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LazyInitializationException with Mapstruct because of cyclic issue

I have a development project using Spring Data JPA and MapStruct to map between Entities and DTOs. Last week I decided it was time to address the FetchType.EAGER vs LAZY issue I have postponed for some time. I choose to use #NamedEntityGraph and #EntityGraph to load properties when needed. However I am stuck with this LazyInitializationExeption problem when doing the mapping from entity to dto. I think I know where this happens but I do not know how to get passed it.
The code
#NamedEntityGraph(name="Employee.full", ...)
#Entity
public class Employee {
private Set<Role> roles = new HashSet<>();
}
#Entity
public class Role {
private Set<Employee> employees = new HashSet<>();
}
public interface EmployeeRepository extends JpaRepository<Employee, Long> {
#EntityGraph(value = "Employee.full")
#Override
Page<Employee> findAll(Pageable pageable);
}
#Service
public class EmployeeService {
public Page<EmployeeDTO> findAll(PageRequest pageRequest) {
Page<Employee> employees = repository.findAll(pageRequest); // ok
Page<EmployeeDTO> dtos = employees.map(emp -> mapper.toDTO(emp, new CycleAvoidMappingContext()); // this is where the exception happens
return dtos;
}
}
// also there is EmployeeDTO and RoleDTO classes mirroring the entity classes
// and there is a simple interface EmployeeMapper loaded as a spring component
// without any special mappings. However CycleAvoidingMappingContext is used.
I have tracked down the LazyInitializationException to happen when the mapper tries to map the roles dependency. The Role object do have Set<Employee> and therefore there is a cyclic reference.
When using FetchType.EAGER new CycleAvoidingMappingContext() solved this problem, but with LAZY this no longer works.
Does anybody know how I can avoid the exception and at the same time get my DTOs mapped correctly?

The problem is that when the code returns from findAll the entities are not managed anymore. So you have a LazyInitializationException because you are trying, outside of the scope of the session, to access a collection that hasn't been initialized already.
Adding eager make it works because it makes sure that the collection has been already initialized.
You have two alternatives:
Using an EAGER fetch;
Make sure that the entities are still managed when you return from the findAll. Adding a #Transactional to the method should work:
#Service
public class EmployeeService {
#Transactional
public Page<EmployeeDTO> findAll(PageRequest pageRequest) {
Page<Employee> employees = repository.findAll(pageRequest);
Page<EmployeeDTO> dtos = employees.map(emp -> mapper.toDTO(emp, new CycleAvoidMappingContext());
return dtos;
}
}
I would say that if you need the collection initialized, fetching it eagerly (with an entity graph or a query) makes sense.
Check this article for more details on entities states in Hibernate ORM.
UPDATE: It seems that this error happens because Mapstruct is converting the collection even if you don't need it in the DTO.
In this case, you have different options:
Remove the field roles from the DTO. Mapstruct will ignore the field in the entity because the DTO doesn't have a field with the same name;
Create a different DTO class for this specific case without the field roles;
Use the #Mapping annotation to ignore the field in the entity:
#Mapping(target = "roles", ignore = true)
void toDTO(...)
or, if you need the toDTO method sometimes
#Mapping(target = "roles", ignore = true)
void toSkipRolesDTO(...) // same signature as toDTO

