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Avoid unncessary relationship Rest Api Response JPA

I have a springboot Application with 10 Tables in my database. When i make a get request on one table, it returns a response with in all the tables having any related records in all the 10 tables. I use Lazy fetch, the problems does not solve. I need to return only response expectated by front end app. For example, if the client wants only students then the REST API should return only students (not its child entities). How can i solve this problem.

You can parse a JSON response and get a particular field/fields from Response by customising it.
The other option is to use #JsonIgnore annotation on the fields which you don't want them to be part of response.

You can solve this by creating DTOs for your use cases and I think this is a perfect use case for Blaze-Persistence Entity Views.
I created the library to allow easy mapping between JPA models and custom interface or abstract class defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure(domain model) the way you like and map attributes(getters) via JPQL expressions to the entity model.
A DTO model for your use case could look like the following with Blaze-Persistence Entity-Views:
#EntityView(Student.class)
public interface StudentDto {
#IdMapping
Long getId();
String getName();
Set<CourseDto> getCourses();
#EntityView(Course.class)
interface CourseDto {
#IdMapping
Long getId();
String getName();
}
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
StudentDto a = entityViewManager.find(entityManager, StudentDto.class, id);
The Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features
Page<StudentDto> findAll(Pageable pageable);
The best part is, it will only fetch the state that is actually necessary!

Kryonet desirialization issue with Hibernate Entities

I hope that someone might have had this or a similar problem before and could help me :)
I have a KryoNet Server / Client architecture, where I am sending messages.
One of these Messages contains an instance of a class "WorldEntity".
This entity looks something like this:
#Entity
#Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class WorldEntity {
#OneToMany
private Collection<EntityComponent> components;
}
I am using spring + hibernate to save this entity to my postgresql database.
This works all fine.
When I am now sending this entity over to the client, the kryonet serializer tries to load the components lazily, which it should not do, since there is no hibernate or database on the client.
In fact, all data is already contained in the entity.
I read somewhere, that a custom Serialization Object could be created and added to the KryoNet client, to disable this hibernate loading:
Kryo kryoSerializer = new Kryo() {
#Override
public Serializer<?> getDefaultSerializer(final Class type) {
if (AbstractPersistentCollection.class.isAssignableFrom(type)) {
return new FieldSerializer(kryoSerializer, type);
}
return super.getDefaultSerializer(type);
}
};
This, unfortunately, can not be used as a Serialization object in the constructor of the Kryo Client.
Any help will be greatly appreciated!
Kind regards
Dustin

I would suggest you start thinking about adding DTOs for this purpose to also avoid exposing too much information to your clients for security but also network performance concerns. I think this is a perfect use case for Blaze-Persistence Entity Views.
I created the library to allow easy mapping between JPA models and custom interface or abstract class defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure(domain model) the way you like and map attributes(getters) via JPQL expressions to the entity model.
A DTO model for your use case could look like the following with Blaze-Persistence Entity-Views:
#EntityView(WorldEntity.class)
public interface WorldEntityDto {
#IdMapping
Long getId();
String getName();
Set<EntityComponentDto> getComponents();
#EntityView(EntityComponent.class)
interface EntityComponentDto {
#IdMapping
Long getId();
String getName();
}
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
WorldEntityDto a = entityViewManager.find(entityManager, WorldEntityDto.class, id);
The Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features
Page<WorldEntityDto> findAll(Pageable pageable);
The best part is, it will only fetch the state that is actually necessary!

hibernate projection for one-to-many mapping set of objects

I have a USER object which has many fields having one-to-many mapping with PERMISSION object as well.
I want to fetch only few fields of user with a set of permissions as well. my code is
public class USER {
private Integer id;
private String username;
...
...
...
private Set<Permission> permissions = new HashSet<Permission>();
//setter - getter methods
}
Permission.java
public class Permission {
private String name;
private Integer id;
//setter - getter methods
}
Projection code:
Criteria criteria = session.createCriteria(User.class);
criteria.setCacheable(true);
criteria.add(eq("username", username).ignoreCase());
criteria.createAlias("permissions ", "perm", LEFT_OUTER_JOIN);
ProjectionList projectedFields = Projections.projectionList();
projectedFields.add(Projections.property("id").as("id"));
projectedFields.add(Projections.property("perm.id").as("permissions.id"));
criteria.setProjection(projectedFields);
criteria.setResultTransformer(new AliasToBeanNestedResultTransformer((User.class)));
User user = (User) criteria.uniqueResult();
I'm getting the following exception:
org.hibernate.PropertyNotFoundException: Could not find setter for id on interface java.util.Set
at org.hibernate.property.ChainedPropertyAccessor.getSetter(ChainedPropertyAccessor.java:66)
at org.hibernate.transform.AliasToBeanResultTransformer.initialize(AliasToBeanResultTransformer.java:121)
at org.hibernate.transform.AliasToBeanResultTransformer.transformTuple(AliasToBeanResultTransformer.java:84)
at ae.gov.adm.saeed.util.AliasToBeanNestedResultTransformer.transformTuple(AliasToBeanNestedResultTransformer.java:80)
at org.hibernate.transform.CacheableResultTransformer.retransformResults(CacheableResultTransformer.java:230)
any idea, how to resolve this issue?

