SPRING JPA Lazy loading data to use in other class - spring

Question

I'm using Spring Boot for a project, I'm stuck with lazy loading.
What I want to do is load data in my controller, then send to presentable object, that will extract needed information and the JSON serializer do the bad work to create my custom HTTP response.
the problem occurs when the UserPresentation class calls the folder getter, the error is the well known: could not initialize proxy - no Session.
Of course the default fetch is LAZY for the folder and I want this, but I don't know how to prepare the object to be usable in the Presentation.
I copy-pasted only Folder set to be clear and short, but I've more collection inside User class, all of them give me the same problem.
I know that I could call getter in controller just to initialize Collections, but I find this like an hardcoding, in fact if I want add something to presentable I need to do in controller too.
I've tried too with #Transactional but not works.
Here are my class:
#Entity
public class User {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "USER_ID")
private Integer id;
#Column(unique = true)
private String email;
private String password;
#Enumerated(EnumType.STRING)
private Authority userAuthority;
#OneToMany(mappedBy = "owner", cascade = CascadeType.ALL)
private Set<Folder> ownFolders = new HashSet<>();
... getter setter
}
#RestController
public class UserController {
#GetMapping(value = "/api/user", produces = APPLICATION_JSON_VALUE)
public CustomResponseEntity userInfo() {
User currentUser = loginService.getCurrentUser();
UserPresentation userPresentation = new UserPresentation(currentUser);
return ResponseManager.respondData(userPresentation);
}
}
public class UserPresentation implements Presentable {
private User user;
public UserPresentation(User user) {
this.user = user;
}
public Integer getId() {
return user.getId();
}
public String getEmail() {
return user.getUsername();
}
public String getAuthority() {
return user.getUserAuthority().name();
}
public boolean isEnabled() {
return user.isEnabled();
}
public Integer getOwnFolders() {
Set<Folder> folderList = user.getOwnFolders();
if (folderList == null)
return 0;
return folderList.size();
}
}
Last two just to be clear
public class ResponseManager {
// DATA
public static ResponseEntity respondData(Presentable presentable, String token) {
CustomResponse response = new DataResponse<>(presentable);
return new ResponseEntity<>(response, HttpStatus.OK);
}
}
public class DataResponse<T extends Presentable> extends CustomResponse {
private T data;
public T getData() {
return data;
}
private void setData(T data) {
this.data = data;
}
public DataResponse(T data) {
this.setData(data);
}
#Override
public String getType() {
return DATA;
}
}

Answer 1

I suppose you load the current user form the database with:
User currentUser = loginService.getCurrentUser();
and the getCurrentUser() method is transactional. You can either:
Use JPQL like this:
"select u from User u join fetch u.ownFolders where ... " to load the user's info (this way ownFolders relation is eagerly fetched)
or
Simply call user.getOwnFolders() inside getCurrentUser() to trigger
the fetch.

Answer 2

I found a way, even is a little bit dirty it allows me to do what I want without big change at the code.
Practically the problem occurs during the JSON serialization, that run outside of my control (somewhere inside Spring classes just before send HTTP response), so I manually serialized every Presentable object inside a #Transactional block just after its creation.
These are the changed classes:
public class UserPresentation implements Presentable {
private User user;
public UserPresentation(User user) {
this.user = user;
this.initialize() //ADDED (called here and in every other class that implements Presentable)
}
...getter and setter (which I want as JSON fields)
}
#RestController
public class UserController {
#Transactional //ADDED
#GetMapping(value = "/api/user", produces = APPLICATION_JSON_VALUE)
public CustomResponseEntity userInfo() {
User currentUser = loginService.getCurrentUser();
UserPresentation userPresentation = new UserPresentation(currentUser);
return ResponseManager.respondData(userPresentation);
}
}
Before this fix, the interface was used only to use Polymorfism inside ResponseManager, so was empty
public interface Presentable {
default void initialize() {
try {
new ObjectMapper().writeValueAsString(this);
} catch (JsonProcessingException e) {
throw new RuntimeJsonMappingException(e.getMessage());
}
}
}

Answer 3

I would suggest you use https://github.com/FasterXML/jackson-datatype-hibernate
The module supports datatypes of Hibernate versions 3.x , 4.x and 5.x; as well as some of the associated behavior such as lazy-loading and detection of transiency (#Transient annotation).
It knows how to handle Lazy loading after the session is closed , it will skip the json conversion for objects marked as Lazy fetch when outside session
<dependency>
<groupId>com.fasterxml.jackson.datatype</groupId>
<artifactId>jackson-datatype-hibernate5</artifactId>
<version>2.9.8</version>
</dependency>
ObjectMapper mapper = new ObjectMapper();
// for Hibernate 4.x:
mapper.registerModule(new Hibernate4Module());
// or, for Hibernate 5.x
mapper.registerModule(new Hibernate5Module());
// or, for Hibernate 3.6
mapper.registerModule(new Hibernate3Module());
#Configuration
public class WebMvcConfig implements WebMvcConfigurer {
/*
* Here we register the Hibernate4Module into an ObjectMapper, then set this * custom-configured ObjectMapper to the MessageConverter and return it to be * added to the HttpMessageConverters of our application
*/
public MappingJackson2HttpMessageConverter jacksonMessageConverter() {
MappingJackson2HttpMessageConverter messageConverter = new MappingJackson2HttpMessageConverter();
ObjectMapper hibernateAwareObjectMapper = new ObjectMapper();
hibernateAwareObjectMapper.enable(MapperFeature.ACCEPT_CASE_INSENSITIVE_ENUMS);
hibernateAwareObjectMapper.enable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
// Registering Hibernate5Module to support lazy objects
hibernateAwareObjectMapper.registerModule(new Hibernate5Module());
messageConverter.setObjectMapper(hibernateAwareObjectMapper);
return messageConverter;
}
}
XML config
<mvc:annotation-driven>
<mvc:message-converters>
<!-- Use the HibernateAware mapper instead of the default -->
<bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
<property name="objectMapper">
<bean class="path.to.your.HibernateAwareObjectMapper" />
</property>
</bean>
</mvc:message-converters>
</mvc:annotation-driven>