Spring Data JPA - bidirectional relation with infinite recursion

First, here are my entities.
Player :
#Entity
#JsonIdentityInfo(generator=ObjectIdGenerators.UUIDGenerator.class,
property="id")
public class Player {
// other fields
#ManyToOne
#JoinColumn(name = "pla_fk_n_teamId")
private Team team;
// methods
}
Team :
#Entity
#JsonIdentityInfo(generator=ObjectIdGenerators.UUIDGenerator.class,
property="id")
public class Team {
// other fields
#OneToMany(mappedBy = "team")
private List<Player> members;
// methods
}
As many topics already stated, you can avoid the StackOverflowExeption in your WebService in many ways with Jackson.
That's cool and all but JPA still constructs an entity with infinite recursion to another entity before the serialization. This is just ugly ans the request takes much longer. Check this screenshot : IntelliJ debugger
Is there a way to fix it ? Knowing that I want different results depending on the endpoint. Examples :
endpoint /teams/{id} => Team={id..., members=[Player={id..., team=null}]}
endpoint /members/{id} => Player={id..., team={id..., members=null}}
Thank you!
EDIT : maybe the question isn't very clear giving the answers I get so I'll try to be more precise.
I know that it is possible to prevent the infinite recursion either with Jackson (#JSONIgnore, #JsonManagedReference/#JSONBackReference etc.) or by doing some mapping into DTO. The problem I still see is this : both of the above are post-query processing. The object that Spring JPA returns will still be (for example) a Team, containing a list of players, containing a team, containing a list of players, etc. etc.
I would like to know if there is a way to tell JPA or the repository (or anything) to not bind entities within entities over and over again?

Here is how I handle this problem in my projects.
I used the concept of data transfer objects, implemented in two version: a full object and a light object.
I define a object containing the referenced entities as List as Dto (data transfer object that only holds serializable values) and I define a object without the referenced entities as Info.
A Info object only hold information about the very entity itself and not about relations.
Now when I deliver a Dto object over a REST API, I simply put Info objects for the references.
Let's assume I deliever a PlayerDto over GET /players/1:
public class PlayerDto{
private String playerName;
private String playercountry;
private TeamInfo;
}
Whereas the TeamInfo object looks like
public class TeamInfo {
private String teamName;
private String teamColor;
}
compared to a TeamDto
public class TeamDto{
private String teamName;
private String teamColor;
private List<PlayerInfo> players;
}
This avoids an endless serialization and also makes a logical end for your rest resources as other wise you should be able to GET /player/1/team/player/1/team
Additionally, the concept clearly separates the data layer from the client layer (in this case the REST API), as you don't pass the actually entity object to the interface. For this, you convert the actual entity inside your service layer to a Dto or Info. I use http://modelmapper.org/ for this, as it's super easy (one short method call).
Also I fetch all referenced entities lazily. My service method which gets the entity and converts it to the Dto there for runs inside of a transaction scope, which is good practice anyway.
Lazy fetching
To tell JPA to fetch a entity lazily, simply modify your relationship annotation by defining the fetch type. The default value for this is fetch = FetchType.EAGER which in your situation is problematic. That is why you should change it to fetch = FetchType.LAZY
public class TeamEntity {
#OneToMany(mappedBy = "team",fetch = FetchType.LAZY)
private List<PlayerEntity> members;
}
Likewise the Player
public class PlayerEntity {
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "pla_fk_n_teamId")
private TeamEntity team;
}
When calling your repository method from your service layer, it is important, that this is happening within a #Transactional scope, otherwise, you won't be able to get the lazily referenced entity. Which would look like this:
#Transactional(readOnly = true)
public TeamDto getTeamByName(String teamName){
TeamEntity entity= teamRepository.getTeamByName(teamName);
return modelMapper.map(entity,TeamDto.class);
}

In my case I realized I did not need a bidirectional (One To Many-Many To One) relationship.
This fixed my issue:
// Team Class:
#OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
private Set<Player> members = new HashSet<Player>();
// Player Class - These three lines removed:
// #ManyToOne
// #JoinColumn(name = "pla_fk_n_teamId")
// private Team team;
Project Lombok might also produce this issue. Try adding #ToString and #EqualsAndHashCode if you are using Lombok.
#Data
#Entity
#EqualsAndHashCode(exclude = { "members"}) // This,
#ToString(exclude = { "members"}) // and this
public class Team implements Serializable {
// ...
This is a nice guide on infinite recursion annotations https://www.baeldung.com/jackson-bidirectional-relationships-and-infinite-recursion

You can use #JsonIgnoreProperties annotation to avoid infinite loop, like this:
#JsonIgnoreProperties("members")
private Team team;
or like this:
#JsonIgnoreProperties("team")
private List<Player> members;
or both.