This doesn't work. This only works for scalar results, but not for nested structures. You would have to build the object graph yourself.
Anyway, this is a perfect use case for Blaze-Persistence Entity Views.
Blaze-Persitence is a query builder on top of JPA which supports many of the advanced DBMS features on top of the JPA model. I created Entity Views on top of it to allow easy mapping between JPA models and custom interface defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure the way you like and map attributes(getters) via JPQL expressions to the entity model. Since the attribute name is used as default mapping, you mostly don't need explicit mappings as 80% of the use cases is to have DTOs that are a subset of the entity model.
A mapping for your model could look as simple as the following
#EntityView(User.class)
interface ApplicationUser {
Integer getId();
#Mapping("permissions.id")
Set<Integer> getPermission();
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
ApplicationUser dto = entityViewManager.find(entityManager, ApplicationUser.class, id);
But the Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features
It will only fetch the mappings that you tell it to fetch

DDD implementation with Spring Data and JPA + Hibernate problem with identities

So I'm trying for the first time in a not so complex project to implement Domain Driven Design by separating all my code into application, domain, infrastructure and interfaces packages.
I also went with the whole separation of the JPA Entities to Domain models that will hold my business logic as rich models and used the Builder pattern to instantiate. This approach created me a headache and can't figure out if Im doing it all wrong when using JPA + ORM and Spring Data with DDD.
Process explanation
The application is a Rest API consumer (without any user interaction) that process daily through Scheduler tasks a fairly big amount of data resources and stores or updates into MySQL. Im using RestTemplate to fetch and convert the JSON responses into Domain objects and from there Im applying any business logic within the Domain itself e.g. validation, events, etc
From what I have read the aggregate root object should have an identity in their whole lifecycle and should be unique. I have used the id of the rest API object because is already something that I use to identify and track in my business domain. I have also created a property for the Technical id so when I convert Entities to Domain objects it can hold a reference for the update process.
When I need to persist the Domain to the data source (MySQL) for the first time Im converting them into Entity objects and I persist them using the save() method. So far so good.
Now when I need to update those records in the data source I first fetch them as a List of Employees from data source, convert Entity objects to Domain objects and then I fetch the list of Employees from the rest API as Domain models. Up until now I have two lists of the same Domain object types as List<Employee>. I'm iterating them using Streams and checking if an objects are not equal() between them if yes a collection of List items is created as a third list with Employee objects that need to be updated. Here I've already passed the technical Id to the domain objects in the third list of Employees so Hibernate can identify and use to update the records that are already exists.
Up to here are all fairly simple stuff until I use the saveAll() method to update the records.
Questions
I alway see Hibernate using INSERT instead of updating the list of
records. So If Im correct Hibernate session is not recognising the
objects that Im throwing into it because I have detached them when I
used the convert to domain object?
Does anyone have a better idea how can I implement this differently or fix
this problem?
Or should I stop using this approach as two different objects and continue use
them as rich Entity models?
Simple classes to explain it with code
EmployeeDO.java
#Entity
#Table(name = "employees")
public class EmployeeDO implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
public EmployeeDO() {}
...omitted getter/setters
}
Employee.java
public class Employee {
private Long persistId;
private Long employeeId;
private String name;
private Employee() {}
...omitted getters and Builder
}
EmployeeConverter.java
public class EmployeeConverter {
public static EmployeeDO serialize(Employee employee) {
EmployeeDO target = new EmployeeDO();
if (employee.getPersistId() != null) {
target.setId(employee.getPersistId());
}
target.setName(employee.getName());
return target;
}
public static Employee deserialize(EmployeeDO employee) {
return new Country.Builder(employee.getEmployeeId)
.withPersistId(employee.getId()) //<-- Technical ID setter
.withName(employee.getName())
.build();
}
}
EmployeeRepository.java
#Component
public class EmployeeReporistoryImpl implements EmployeeRepository {
#Autowired
EmployeeJpaRepository db;
#Override
public List<Employee> findAll() {
return db.findAll().stream()
.map(employee -> EmployeeConverter.deserialize(employee))
.collect(Collectors.toList());
}
#Override
public void saveAll(List<Employee> employees) {
db.saveAll(employees.stream()
.map(employee -> EmployeeConverter.serialize(employee))
.collect(Collectors.toList()));
}
}
EmployeeJpaRepository.java
#Repository
public interface EmployeeJpaRepository extends JpaRepository<EmployeeDO, Long> {
}

