REST controller and fails with an error because clazz is missing:
Error while extracting response for type [class com.example.demo.MySubEntity] and content type [application/json;charset=UTF-8]; nested exception is org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Missing type id when trying to resolve subtype of [simple type, class com.example.demo.MySubEntity]: missing type id property 'clazz'; nested exception is com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Missing type id when trying to resolve subtype of [simple type, class com.example.demo.MySubEntity]: missing type id property 'clazz'
This, indeed, looks like a bug. There is one workaround that I can suggest for this case, is to use JsonTypeInfo.As.EXISTING_PROPERTY and add field clazz to your Entity. There only one case with this approach is that the clazz must be set in java code manually. However this is easy to overcome.
Here is the full code for suggested workaround:
#JsonIdentityInfo(
generator = ObjectIdGenerators.IntSequenceGenerator.class,
property = "oid"
)
#JsonTypeInfo(
use = JsonTypeInfo.Id.NAME,
include = JsonTypeInfo.As.EXISTING_PROPERTY, //field must be present in the POJO
property = "clazz")
#JsonSubTypes({
#JsonSubTypes.Type(value = MySubEntity.class, name = "MySubEntity"),
})
public abstract class Entity {
#JsonProperty
private String uuid;
//Here we have to initialize this field manually.
//Here is the simple workaround to initialize in automatically
#JsonProperty
private String clazz = this.getClass().getSimpleName();
public String getUuid() {
return uuid;
}
public void setUuid(String uuid) {
this.uuid = uuid;
}
public String getClazz() {
return clazz;
}
public void setClazz(String clazz) {
this.clazz = clazz;
}
}
I would like to know how to access a deep collection class attribute in a GET request. My endpoint maps my query strings through #ModelAttribute annotation:
Given that:
public class MyEntity
{
Set<Item> items;
Integer status;
// getters setters
}
public class Item
{
String name;
// getters setters
}
And my GET request: localhost/entities/?status=0&items[0].name=Garry
Produces bellow behavior?
#RequestMapping(path = "/entities", method = RequestMethod.GET)
public List<MyEntity> findBy(#ModelAttribute MyEntity entity) {
// entity.getItems() is empty and an error is thrown: "Property referenced in indexed property path 'items[0]' is neither an array nor a List nor a Map."
}
Should my "items" be an array, List or Map? If so, there´s alternatives to keep using as Set?
Looks like there is some problem with the Set<Item>.
If you want to use Set for the items collection you have to initialize it and add some items:
e.g. like this:
public class MyEntity {
private Integer status;
private Set<Item> items;
public MyEntity() {
this.status = 0;
this.items = new HashSet<>();
this.items.add(new Item());
this.items.add(new Item());
}
//getters setters
}
but then you will be able to set only the values of this 2 items:
This will work: http://localhost:8081/map?status=1&items[0].name=asd&items[1].name=aaa
This will not work: http://localhost:8081/map?status=1&items[0].name=asd&items[1].name=aaa&items[2].name=aaa
it will say: Invalid property 'items[2]' of bean class MyEntity.
However if you switch to List:
public class MyEntity {
private Integer status;
private List<Item> items;
}
both urls map without the need to initialize anything and for various number of items.
note that I didn't use #ModelAttribute, just set the class as paramter
#GetMapping("map")//GetMapping is just a shortcut for RequestMapping
public MyEntity map(MyEntity myEntity) {
return myEntity;
}
Offtopic
Mapping a complex object in Get request sounds like a code smell to me.
Usually Get methods are used to get/read data and the url parameters are used to specify the values that should be used to filter the data that has to be read.
if you want to insert or update some data use POST, PATCH or PUT and put the complex object that you want to insert/update in the request body as JSON(you can map that in the Spring Controller with #RequestBody).
I do a replication mechanism where I synchronize two databases. For communicating between databases I serialize the objects into JSON using Gson. Each object has a UUID to identify it. To avoid having to send the items that are up to date I use the objects UUID when an object is included in a field in an object to be replicated.
We got the following classes:
public class Entity {
String uuid;
// Getters and setters..
}
public class Article extends Entity {
String name;
Brand brand;
// Getters and setters..
}
public class Brand extens Entity {
String name;
Producer producer
// Getters and setters..
}
public class Producer extends Entity {
String name;
// Getters and setters..
}
If I serialize Article its JSON representation will look like this:
{"brand":"BrandÖ179d7798-aa63-4dd2-8ff6-885534f99e77","uuid":"5dbce9aa-4129-41b6-a8e5-d3c030279e82","name":"Sun-Maid"}
where "BrandÖ179d7798-aa63-4dd2-8ff6-885534f99e77" is the class ("Brand") and the UUID.
If I serialize Brand I expect:
{"producer":"ProducerÖ173d7798-aa63-4dd2-8ff6-885534f84732","uuid":"5dbce9aa-4129-41b6-a8e5-d3c0302w34w2","name":"Carro"}
In Jackson I would change Article class to:
public class Article {
String uuid;
String name;
#JsonDeserialize (using = EntityUUIDDeserializer.class) # JsonSerialize (using = EntityUUIDSerializer.class)
Brand brand;
// Getters and setters..
}
and implement custom serializer and deserializer to return the UUID instead of the object.
Gson do not have a #JsonDeserialize annotation.
If we install the serializer and deserializer doing like this:
Gson gson = new GsonBuilder()
.registerTypeAdapter(Producer.class, new EntityUUIDDeserializer())
.registerTypeAdapter(Brand.class, new EntityUUIDDeserializer())
.registerTypeAdapter/Producer.class, new EntityUUIDSerializer())
.registerTypeAdapter(Brand.class, new EntityUUIDSerializer())
.create();
We can serialize Article and Brand ok.
Deserialize Article by
Article article= gson.fromJson(inputJSONString, Article.class);
works fine but
Brand brand= gson.fromJson(inputJSONString, Brand.class);
do not work. I guess the probem is that when we deserialize a Brand we get the deserializer for Brand to kick in trying to return an UUID string, but we want the deserializer to return a Brand-object instead.
Is there a way to avoid creating two different Gson objects? The problem with two diffrent Gson objects is when you want to deserialize an object that contains both Article and Brand.
You write the serializer/deserializer and register it with Gson (using the GsonBuilder).
https://sites.google.com/site/gson/gson-user-guide#TOC-Custom-Serialization-and-Deserialization
Gson g = new GsonBuilder()
.registerTypeAdapter(Producer.class, new MyProducerDeserializer())
.registerTypeAdapter(Producer.class, new MyProducerSerializer())
.create();
When you serialize/deserialize your Brand class, it will use them for the Producer field contained therein.