JPA #ManyToOne #JoinFormula throwing ClassCastException

I'm trying to model a many-to-one bi-directional association using JPA. The join uses a formula. I've tried it a couple of ways, as shown below. Once with just JoinFormula and another with JoinColumnsOrFormulas.
public class JobOperation
{
private Operation operation;
#ManyToOne
// #JoinFormula("CASE WHEN attribute7 IS NULL OR TO_NUMBER(attribute7) = 0 THEN standard_operation_id ELSE TO_NUMBER(attribute7) END")
#JoinColumnsOrFormulas(
{
#JoinColumnOrFormula(formula = #JoinFormula(//
value = "(CASE WHEN this_.attribute7 IS NULL OR TO_NUMBER(this_.attribute7) = 0 THEN this_.standard_operation_id ELSE TO_NUMBER(this_.attribute7) END)", //
referencedColumnName = "standard_operation_id"))
})
#Fetch(FetchMode.SELECT)
#NotFound(action = NotFoundAction.IGNORE)
public Operation getOperation()
{
return this.operation;
}
}
I originally was using Hibernate 4.3.9 and then tried with Hibernate 5.1.0. Both throw the same exception:
15:55:21,408 DEBUG [org.hibernate.cfg.annotations.TableBinder] Retrieving property com.icumed.ifactory3.dto.wip.JobOperation.operation
15:55:21,409 DEBUG [org.hibernate.jpa.HibernatePersistenceProvider] Unable to build entity manager factory
java.lang.ClassCastException: org.hibernate.mapping.Formula cannot be cast to org.hibernate.mapping.Column
at org.hibernate.cfg.annotations.TableBinder.bindFk(TableBinder.java:584)
Nothing in Hibernate's TableBinder class refers to a formula. Does Hibernate just not support this or am I using the wrong annotations, or is there something else going on?

The root cause of the problem seems to be on the other side of the association. I originally had this
public class Operation extends AbstractOperation
{
#OneToMany(mappedBy="operation")
public Set<JobOperation> getJobOperations()
{
return this.jobOperations;
}
}
and when I changed it to the following, it worked.
public class Operation extends AbstractOperation
{
#OneToMany
#JoinColumn(name="STANDARD_OPERATION_ID")
public Set<JobOperation> getJobOperations()
{
return this.jobOperations;
}
}

Jackson: referencing an object as a property

In my java spring application, I am working with hibernate and jpa, and i use jackson to populate data in DB.
Here is the User class:
#Data
#Entity
public class User{
#Id
#GeneratedValue
Long id;
String username;
String password;
boolean activated;
public User(){}
}
and the second class is:
#Entity
#Data
public class Roles {
#Id
#GeneratedValue
Long id;
#OneToOne
User user;
String role;
public Roles(){}
}
In the class Roles i have a property of User
and then i made a json file to store the data:
[ {"_class" : "com.example.domains.User", "id": 1, "username": "Admin", "password": "123Admin123","activated":true}
,
{"_class" : "com.example.domains.Roles", "id": 1,"user":1, "role": "Admin"}]
Unfortunately, when i run the app it complains with:
.RuntimeException: com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of com.example.domains.User: no int/Int-argument constructor/factory method to deserialize from Number value (1)
at [Source: N/A; line: -1, column: -1] (through reference chain: com.example.domains.Roles["user"])
The problem comes from
{"_class" : "com.example.domains.Roles", "id": 1,"user":1, "role": "Admin"}
and when i remove the above line the app works well.
I think, it complains because it cannot make an instance of user.
So, how can i fix it?