I use the same approach on my project: two different models for the domain and the persistence.
First, I would suggest you to don't use the converter approach but use the Memento pattern. Your domain entity exports a memento object and it could be restored from the same object. Yes, the domain has 2 functions that aren't related to the domain (they exist just to supply a non-functional requirement), but, on the other side, you avoid to expose functions, getters and constructors that the domain business logic never use.
For the part about the persistence, I don't use JPA exactly for this reason: you have to write a lot of code to reload, update and persist the entities correctly. I write directly SQL code: I can write and test it fast, and once it works I'm sure that it does what I want. With the Memento object I can have directly what I will use in the insert/update query, and I avoid myself a lot of headaches about the JPA of handling complex tables structures.
Anyway, if you want to use JPA, the only solution is to:
load the persistence entities and transform them into domain entities
update the domain entities according to the changes that you have to do in your domain
save the domain entities, that means:
reload the persistence entities
change, or create if there're new ones, them with the changes that you get from the updated domain entities
save the persistence entities
I've tried a mixed solution, where the domain entities are extended by the persistence ones (a bit complex to do). A lot of care should be took to avoid that domain model should adapts to the restrictions of JPA that come from the persistence model.
Here there's an interesting reading about the splitting of the two models.
Finally, my suggestion is to think how complex the domain is and use the simplest solution for the problem:
is it big and with a lot of complex behaviours? Is expected that it will grow up in a big one? Use two models, domain and persistence, and manage the persistence directly with SQL It avoids a lot of caos in the read/update/save phase.
is it simple? Then, first, should I use the DDD approach? If really yes, I would let the JPA annotations to split inside the domain. Yes, it's not pure DDD, but we live in the real world and the time to do something simple in the pure way should not be some orders of magnitude bigger that the the time I need to to it with some compromises. And, on the other side, I can write all this stuff in an XML in the infrastructure layer, avoiding to clutter the domain with it. As it's done in the spring DDD sample here.

When you want to update an existing object, you first have to load it through entityManager.find() and apply the changes on that object or use entityManager.merge since you are working with detached entities.
Anyway, modelling rich domain models based on JPA is the perfect use case for Blaze-Persistence Entity Views.
Blaze-Persistence is a query builder on top of JPA which supports many of the advanced DBMS features on top of the JPA model. I created Entity Views on top of it to allow easy mapping between JPA models and custom interface defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure the way you like and map attributes(getters) via JPQL expressions to the entity model. Since the attribute name is used as default mapping, you mostly don't need explicit mappings as 80% of the use cases is to have DTOs that are a subset of the entity model.
The interesting point here is that entity views can also be updatable and support automatic translation back to the entity/DB model.
A mapping for your model could look as simple as the following
#EntityView(EmployeeDO.class)
#UpdatableEntityView
interface Employee {
#IdMapping("persistId")
Long getId();
Long getEmployeeId();
String getName();
void setName(String name);
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
Employee dto = entityViewManager.find(entityManager, Employee.class, id);
The Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features and it can also be saved back. Here a sample repository
#Repository
interface EmployeeRepository {
Employee findOne(Long id);
void save(Employee e);
}
It will only fetch the mappings that you tell it to fetch and also only update the state that you make updatable through setters.
With the Jackson integration you can deserialize your payload onto a loaded entity view or you can avoid loading alltogether and use the Spring MVC integration to capture just the state that was transferred and flush that. This could look like the following:
#RequestMapping(path = "/employee/{id}", method = RequestMethod.PUT, consumes = MediaType.APPLICATION_JSON_VALUE)
public ResponseEntity<String> updateEmp(#EntityViewId("id") #RequestBody Employee emp) {
employeeRepository.save(emp);
return ResponseEntity.ok(emp.getId().toString());
}
Here you can see an example project: https://github.com/Blazebit/blaze-persistence/tree/master/examples/spring-data-webmvc