Do yourself a favor and stop using your Entities as DTOs!
JPA entities have bidirectional relations, JSON objects don't, I also believe that the responsibilities of an Entity is very different from a DTO, and although joining these responsibilities into a single Java class is possible, in my experience it is a very bad idea.
Here are a couple of reasons
You almost always need more flexibility in the DTO layer, because it is often related to a UI.
You should avoid exposing primary keys from your database to the outside, including your own UI. We always generate an additional uniqueId (UUID) for every publicly exposed Entity, the primary key stays in the DB and is only used for joins.
You often need multiple views of the same Entity. Or a single view of multiple entities.
If you need to add a new entity to a relation with an existing, you will need find the existing one in the database, so posting the new and old object as a single JSON structure has no advantage. You just need the uniqueId of the existing, and then new.
A lot of the problems developers have with JPA, specifically with regards to merging comes from the fact that they receive a detached entity after their json has been deserialized. But this entity typically doesn't have the OneToMany relations (and if it does, it's the parent which has a relation to the child in JSON, but in JPA it is the child's reference to the parent which constitutes the relationship). In most cases you will always need to load the existing version of the entity from the database, and then copy the changes from your DTO into the entity.
I have worked extensively with JPA since 2009, and I know most corner cases of detachment and merging, and have no problem using an Entity as a DTO, but I have seen the confusion and types of errors that occur when you hand such code over to some one who is not intimately familiar with JPA. The few lines you need for a DTO (especially since you already use Lombok), are so simple and allows you much more flexibility, than trying to save a few files and breaking the separation of concerns.

Jackson provide ObjectIdResolver interface for resolving the objects from ids during de-serialization.
In your case you want to resolve the id based from the JPA/hibernate. So you need to implement a custom resolver to resolve id by calling the JPA/hierbate entity manager.
At high level below are the steps:
Implement a custom ObjectIdResolver say JPAEntityResolver (you may extends from SimpleObjectIdResolver). During resolving object it will call JPA entity manager class to find entity by given id and scope(see. ObjectIdResolver#resolveId java docs)
//Example only;
#Component
#Scope("prototype") // must not be a singleton component as it has state
public class JPAEntityResolver extends SimpleObjectIdResolver {
//This would be JPA based object repository or you can EntityManager instance directly.
private PersistentObjectRepository objectRepository;
#Autowired
public JPAEntityResolver (PersistentObjectRepository objectRepository) {
this.objectRepository = objectRepository;
}
#Override
public void bindItem(IdKey id, Object pojo) {
super.bindItem(id, pojo);
}
#Override
public Object resolveId(IdKey id) {
Object resolved = super.resolveId(id);
if (resolved == null) {
resolved = _tryToLoadFromSource(id);
bindItem(id, resolved);
}
return resolved;
}
private Object _tryToLoadFromSource(IdKey idKey) {
requireNonNull(idKey.scope, "global scope does not supported");
String id = (String) idKey.key;
Class<?> poType = idKey.scope;
return objectRepository.getById(id, poType);
}
#Override
public ObjectIdResolver newForDeserialization(Object context) {
return new JPAEntityResolver(objectRepository);
}
#Override
public boolean canUseFor(ObjectIdResolver resolverType) {
return resolverType.getClass() == JPAEntityResolver.class;
}
}
Tell Jackson to use a custom id resolver for a class, by using annotation JsonIdentityInfo(resolver = JPAEntityResolver.class). For e.g.
#JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class,
property = "id",
scope = User.class,
resolver = JPAObjectIdResolver.class)
public class User { ... }
JPAObjectIdResolver is a custom implementation and will have dependency on other resources( JPA Entity Manager) which might not be known to Jackson. So Jackson need help to instantiate resolver object. For this purpose, you need to supply a custom HandlerInstantiator to ObjectMapper instance. (In my case I was using spring so I asked spring to create instance of JPAObjectIdResolver by using autowiring)
Now de-serialization should work as expected.
Hope this helps.

I have changed the json file to :
[
{"_class" : "com.example.domains.User",
"id": 1,
"username": "Admin",
"password": "123Admin123",
"activated":true
},
{
"_class" : "com.example.domains.Roles",
"id": 1,
"user":{"_class" : "com.example.domains.User",
"id": 1,
"username": "Admin",
"password": "123Admin123",
"activated":true
},
"role": "Admin"
}
]
But i still think, the best ways is using a foreign key to user record.
Any solution is welcomed

If your bean doesn't strictly adhere to the JavaBeans format, Jackson has difficulties.
It's best to create an explicit #JsonCreator constructor for your JSON model bean, e.g.
class User {
...
#JsonCreator
public User(#JsonProperty("name") String name,
#JsonProperty("age") int age) {
this.name = name;
this.age = age;
}
..
}

1-1 mapping of fields works well , but when it comes to complex object mapping , better to use some API.
You can use Dozer Mapping or Mapstruct to map Object instances.
Dozer has spring integration also.

You could specify non default constructors and then use a custom deserialiser.
Something like this should work (it has not been tested).
public class RolesDeserializer extends StdDeserializer<Roles> {
public RolesDeserializer() {
this(null);
}
public RolesDeserializer(Class<?> c) {
super(c);
}
#Override
public Roles deserialize(JsonParser jp, DeserializationContext dsctxt)
throws IOException, JsonProcessingException {
JsonNode node = jp.getCodec().readTree(jp);
long id = ((LongNode) node.get("id")).longValue();
String roleName = node.get("role").asText();
long userId = ((LongNode) node.get("user")).longValue();
//Based on the userId you need to search the user and build the user object properly
User user = new User(userId, ....);
return new Roles(id, roleName, user);
}
}
Then you need to register your new deserialiser (1) or use the #JsonDeserialize annotation (2)
(1)
ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(Item.class, new RolesDeserializer());
mapper.registerModule(module);
Roles deserializedRol = mapper.readValue(yourjson, Roles.class);
(2)
#JsonDeserialize(using = RolesDeserializer.class)
#Entity
#Data
public class Roles {
...
}
Roles deserializedRol = new ObjectMapper().readValue(yourjson, Roles.class);

public class Roles {
#Id
#GeneratedValue
Long id;
#JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
#JsonIdentityReference(alwaysAsId = true)
#OneToOne
User user;
String role;
public Roles(){}
}

JPA OneToMany - Collection is null

I'm trying to set up a bidirectional relationship using JPA. I understand that it's the responsability of the application to maintain both sides of the relationship.
For example, a Library has multiple Books. In the Library-entity I have:
#Entity
public class Library {
..
#OneToMany(mappedBy = "library", cascade = CascadeType.ALL)
private Collection<Book> books;
public void addBook(Book b) {
this.books.add(b);
if(b.getLibrary() != this)
b.setLibrary(this);
}
..
}
The Book-entity is:
#Entity
public class Book {
..
#ManyToOne
#JoinColumn(name = "LibraryId")
private Library library;
public void setLibrary(Library l) {
this.library = l;
if(!this.library.getBooks().contains(this))
this.library.getBooks().add(this);
}
..
}
Unfortunately, the collection at the OneToMany-side is null. So for example a call to setLibrary() fails because this.library.getBooks().contains(this) results in a NullPointerException.
Is this normal behavior? Should I instantiate the collection myself (which seems a bit strange), or are there other solutions?

Entities are Java objects. The basic rules of Java aren't changed just because there is an #Entity annotation on the class.
So, if you instantiate an object and its constructor doesn't initialize one of the fields, this field is initialized to null.
Yes, it's your responsibility to make sure that the constructor initializes the collection, or that all the methods deal with the nullability of the field.
If you get an instance of this entity from the database (using em.find(), a query, or by navigating through associations of attached entities), the collection will never be null, because JPA will always initialize the collection.

It seems that books type of Collection in Library class is not initilized. It is null;
So when class addBook method to add a book object to collection. It cause NullPointerException.
#OneToMany(mappedBy = "library", cascade = CascadeType.ALL)
private Collection<Book> books;
public void addBook(Book b) {
this.books.add(b);
if(b.getLibrary() != this)
b.setLibrary(this);
}
Initilize it and have a try.
Change
private Collection<Book> books;
To
private Collection<Book> books = new ArrayList<Book>();

Try to set the fetch type association property to eager on the OneToMany side. Indeed, you may leave this part (this.library.getBooks().add(this)) to be written within a session:
Library l = new Library();
Book b = new Book();
b.setLibrary(l);
l.getBooks().add(b);