Preserve Hibernate Lazy loading in data transfer object design pattern

I usually work on 3-tier applications using Hibernate in the persistence layer and I take care to not use the domain model classes in the presentation layer. This is why I use the DTO (Data Transfer Object) design pattern.
But I always have a dilemma in my entity-dto mapping. Whether I lose the lazy loading benefict, or I create complexity in the code by introducing filters to call or not the domain model getters.
Example : Consider a DTO UserDto that corresponds to the entity User
public UserDto toDto(User entity, OptionList... optionList) {
if (entity == null) {
return null;
}
UserDto userDto = new UserDto();
userDto.setId(entity.getId());
userDto.setFirstname(entity.getFirstname());
if (optionList.length == 0 || optionList[0].contains(User.class, UserOptionList.AUTHORIZATION)) {
IGenericEntityDtoConverter<Authorization, AuthorizationDto> authorizationConverter = converterRegistry.getConverter(Authorization.class);
List<AuthorizationDto> authorizations = new ArrayList<>(authorizationConverter.toListDto(entity.getAuthorizations(), optionList));
userDto.setAuthorizations(authorizations);
...
}
OptionList is used to filter the mapping and map just what is wanted.
Although the last solution allow lazy loading but it's very heavy because the optionList must be specified in the service layer.
Is there any better solution to preserve lazy loading in a DTO design pattern ?

For the same entity persistent state, I don't like having fields of an object un-initialized in some execution path, while these fields might also be initialized in other cases. This cause too much headache to maintain :
it will cause Nullpointer in the better cases
if null is also a valid option (and thus not cause NullPointer), it could mean the data was removed and might trigger unexpected removal business rules, while the data is in fact still there.
I would rather create a DTO hierarchy of interfaces and/or classes, starting with UserDto. All of the actual dto implementation fields are filled to mirror the persistent state : if there is data, the field of the dto is not null.
So then you just need to ask the service layer which implementation of the Dto you want :
public <T extends UserDto> T toDto(User entity, Class<T> dtoClass) {
...
}
Then in the service layer, you could have a :
Map<Class<? extends UserDto>, UserDtoBUilder> userDtoBuilders = ...
where you register the different builders that will create and initialize the various UserDto implementations.

I'm not sure why you would want lazy loading, but I guess because your UserDto serves multiple representations through optionList configuration?
I don't know how your presentation layer code looks like, but I guess you have lot's of if-else code for each element in optionList?
How about having different representations i.e. subclasses instead? I'm asking this because I'd like to suggest giving Blaze-Persistence Entity Views a try. Here a little code example that fits to your domain.
#EntityView(User.class)
public interface SimpleUserView {
// The id of the user entity
#IdMapping("id") int getId();
String getFirstname();
}
#EntityView(Authorization.class)
public interface AuthorizationView {
// The id of the authorization entity
#IdMapping("id") int getId();
// Whatever properties you want
}
#EntityView(User.class)
public interface AuthorizationUserView extends SimpleUserView {
List<AuthorizationView> getAuthorizations();
}
These are the DTOs with some metadata about the mapping to the entity model. And here comes the usage:
#Transactional
public <T> T findByName(String name, EntityViewSetting<T, CriteriaBuilder<T>> setting) {
// Omitted DAO part for brevity
EntityManager entityManager = // jpa entity manager
CriteriaBuilderFactory cbf = // query builder factory from Blaze-Persistence
EntityViewManager evm = // manager that can apply entity views to query builders
CriteriaBuilder<User> builder = cbf.create(entityManager, User.class)
.where("name").eq(name);
List<T> result = evm.applySetting(builder, setting)
.getResultList();
return result;
}
Now if you use it like service.findByName("someName", EntityViewSetting.create(SimpleUserView.class)) it will generate a query like
SELECT u.id, u.firstname
FROM User u
WHERE u.name = :param_1
and if you use the other view like service.findByName("someName", EntityViewSetting.create(AuthorizationUserView.class)) it will generate
SELECT u.id, u.firstname, a.id
FROM User u LEFT JOIN u.authorizations a
WHERE u.name = :param_1
Apart from being able to get rid of the manual object mapping, the performance will improve because of using optimized queries!